When aid or assistance in the use of force turns into an indirect use of force

ABSTRACT

This article explores when aiding or assisting another state in the use of force against a third state may turn into a ‘use of force’ within the meaning of Article 2(4) of the UN Charter. The provision of weapons to a belligerent state is usually not considered to pass this threshold. But one can imagine extreme cases where it is appropriate to assume that Article 2(4) is implicated. This article argues that a contribution that directly materialises in another state’s direct use of force against a third state, and is essential to that use of force, should be treated as an indirect use of force if the contributing state itself intends to force the will of the third state. Military support to Ukraine has come to a point where these criteria would be met.

KEYWORDS:

• Use of force
• indirect force
• aid or assistance
• coercive intent
• Ukraine
• weapons

1. Introduction

A state that supports another state in the commission of an act amounting to an illegal use of force within the meaning of Article 2(4) of the UN Charter and customary international law may be internationally responsible for doing so under the concept of ‘aid or assistance’. This concept, also known as ‘complicity’, is embodied in Article 16 of the International Law Commission’s articles on state responsibility (ASR).¹ Article 3(f) of the UN General Assembly’s 1974 Definition of Aggression² addresses a specific situation of complicity. It stipulates that ‘[t]he action of a State in allowing its territory, which it has placed at the disposal of another State, to be used by that other State for perpetrating an act of aggression against a third State’ shall qualify as an act of aggression. While some scholars regard Article 3(f) as reflecting an aggression-related complicity rule,³ others read the provision as defining a particular type of aggression.⁴ Sometimes, Article 3(f) is associated with the notion of indirect aggression.⁵ In any case, the state allowing its territory to be used is responsible only for its own action in doing so. It is not responsible for the aggressive conduct of the state that uses the territory.

‘Allowing territory to be used’ is the only form of state-to-state aid or assistance in the commission of an act of aggression subject to a special norm of the jus contra bellum.⁶ Other activities in support of an unlawful use of force are generally treated as complicity under Article 16 ASR.⁷ However, at some point supportive action can become so intense and critical that it reaches the level of a ‘use of force’ in the sense of Article 2(4) of the UN Charter. The idea that a ‘use of force’ encompasses indirect uses of force is reflected in several General Assembly resolutions⁸ and has been fleshed out by the International Court of Justice.⁹ The notion of indirect use of force is mostly applied in cases in which a state is actively involved in subversive activities against another state, especially by organising irregular forces or armed bands for incursion into foreign territory,¹⁰ by sending armed bands that carry out acts of armed force against another state,¹¹ or by arming and training rebels.¹² But it is not apparent why the conception of an indirect use of force should not be adapted to situations where a state is involved in another state’s use of force against a third state.¹³ As Michael Schmitt and W. Casey Biggerstaff noted, the harm caused by supporting another state (even if lawful) could be more severe than that caused by supporting an armed group.¹⁴ Such a setting could also be dangerous to third states because the provision of arms, military services, and intelligence in an environment where states use force against each other carries a high risk of escalation. This could seriously threaten international peace and security. Article 2(4) of the UN Charter does not only protect the territorial integrity and political independence of states but ultimately serves the purpose of maintaining international peace and security. As Schmitt and Biggerstaff explained, ‘limiting the prohibition of indirect force to situations involving non-State groups would run counter to the object and purpose of the rule prohibiting the use of force between States’.¹⁵ In their view, the inclusion of indirect force in the concept of the use of force was even more compelling when all the actors were states.¹⁶ These considerations speak in favour of extending the concept of indirect use of force to cases in which one state contributes to another state’s use of force against a third state.

What are the implications when the action of a state in supporting another state’s use of force qualifies as an indirect use of force in the sense of Article 2(4) of the UN Charter? The contributing state would risk violating a fundamental norm of international law, which is, at least to a certain extent, part of jus cogens.¹⁷ This would have various consequences under the law of state responsibility, potentially even consequences stipulated in Article 41 ASR, which are triggered by a serious breach of an obligation arising under a peremptory norm of general international law.¹⁸ A state that – directly or indirectly – uses force falling within the scope of Article 2(4) of the UN Charter must demonstrate that its conduct is justified under the jus contra bellum, either as a lawful exercise of the right of individual or collective self-defence or as a measure authorised by the UN Security Council on the basis of Chapter VII of the UN Charter.

A state merely aiding or assisting another state in the use of force, without itself carrying out acts that would qualify as a use of force, does not need to justify its conduct as long as the use of force by the other state is lawful.¹⁹ Due to the accessory character of Article 16 ASR, the question of responsibility of the aiding or assisting state arises only if the state that benefits from this support uses force in breach of Article 2(4) of the UN Charter. A state that is accused of having provided aid or assistance to another state’s unlawful use of force can plead that the wrongfulness of its conduct was precluded because it constituted a lawful countermeasure (Article 22 ASR). Or it can invoke, subject to strict conditions, the plea of necessity (Article 25 ASR). These two options are arguably not available to a state whose conduct amounts to a direct or indirect use of force within the meaning of Article 2(4) of the UN Charter.²⁰

The states that provide Ukraine with arms, training, and intelligence do not consider such aid and assistance to pass the threshold of a use of force. Otherwise, they would have to justify their activities on the basis of Article 51 of the UN Charter as measures of collective self-defence and immediately report this to the Security Council (which has not happened). Germany has most clearly articulated its position on the matter. In the view of the Federal Government, Germany and its partners are supporting Ukraine through weapons deliveries in the exercise of the right to individual self-defence against Russia’s illegal war of aggression.²¹ Such internationally lawful aid, the Federal Government has stated, does not cross the threshold of a collective exercise of the right to self-defence.²² James Green recently expounded in this Journal that NATO and most of the states that militarily support Ukraine have argued in the same direction, stopping short of considering the provision of weapons and training to Ukraine as a measure of collective self-defence.²³ Hence, the subtle distinction between, on the one hand, supporting another state in the exercise of its right to individual self-defence and, on the other hand, collective self-defence plays an important role in the official legal framing of the military aid and assistance rendered to Ukraine.

The decisive question is at what point the provision of support to another state using force against a third state turns into a use of force by the supporting state. In the literature, this transition has not received much attention to date.²⁴ The following sections address the issue more closely. Sections 2 and 3 give an overview of international practice relating to state-to-state support in the use of force. In particular, it is demonstrated that states frequently refrain from treating the provision of weapons to a belligerent as a sub-category of the use of force. Section 4 unfolds the argument that, in extreme cases, providing aid or assistance to another state’s use of force may reach the level of an indirect use of force falling under Article 2(4) of the Charter. That section discusses when the threshold might be crossed. The proposed criteria are tested in section 5, which focuses on the military support rendered to Ukraine. Section 6 contains the main conclusions of this study.

2. The UN General Assembly and the issue of state-to-state support in the use of force

In 1951, the UN General Assembly, acting under the Uniting for Peace Resolution, found ‘that the Central People’s Government of the People’s Republic of China, by giving direct aid and assistance to those who were already committing aggression in Korea and by engaging in hostilities against United Nations forces there, has itself engaged in aggression in Korea’.²⁵ This statement could be read as an expression of a broader understanding of the concept of aggression, incorporating direct assistance in the commission of an act of aggression.²⁶

During the deliberations that led to the adoption of the Definition of Aggression in 1974, it was not controversial that allowing one’s territory to be used for aggression would constitute an act of aggression.²⁷ Erin Kimberley Pobjie has suggested that the category of unlawful use of force specified in Article 3(f) of the Definition was a result of subsequent agreement between states regarding the interpretation of Article 2(4) of the UN Charter, covering a special case of indirect force that was characterised neither by the use of direct physical means nor by direct physical effects.²⁸ Prior to the adoption of the Definition of Aggression, states had already followed an unwritten rule prohibiting such conduct.²⁹ In 1958, Germany was accused by the Soviet Union of having participated in US aggression against Lebanon by allowing US armed forces to use airfields on German territory. In response, the Federal Government of Germany asserted that it did not view the measures taken by the United States and the United Kingdom in the Middle East as an unlawful intervention. Moreover, the Federal Government affirmed that, in accordance with its obligations under general international law, it would never place German territory at the disposal of another state for perpetrating acts of aggression.³⁰

But what about other categories of support? In 1963, Rosalyn Higgins posed the question of whether giving military aid in the form of arms, training etc. to a state aggressor in its attack on the victim-state constituted indirect aggression or aggression proprio suo.³¹ The International Law Commission, however, noted that the supply of weapons or other means by one state to another state in order to facilitate the commission of an act of aggression by that state did not necessarily, and in every case, constitute aggression.³² In the Commission’s view, it would be inadmissible to generalise the idea embodied in Article 3(f) of the Definition of Aggression.³³

Nevertheless, it is widely acknowledged that Article 2(4) of the UN Charter and corresponding customary international law must be interpreted broadly to cover a variety of different manifestations of inter-state armed force.³⁴ Theoretically, it would have been possible for the UN General Assembly to include under the first principle of the 1970 Friendly Relations Declaration a paragraph dealing with the involvement of one state in the use of force by another. The negotiations, which led to the adoption of the Declaration, concentrated on a host of problematic issues relating to the prohibition of the use of force.³⁵ But the implications of the participation by one state in another state’s use of force were not discussed.

In the 1987 Declaration on the Enhancement of the Effectiveness of the Principle of Refraining from the Threat or Use of Force in International Relations (‘1987 Declaration’), the UN General Assembly declared that ‘States have the duty not to urge, encourage or assist other States to resort to the threat or use of force in violation of the UN Charter’.³⁶ The impulse for including this paragraph in the Declaration came from the Soviet Union, which actually pursued the adoption of a World Treaty on the Non-Use of Force in International Relations.³⁷ The draft World Treaty submitted by the Soviet Union in 1979 contained a provision according to which the High Contracting Parties would have agreed ‘not to assist, encourage or induce any States or groups of States to use force or the threat of force in violation of the provisions of this Treaty’.³⁸ The representative of the Soviet Union pointed out that Article 3(f) of the Definition of Aggression was an example of action which would contravene this particular provision of the draft World Treaty. A similar infringement, the representative stressed, was the sale by states of weapons to an aggressor state or to a state preparing for aggression.³⁹ Moreover, he explained that the proposed World Treaty would not only reaffirm the ban on providing assistance to states that had already used force, but was also intended to avert the use of force through a prohibition on encouraging and inciting other states to illegal conduct. Ultimately, the initiative to conclude such a treaty did not receive sufficient support within the international community. Remarkably, five Western European countries, which were among the states that objected to the Soviet proposal for a World Treaty on the Non-Use of Force, played an important part in pushing for the inclusion of a paragraph on state-to-state assistance in the text of the 1987 Declaration. They asserted that such a declaration by the UN General Assembly should affirm that ‘no State shall assist, encourage or induce any State or group of States to use force or the threat of force in violation of the political independence, territorial integrity or sovereignty of other States’.⁴⁰ Since that time, however, the issue of state-to-state assistance in the use of force has not received further attention in landmark resolutions of the UN General Assembly.

3. State practice on state-to-state support in the use of force

States have legitimate political, security, and economic interests in trading arms and military technology and equipment. And every sovereign state has the right to acquire the means for defending itself against armed attacks, consistent with the right of self-defence recognised by the UN Charter.⁴¹ International arms control and non-proliferation treaties as well as UN Security Council sanctions regimes establish further conditions for such transactions.

States involved in weapons sales usually do not feel compelled to comment on the legal basis and legal implications of a particular transaction, not even when the recipient state is a party to an inter-state armed conflict.⁴² If at all, states rely on the concept of qualified neutrality or non-belligerency to escape the strict obligations under the law of neutrality applicable in situations of war.⁴³ However, there is no documented case of a state invoking collective self-defence to justify the provision of arms and military equipment to a belligerent.⁴⁴ State practice in this field may be illustrated by the following episodes.

1. In October 1973, during the Yom Kippur War, the Soviet Union delivered weapons to Egypt and Syria. In the UN Security Council, the representative of the Soviet Union asserted that ‘faced with Israel’s stubborn refusal to accept a just political settlement in the Middle East’, the Arab states were fully entitled to fight for the liberation of their occupied territories and were also entitled under Article 51 of the Charter to self-defence in the event of aggression.⁴⁵ Moscow, however, did not comment on its own part as an arms provider in this war. When evaluating the Soviet Union’s role, two aspects are of particular importance. First, Egypt and Syria had steadily received arms from the Soviet Union prior to the outbreak of the war. According to analysts, the arsenals of both states had been filled even before hostilities had begun, and the resupply during the war had not made much difference on the ground.⁴⁶ Second, in the analysts’ assessment, the Soviet Union had no particular interest in assisting its clients in the preparation of a future war and in carrying out such a war.⁴⁷ Instead, the Soviets had merely been motivated by the desire ‘to maintain their credibility as a superpower capable of defending the interests of its clients’⁴⁸ while trying to avoid direct confrontation with the United States.⁴⁹

2. During the Iran-Iraq War (1980–1988), France systematically and openly supplied Iraq with combat aircraft, tanks, and other weapon systems. Initially, the French Government emphasised that it had authorised the delivery of Mirage aircraft to Iraq based on the consideration that it was necessary to honour a signed contract, which had been concluded prior to the outbreak of the war. The execution of this contract, the French Minister for Foreign Affairs said, would not be tantamount to an intervention in the conflict.⁵⁰ Two years later, France began to consider Iran to be the aggressor in the war with Iraq. Henceforth, Paris argued that Iraq was under attack on its soil.⁵¹ According to the French Minister of Defence, France’s role was not to participate in the war with Iran nor to give Iraq the instruments for aggression.⁵² This new line of argument was based on the assumption that Iraq was in a situation of self-defence against Iran and had the right to receive weapons from other states to defend itself. But France never went so far as to justify its policy of arms exports to Iraq.

3. In the Oil Platforms case before the International Court of Justice, Iran blamed the United States for having supported Iraq diplomatically, politically, economically, and militarily during the war.⁵³ In Iran’s view, the activities of the United States ‘were not only violations of the law of neutrality; they also constituted unlawful assistance to an aggression, i.e. a violation of the prohibition of the use of force’.⁵⁴ This statement is ambiguous. In the preceding paragraph of its response to the Court, Iran explicitly referred to Article 16 ASR.⁵⁵ This could be interpreted as an argument that the United States was responsible for having provided ‘aid or assistance’ to an aggression (and also to an unlawful use of force) committed by Iraq. At the same time, the statement could be read as equating (‘i.e.’) the provision of assistance to an aggressor with a violation of the prohibition of the use of force. In that latter sense, Iran might have insinuated that the United States itself committed an unlawful use of force by assisting Iraq’s aggression.

4. In the armed conflict with Argentina over the Falkland Islands (1982), the United Kingdom received considerable military aid and assistance from the United States, which included ammunition, missiles, and intelligence.⁵⁶ Already on the eve of the Argentine invasion, US President Ronald Reagan reportedly told British Prime Minister Margaret Thatcher that the United States, while having a policy of neutrality on the question of Falklands sovereignty, would not be neutral regarding the use of military force by Argentina.⁵⁷ After a mediation attempt by US Secretary of State Alexander Haig had failed, President Reagan admitted that the UK Government ‘was left with no choice but to proceed with military action based on the right of self-defense’.⁵⁸ Referring to the Argentine invasion, he made clear that ‘armed aggression of that kind must not be allowed to succeed’.⁵⁹ In a resolution, the US Senate also recognised Britain’s right to self-defence and called on the administration to further all efforts to achieve full withdrawal of Argentine forces from the Islands.⁶⁰ The US Government, however, was far from considering the provision of military aid and assistance to the United Kingdom as amounting to an act of collective self-defence.

5. The US-led invasion of Iraq in 2003 was actively supported by many states in various forms. Some states participated with their armed forces in the intervention, while others provided logistical assistance below that level. A few countries granted overflight rights and allowed their territories to be used by coalition members for preparing and conducting military operations. The United States, the United Kingdom, and Australia were the only countries that formally reported to the UN Security Council on their military engagement in Iraq.⁶¹ In their letters to the Security Council, the three states conveyed their positions concerning the legal bases of the measures taken. Other members of the coalition, as well as those states that supported the campaign on a lower level, were more hesitant in communicating their views on the legality of the invasion. Some of these countries limited themselves to endorsing the campaign in political terms, whereas others even expressed political objections against it.⁶² Germany, for instance, granted overflight rights to the United States and the United Kingdom to transport troops, weapons, and military equipment to the theatre of war. Moreover, Germany allowed US military bases and installations located on German soil to be used as a hub for transporting troops and military material to the Gulf region. But Germany neither adopted the US or British legal position to justify its contribution under the jus contra bellum nor did it attempt to fend off accusations that such assistance was at odds with the law of neutrality. Instead, the German Government, while politically disapproving of the intervention in Iraq, argued that it would fulfil its commitments as a host nation vis-à-vis its partners under the NATO Treaty and accompanying international agreements.⁶³ The Netherlands pursued a different communication strategy. The Dutch Government frequently emphasised that it offered political (but not military) support to the intervening states. At the same time, it ‘apparently thought it important to bring to the fore an international law justification for possible military intervention’.⁶⁴ Like the governments of the United States, the United Kingdom, and Australia, the Dutch Government referred to the corpus of resolutions that had been adopted by the UN Security Council in the aftermath of the Iraqi invasion of Kuwait. In addition, the Dutch Government also repeatedly invoked the material breach by Iraq of its obligations to disarm and cooperate with the inspections.⁶⁵ From these statements, however, it cannot be inferred that the Netherlands deemed it necessary, as a matter of international law, to formally justify the support it provided to the coalition. The case simply illustrates that the governments of those states that backed the intervention felt enormous political pressure to explain their decisions.

6. Germany’s position concerning the military aid and assistance given to Ukraine in support of that country’s struggle against Russia’s war of aggression is particularly instructive. As mentioned previously, the Federal Government has asserted that Germany and its partners are supporting Ukraine through weapons deliveries in the exercise of the right to individual self-defence.⁶⁶ Such internationally lawful aid, in the view of the Federal Government, does not reach the threshold of a collective exercise of the right to self-defence.⁶⁷ This classification implies that the sending of weapons to Ukraine is not considered to amount to a use of force in the sense of Article 2(4) of the UN Charter.⁶⁸ If this threshold was crossed, Germany and other sending states would have to justify their activities as an exercise of the right to collective self-defence and immediately report to the UN Security Council the measures taken in accordance with Article 51 of the UN Charter. So far, however, no such report has been submitted to the Security Council. This may be regarded as evidence that Germany’s position is widely shared among NATO partners and other states that provide military support to Ukraine. But then again, the decision not to rely on the right to collective self-defence is not necessarily an expression of opinio juris. Many Western governments have serious concerns that framing military assistance for Ukraine in terms of collective self-defence would contribute to an escalation of the conflict because it would make it easier for Russia to consider NATO and ‘the West’ as parties to the armed conflict. In the final section of this article, I will return to this issue.

4. The transition from supporting another state’s use of force to an indirect use of force: a threshold proposal

The following discussion is based on the assumption that a state aiding or assisting another state in the use of force against a third state may eventually cross the threshold of a use of force within the meaning of Article 2(4) of the UN Charter. A contribution that reaches this level could be categorised as a form of indirect use of force.

4.1. Preliminary remarks

In the literature, it is generally recognised that there is a fine line between complicity in an illegal use of force and joint responsibility for such a use of force. Ian Brownlie noted that the supply of weapons, military aircraft, and military equipment could in certain situations add up to ‘aid and assistance’ in the commission of an act of aggression. In contrast, the supply of combat units, vehicles, equipment, and personnel, for the specific purpose of assisting an aggressor, he argued, would constitute a joint responsibility.⁶⁹ John Quigley pointed out that where a state was ‘a nearly full partner in aggression, contributing troops and equipment’, joint liability was called for.⁷⁰ But, in his opinion, permitting territory to be used as a base for aggression should rather be subsumed in the complicity category because it was more akin to supplying weapons than it was to ‘full participation’ in aggression.⁷¹ Similarly, Christian Dominicé held that a state which provided military assistance to another state could become a co-author in an internationally wrongful act if the character of the assistance amounted to ‘true participation’ in the act.⁷² Helmut Aust put it in more abstract terms: Where a state contributed to such extent to another state’s wrongful act that it was no longer sufficient to attribute to it the role of a complicit state, joint commission of the wrongful act had to be assumed.⁷³ And in Harriet Moynihan’s words, if the aid was ‘sufficiently significant’, the contributing state’s role could transcend that of mere assistance and the state could become jointly responsible for the act.⁷⁴

All these statements are drawn from works dealing with issues of state responsibility. However, conceptualising the transition from complicity in the use of force to the (joint) commission of a use of force is a matter falling within the realm of the primary norms of the jus contra bellum. It is not for the secondary rules of state responsibility to determine when a state commits an act triggering Article 2(4) of the UN Charter. Therefore, this section concentrates on deriving possible threshold criteria for an indirect use of force, as far as possible, from established principles and concepts of the jus contra bellum. To be clear, this exercise is tentative, due to the fact that there is no conclusive evidence concerning a general practice accepted as law on which we could rely to formulate specific criteria.

4.2. Not setting the threshold too low

The standard for an indirect use of force reflected in the Friendly Relations Declaration is not very high. According to the Declaration, the prohibition of the use of force is implicated even when a state instigates or assists in acts of civil strife or terrorist acts in another state, as long as the acts involve a threat or use of force.⁷⁵ It would also be sufficient if a state acquiesces in organised activities within its territory directed towards the commission of such acts.⁷⁶ The fact that such broad categories and low-threshold activities as ‘assisting’, ‘instigating’, and ‘acquiescing’ were included in the Declaration to specify the prohibition of the use of force shows that it was important to states to affirm that Article 2(4) of the UN Charter takes effect whenever a state makes use of non-state armed groups to destabilise another state through violent action. The formula used by the International Court of Justice in the 1986 Nicaragua judgment – ‘assistance to rebels in the form of the provision of weapons or logistical or other support’ (in particular ‘arming and training’)⁷⁷ – leaves room for interpretation as well. According to Christian Henderson, a state needed to ‘take some practical steps in facilitating the physical coercion’ of the other state.⁷⁸ Simply encouraging physical coercion through verbal means was not sufficient. In his view, the standard formulated by the Court at least required ‘an intentional and material contribution towards others carrying out the direct violence that ensues’.⁷⁹

However, when it comes to the category of state-to-state support in the use of force, the threshold will probably be higher. As demonstrated in section 3, states are generally not willing to treat the provision of weapons by one state to another state using force against a third state as a sub-category of the use of force under Article 2(4) of the UN Charter. In each of the cases under review, the aid provided may be described as ‘an intentional and material contribution’. Yet, none of the states that supplied weapons saw any reason to rely on the right to collective self-defence. And, arguably with the exception of Iran’s statement in the Oil Platforms case,⁸⁰ there are no indications that other states considered a particular act of support in these cases to have reached the level of a use of force. This fact should be duly considered when exploring where the boundary sits between the provision of aid or assistance in the use of force and an indirect use of force. The standard of an ‘intentional and material contribution’ may be sufficient under Article 16 ASR, but would be too low for an indirect use of force in a state-to-state context.

4.3. The material dimension of an indirect use of force

The first element to be considered in this exercise is the nexus between the contribution and the other state’s use of force. After that, I will turn to the quality of the contribution.

4.3.1. The nexus element

The nexus criterion finds expression in the traditional doctrine of indirect use of force. The International Court of Justice, while classifying the arming and training of the contras in Nicaragua by the United States as a use of force against Nicaragua, made clear that the mere supply of funds to the contras did not in itself amount to a use of force.⁸¹ It seems that the Court considered the connection between the funding and the execution of the use of force as too remote. Accordingly, a state that provides substantial financial resources to another state to enable that state to use force against a third state would probably not be regarded as carrying out an indirect use of force.

The nexus element also plays an important role in the classification of assistance to an act of war. Sir Christopher Greenwood explained that ‘[s]upport for a third party’s acts of war shall generally be rated as an act of war of the supporting state if it is directly related, i.e. closely related in space and time, to measures harmful to the adversary’.⁸² Hence, in order that the threshold not be set too low for an indirect use of force in a state-to-state context, it should be required that the nexus be direct,⁸³ implying that the contribution must be directly integrated into a particular use of force and must directly materialise in that use of force.

Moreover, I submit that, to reach the level of an indirect use of force coming within the scope of Article 2(4) of the UN Charter, a contribution must be made to another state’s direct use of force. In my view, contributing to another state’s indirect use of force would not be sufficient to hold the assisting state responsible for an indirect use of force. Imagine state A allowing its territory to be used by state B for arming and training rebels that carry out violent acts against state C. In this case, state B indirectly uses force against state C. But the nexus between the action of state A (in allowing its territory to be used by state B) and the execution of the use of force by the rebels against state C would not suffice to make the action of State A fall under Article 2(4).

Furthermore, it is at least worth discussing whether the direct use of force by the supported state must be of a certain gravity to warrant the assumption that the act of support itself has crossed – in the form of an indirect use of force – the threshold of Article 2(4) of the UN Charter. It is difficult to imagine that a contribution to a small-scale incursion, a targeted counter-terrorism operation, or another ‘minimal’ use of force⁸⁴ would be considered as an indirect use of force. On the other end of the spectrum are situations in which a state contributes to an act of aggression or to an armed attack within the meaning of Article 51 of the UN Charter. In such cases, the contribution could very well constitute an indirect use of force.⁸⁵

The criterion of a direct nexus is always important when a state provides another state with arms, military services, and intelligence. Such transfer may occur within permanent institutionalised structures, on the basis of special treaties or political agreements, or in the form of ad hoc arrangements. Depending on the circumstances, the nexus between an arms shipment and an actual use of force by the recipient state may be quite remote, for example if the recipient state merely stores the weapons and does not use them against its adversary. In this case, according to the standard I propose, the provider-state cannot be held responsible for an indirect use of force. On the other hand, a direct nexus exists if weapons are delivered virtually onto the battlefield, where they are immediately employed by the recipient state.

4.3.2. The quality element: a contribution essential to the actual use of force

When exploring the quality that a contribution to another state’s use of force should have in order to constitute an indirect use of force, it is tempting to draw conclusions from the parallel discussion concerning the quality of ‘aid or assistance’ under Article 16 ASR. From the benchmark of Article 16 ASR,⁸⁶ an appropriate higher standard for the concept of indirect use of force could be deduced. Vladyslav Lanovoy pointed out that one way of distinguishing between complicity and joint and several responsibility was by looking at the contribution’s actual impact on the wrongful act and on its operational involvement in that act.⁸⁷ In his view, a distinction had to be made between contributions that are essential to and those that merely facilitate the commission of the wrongful act.⁸⁸ An essential contribution would establish joint responsibility, whereas mere facilitation would qualify as complicity.

As already emphasised, the law of state responsibility cannot determine the lower threshold of an indirect use of force. Nevertheless, ‘essentiality’ could become a key concept to describe the quality that a contribution to another state’s use of force should have to amount to an indirect use of force. In the absence of any established doctrine on this issue, the following discussion is particularly explorative.

‘Essential’ may be understood as being of fundamental or extreme importance for the other state’s direct use of force against the third state. Schmitt and Biggerstaff suggest to examine the extent to which the support ‘meaningfully contributes to, and sometimes enables’ the supported state’s direct use of force.⁸⁹ However, the term ‘meaningfully contributing’ seems to fit better with the concept of aid or assistance under Article 16 ASR.⁹⁰ If the benchmark for an indirect use of force was ‘meaningfully contributing’, it would become impossible to draw a clear line between, on the one hand, the type of contribution required by Article 16 ASR (i.e. a ‘significant’, ‘substantial’, or ‘material’ contribution) and, on the other hand, a ‘meaningful’ contribution as required for an indirect use of force. This approach would be practically unworkable. But again, determining the lower threshold of an indirect use of force within the meaning of Article 2(4) is not a matter of bringing the jus contra bellum in line with the law of state responsibility. The crucial point is that, as explained in subsection 4.2, state practice indicates that the standard for an indirect use of force in a state-to-state context must not be defined too broadly. In all of the cases discussed in section 3, the support provided could be characterised as ‘meaningfully contributing’ to the supported state’s use of force. It should be recalled, however, that in these cases no state (perhaps with the exception of Iran in the Oil Platforms case) considered such meaningful contribution sufficient to trigger Article 2(4) of the UN Charter.

In light of relevant state practice, it is appropriate to construe ‘essentiality’ as a more demanding standard, requiring that the contribution is a sine qua non for the other state’s direct use of force. To be precise, one should distinguish between a sine qua non for the other state’s ability to use force as such in the given situation, and a sine qua non for the actual use of force defined by its specific intensity or character. The first alternative – essential to the other state’s general ability to use force – implies that, without the contribution, the other state would not have been in a position to use force at all against the third state in that particular situation. This scenario is somewhat unrealistic, as states will always have at their disposal instruments to use force against each other (no matter how effective or ineffective that use of force would be and what consequences it would have). The second alternative – essential to the actual use of force – reflects the assumption that the other state is generally capable of using force against the third state, but that it is only through the contribution that the actual use of force acquires its specific intensity or character. To operationalise this standard, one should ask whether the contribution actually makes a difference with regard to the intensity or character of the other state’s direct use of force against the third state.

The impact of a contribution can result from the quantity and/or quality of the goods and services provided. Sending large amounts of conventional weapons and ammunition could increase the intensity of a use of force to a whole new level. In certain circumstances, the same effect may be achieved by delivering only a limited number of highly advanced and particularly powerful weapons in combination with special training. In both cases, the contribution can be regarded as essential if the recipient state would not have been able to use force of such high intensity on its own. Placing military bases and infrastructure at the disposal of another state, to be used by that state for conducting a large-scale drone campaign against a third state, could also fall into this category if the state that deploys the drones would otherwise not be in a position to carry out such an attack. The same applies to the provision of combat aircraft and pilot training to a state that does not itself have a powerful air force. Where biological, chemical, or nuclear weapons are made available to a belligerent, the use of these weapons would completely change the character of the confrontation. Moreover, the criterion of essentiality can be fulfilled if intelligence is shared that is necessary to select, locate, and verify targets of a particularly high tactical or strategic value.

However, two qualifications are in order. First, the tipping point at which a contribution may be considered as actually making a difference in increasing the intensity or in changing the character of a particular use of force must be identified case by case, based on the specific circumstances at hand. This requires in every case an assessment of how the use of force by the recipient state would have hypothetically looked without the contribution (assuming that all the other circumstances were identical). If the use of force would have reached the same intensity and had the same character without the contribution, the contribution would not qualify as ‘essential’. Not every delivery of weapons, military equipment, training, or intelligence, while helping the recipient state to conduct a military campaign, will necessarily increase the intensity of the campaign or even change its character. The provision of such goods and services to a poorly-equipped and ill-trained army may significantly enhance the fighting power and combat effectiveness of that army and, indeed, have an impact on the actual use of force. However, if rendered to highly professional armed forces that already possess state-of-the-art weapon systems, the same kind and same amount of support may not make a difference at all. In this sense, impact is relative and contextual.⁹¹

Second, from a conceptual perspective, it is important to distinguish cases in which one state essentially contributes to another state’s use of force from situations of ‘direction and control’. If a state takes an active part in planning, organising, and coordinating a use of force executed by another state, it may also have the opportunity to define the character of that use of force. However, where such involvement rises to the level of direction and control, the actions committed by the state that directly uses force against the third state will be attributed to the controlling state. ‘Direction and control’ as reflected in Article 17 ASR is an attributional concept. In order to incur responsibility for an internationally wrongful act under Article 17 ASR, a state must exercise both direction and control over the wrongful conduct. According to the International Law Commission’s Commentary, ‘control’ within the meaning of Article 17 refers to domination over the commission of wrongful conduct, whereas ‘direction’ connotes actual direction of an operative kind.⁹² By contrast, the concept of indirect use of force is not about attribution. It denotes a certain conduct that qualifies itself as a use of force.

4.4. The subjective dimension

Schmitt and Biggerstaff recently argued that a key factor likely to prove influential when states assess whether support amounted to an indirect use of force was ‘the degree to which the supporting State objectively intends to contribute to the supported State’s direct use of force’.⁹³ In their understanding, ‘a supporting State objectively intends to assist another’s direct use of force when its actions are purposely and consciously designed to enhance the supported State’s forcible operations’.⁹⁴ At the same time, they concede that states might find that support already crossed the indirect use of force threshold ‘when a supporting State knows that its assistance will contribute to a direct use of force, even if such a result is not the conscious purpose of the supporting State’.⁹⁵ Going one step further, the two authors suggest that ‘States might even conclude that aid or assistance qualifies as an indirect use of force when a supporting State consciously disregards a substantial risk that it will enable or contribute to a supported State’s use of force’.⁹⁶ In their view, ‘even if a supporting State does not know that its aid or assistance will enhance a supported State’s use of force, if it should have known … that it would do so, that fact may contribute to the conclusion by other States that it has used force indirectly’.⁹⁷

On this point, I strongly disagree. I do not believe that states will regard a certain contribution as an indirect use of force if the state in question merely intends to assist another state’s direct use of force, let alone if it acts with indirect intent or any lesser degree of knowledge and/or intent regarding the supporting effect of its conduct. The different manifestations of an ‘intent to assist’ addressed by Schmitt and Biggerstaff have been discussed in detail by Marko Milanovic with a view to interpreting the mental element of the complicity rule embodied in Article 16 ASR.⁹⁸ Framing the mental element of an indirect use of force in a state-to-state context as ‘intent to assist another state’s direct use of force’ would result in conflating, as far as the subjective dimension is concerned, the concept of indirect use of force with the concept of aid or assistance. If both concepts presupposed the same subjective element, the material element alone would make the difference. In any case, for an act to qualify as an indirect use of force falling within the scope of the primary rule of Article 2(4) of the UN Charter, it should be required that the act is carried out with use-of-force-specific intent.

4.4.1. The use of force and the element of intent

In the negotiations leading to the adoption of the Definition of Aggression, it was contested whether intention and purpose should be included in the Definition as criteria for determining the existence of an act of aggression.⁹⁹ The acts listed in Article 3 of the Definition of Aggression are typically committed with hostile intent. And yet, the final text of the Definition does not contain any reference to intention or purpose. Article 2 of the Definition merely states that ‘the Security Council may … conclude that a determination that an act of aggression has been committed would not be justified in the light of other relevant circumstances’. This passage remained subject to varying interpretations. Japan, for instance, made clear that in the negotiations it had not insisted on explicit reference to the element of intent, on the understanding that intent was one of the factors to be considered by the UN Security Council in determining whether an act of aggression had been committed.¹⁰⁰ Likewise, the United States asserted that it had been agreed not to make special reference to intent or purpose as that notion was covered by the phrase ‘other relevant circumstances’ in Article 2.¹⁰¹ This position was shared by Canada and other Western states.¹⁰² The Soviet Union maintained that the Security Council would always have to assess the intentions of states in order to identify an aggressor.¹⁰³

When claiming to be the victim of an armed attack, states often emphasise that the attacker acted deliberately.¹⁰⁴ However, as far as Article 2(4) of the UN Charter is concerned, it is not settled whether a use of force generally requires that the state acts with hostile intent.¹⁰⁵ Olivier Corten and Christian Henderson both claim that Article 2(4) is engaged only if the state intends to compel the other state to do or to refrain from doing something.¹⁰⁶ Corten holds the view that even when a state takes only limited military measures against another state, but admits that these measures are part of a policy conducted against the other state, there is no doubt that Article 2(4) is applicable.¹⁰⁷ Tom Ruys differentiates as follows: While hostile intent is sufficient to qualify an unlawful small-scale incursion as a use of force,¹⁰⁸ no express authority supports the view that intent is generally needed to establish a violation of Article 2(4).¹⁰⁹

The Independent International Fact-Finding Mission on the Conflict in Georgia asserted in 2009 that, according to state practice, not all militarised acts entail a violation of Article 2(4) of the UN Charter.¹¹⁰ The Mission’s experts made clear that many activities are of a routine character, ‘devoid of any hostile intent’ and are ‘meaningless in the absence of a sizeable dispute’. But as soon as they are ‘non-routine, suspiciously timed, scaled up, intensified, geographically proximate, staged in the exact mode of a potential military clash, and easily attributable to a foreign policy message’, the hostile intent is considered present and the demonstration of force manifest.¹¹¹

In any case, intention is not to be confused with motivation. An intent to use force need not be ‘hostile’ in the literal sense. It may also be based on economic or humanitarian motives. Even a state motivated to help the victim of an aggression to defend itself may act with hostile intent against the aggressor. What matters, according to Corten and Henderson, is that the state intends to force the will of the other state through physical coercion.¹¹² This view is based on the presumption that the use of force is a particular kind of intervention, which implies that force is defined by the element of coercion.¹¹³ For the purpose of the present analysis, the term ‘coercive intent’ will be used instead of ‘hostile intent’.

4.4.2. Coercive intent as the defining criterion of an indirect use of force

While it is not settled whether Article 2(4) of the UN Charter generally requires that a use of force is carried out with coercive intent, the doctrine of indirect use of force – as traditionally applied to situations in which a state is involved in subversive activities – clearly incorporates such a subjective element. A state that arms, trains, and sends irregulars to carry out acts of armed force against another state always pursues a political agenda against the other state. For such activities, coercive intent is constitutive.

The mens rea standard for an indirect use of force in settings in which a state contributes to another state’s use of force cannot be lower than the standard applied under the traditional doctrine of indirect use of force. Without the element of coercive intent, the conception of an indirect use of force in a state-to-state context would be overly broad. Schmitt and Biggerstaff assume that states could be inclined to identify an indirect use of force where a state delivers weapons to another state with the intent to facilitate that state’s use of force against a third state (and possibly even where the state does not know but should have known that its aid or assistance will enhance the recipient state’s use of force).¹¹⁴ However, state practice, as demonstrated in section 3, suggests that the bar will probably be much higher. I submit that it would not be sufficient if the contributing state simply wanted to help the other state to use force. An indirect use of force in such situations should presuppose, in accordance with the traditional doctrine of indirect use of force, that the contributing state itself intends to force, through its contribution, the will of the third state. In this sense, the defining feature of an indirect use of force is the intention to enforce a certain policy vis-à-vis the third state. Hence, the context of the case may serve as an important indicator for determining whether the contribution expresses coercive intent.¹¹⁵ If the relationship between the contributing state and the third state is extremely tense and the contributing state has already tried to force the will of the third state by other means, there is reason to assume that the contributing state acts with coercive intent. All in all, it is appropriate to assume that it is the element of coercive intent that elevates an essential contribution to another state’s direct use of force from the level of aid or assistance in the use of force to an indirect use of force.

5. The provision of military support to Ukraine as a test case

The invasion of Ukraine by Russia on 24 February 2022 set in motion a campaign by around thirty states to provide arms and military equipment to Ukraine. Ukraine has been progressively receiving more advanced and more powerful weapon systems. By the end of April 2023, this included, among other things, infantry fighting vehicles, battle tanks, howitzers, and air defence systems, as well as MIG-29 aircraft.¹¹⁶ Even the delivery of modern Western fighter jets is under discussion. NATO member states bordering Ukraine serve as hubs for the transport of the weapons,¹¹⁷ which are largely combat-ready when they reach Ukrainian territory. To ensure that the systems can be properly operated, Ukrainian soldiers are trained on military bases in Western countries. Moreover, a remote maintenance system has been established, which allows Ukrainian technicians in the war zone to communicate directly with Western specialists via video or chat.¹¹⁸

In the first months of the war, it was reported that the United States and other countries had provided intelligence, which had been used by the Ukrainian army to carry out specific military operations.¹¹⁹ In February 2023, the Washington Post described a ‘previously undisclosed practice that reveals a deeper and more operationally active role for the Pentagon in the war’.¹²⁰ According to the newspaper, Ukrainian officials had admitted that they almost never launched HIMARS rocket strikes without specific coordinates provided or confirmed by US military personnel. The targeting all went through US installations on NATO territory, a Ukrainian official reportedly told the newspaper.¹²¹

The question is whether such assistance is essential to Ukraine’s use of force against Russia – ‘essential’ being understood as sine qua non for the actual use of force defined by its specific intensity or character. According to statements frequently made by the Ukrainian Government as well as by Western governments, permanent supplies of advanced weapon systems are absolutely vital to uphold Ukrainian defence against Russian attacks. Recent expert studies confirm this assessment.¹²² Without adequate training and remote maintenance, these systems could probably not be effectively operated by the Ukrainian Army. And the intelligence provided to Ukraine not only ensures more precise striking and a higher hit ratio but also appears to help select more valuable targets. From an outside view, it seems plausible to assume that Ukraine, without the military assistance provided by Western states, would not be able to use force against Russia at the same level as it currently does. But in the absence of more detailed knowledge about the actual impact that such support has on the battlefield, it is difficult to make a more definitive statement on this point.

What is easier to establish is the coercive intent of the states that help Ukraine to defend itself. Many Western leaders and politicians openly proclaim that Ukraine had to ‘win’ the war. Others are more cautious and assert that Ukraine must not lose, or that Russia had to be prevented from achieving its goals. Sometimes, it is heard that Ukraine needed to be put in a better position for future negotiations, which implied that the country should be enabled to recapture, as much as possible, territory occupied by Russia. A common consensus among the members of the pro-Ukraine coalition appears to be that Russia, at a minimum, must be kept from committing further atrocities against the civilian population. Whatever the underlying strategy, the final objective, or the motives pursued by Western states, the general purpose of the military aid and assistance provided to Ukraine is to force Russia to end its aggression.

On a more abstract level, it may be argued that the support provided to Ukraine by individual states is part of a concerted effort aimed at containing an aggressor through the use of force executed by the victim-state. There are concerns that Russia’s aggressive imperialist stance could also pose a threat to other states in the region, especially to the Republic of Moldova and to the Baltic countries. For these states, it is vital for the protection of their own territorial integrity and political independence to stop Russian aggression in Ukraine. From their perspective, Ukraine’s resistance is virtually the first line of defence. A statement by Annalena Baerbock, the Federal Foreign Minister of Germany, illustrates the general mindset that drives Western states’ support for Ukraine. In January 2023, she said at a meeting of the Parliamentary Assembly of the Council of Europe:

Ukraine needs more military support. But not only by one country, like mine, or the U.S., by all of us. We can fight this war only together. This is about our common European peace order, and this is why we are calling with regard to tanks and other support for a big big international group … Yes, we have to do more to defend Ukraine. Yes, we have to do more also on tanks but the most important – and the crucial part is – that we do it together and that we do not do the blame game in Europe because we are fighting a war against Russia and not against each other.¹²³

All in all, the provision of weapons, training, and intelligence to Ukraine is carried out with an explicitly articulated intent to force Russia to back off, which satisfies the subjective requirement of an indirect use of force as suggested in the previous section. That being said, no in-depth discussion is needed to show that the conditions of collective self-defence would be fulfilled (with the exception of the formal reporting requirement contained in Article 51 of the UN Charter).

6. Conclusion

The key argument developed in this article is that a state which contributes to another state’s direct use of force against a third state will itself pass the threshold of a ‘use of force’ within the meaning of Article 2(4) of the UN Charter if: (1) there is a direct nexus between the contribution and the direct use of force; (2) the contribution is essential to the direct use of force (‘essential’ being understood as sine qua non for the actual use of force defined by its specific intensity or character), which means that it is only through the contribution that the direct use of force acquires its particular intensity or character; and (3) the contributing state acts with the intent to force the will of the third state against which the use of force is directed (coercive intent). According to the standard proposed in this study, coercive intent would elevate an essential contribution from the level of aid or assistance in the use of force to an indirect use of force. Under these conditions, even the provision of weapons, training, and intelligence to a state using force against a third state may come within the ambit of Article 2(4) of the UN Charter. The military support rendered to Ukraine by some states might have already crossed this threshold.¹²⁴

The risk of becoming a party to the conflict has caused great concern in a number of countries that help Ukraine to defend itself against Russian aggression. At the Brussels NATO Summit in March 2022, French President Emmanuel Macron insisted that the NATO members’ position was to continue to provide defensive and lethal weapons to Ukraine, but with the ‘red line’ of not becoming a ‘co-belligerent’. Delivering planes and tanks, according to President Macron, would cross that line, making NATO a co-belligerent.¹²⁵ And German Chancellor Olaf Scholz repeatedly stressed that nothing would be done which would turn NATO into a warring party. The jus contra bellum and international humanitarian law are two completely separate regimes, which must not be mixed. Nevertheless, the question arises whether providing a belligerent state with aid and assistance of such quality as to meet the criteria of an indirect use of force would not also be sufficient to make the assisting state a party to the armed conflict under international humanitarian law.¹²⁶ The International Committee of the Red Cross has developed an approach to determine when a state that supports a party to an existing armed conflict itself becomes a party to the conflict.¹²⁷ Prima facie, the conditions under this support-based approach appear to be easier to meet than the standard of an indirect use of force proposed in the present article. It would take a more detailed analysis to clarify the relationship between both thresholds. However, should the military aid and assistance provided to Ukraine really qualify as an indirect use of force, it would be much more difficult to uphold the argument that Western states are not already party to the armed conflict between Russia and Ukraine.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Notes

1 ILC, ‘Draft Articles on Responsibility of States for Internationally Wrongful Acts’ (text with commentaries thereto), Report of the International Law Commission on the work of its 53rd session (23 April–1 June and 2 July–10 August 2001) UN Doc A/56/10, 43. According to Article 16 ASR: ‘A State which aids or assists another State in the commission of an internationally wrongful act by the latter is internationally responsible for doing so if: (a) that State does so with knowledge of the circumstances of the internationally wrongful act; and (b) the act would be internationally wrongful if committed by that State’. In 2007, the International Court of Justice found that Article 16 ASR reflected a customary rule. See Application of the Convention on the Prevention and Punishment of the Crime of Genocide (Bosnia and Herzegovina v Serbia and Montenegro) (judgment) [2007] ICJ Rep 43, 217, para 420.

2 UNGA Res 3314 (XXIX), UN Doc A/RES/3314 (14 December 1974) annex.

3 See, e.g. John Quigley, ‘Complicity in International Law: A New Direction in the Law of State Responsibility’ (1986) 57 British Yearbook of International Law 77, 86; Miles Jackson, Complicity in International Law (Oxford University Press, 2015) 142–4; Claus Kreß, ‘The State Conduct Element’ in Claus Kreß and Stefan Barriga (eds), The Crime of Aggression: A Commentary (Cambridge University Press, 2017) 412, 446.

4 See, e.g. Nele Verlinden, ‘“Are We at War?” State Support to Parties in Armed Conflict: Consequences under Jus in Bello, Jus ad Bellum and Neutrality Law’ (Katholieke Universiteit Leuven, Doctoral Thesis, November 2019) https://ghum.kuleuven.be/ggs/people/00099480, 233–4.

5 See, e.g. Ahmed M Rifaat, International Aggression: A Study of the Legal Concept: Its Development and Definition in International Law (Almqvist and Wiksell, 1979) 273.

6 Article 3(f) of the Definition of Aggression has attained the status of customary international law: see, e.g. Jackson (n 3) 137–9.

7 See, e.g. Quigley (n 3) 87–90.

8 In light of the experience of World War II, the drafters of the UN Charter were particularly concerned with the suppression of large-scale acts of aggression conducted by states using their entire armies to attack other states. One of the most important achievements of the San Francisco Conference was the adoption of UN Charter Article 2(4). However, while officially endorsing Article 2(4), states increasingly turned to non-overt and indirect methods of using force to escape the prohibition. Fomenting civil strife, providing assistance to rebel groups, and sending volunteers, mercenaries, or armed bands soon proved to be effective ways of exerting pressure on foreign governments. To counter this dangerous trend, various initiatives were launched within the United Nations. These efforts resulted in a series of resolutions and other documents, which affirm that Article 2(4) of the UN Charter also covers certain forms of indirect use of force and that aggression may even be committed without directly applying armed force: see, e.g. UNGA Res 290 (IV), UN Doc A/RES/290(IV) (1 December 1949) para 3; UNGA Res 380 (V), UN Doc A/RES/380(V) (17 November 1950) para 1; UNGA Res 2131 (XX), UN Doc A/RES/2131(XX) (21 December 1965); UNGA Res 2625 (XXV), UN Doc A/RES/2625(XXV) (24 October 1970) annex, first principle, paras 8–9; UNGA Res 3314 (XXIX) (n 2) annex, Article 3(g); UNGA Res 42/22, UN Doc A/RES/42/22 (18 November 1987) annex, para 6.

9 Military and Paramilitary Activities in and against Nicaragua (Nicaragua v United States of America) (merits) [1986] ICJ Rep 14. In this judgment, the ICJ pointed out that a breach of the principle of non-use of force could be effectuated by acts of intervention that directly or indirectly involved the use of force (para 209) – either in the direct form of military action, or in the indirect form of support for subversive or terrorist armed activities within another state (para 205). Referring to the Friendly Relations Declaration, the Court emphasised that assistance of this kind was equated with the use of force by the assisting state if the acts committed in the other state involved a threat or use of force (ibid). In the Court’s view, assistance to rebels in the form of the provision of weapons or logistical or other support could be regarded as a threat or use of force (para 195). In particular, the Court found that while the arming and training of the contras in Nicaragua by the United States could certainly be said to involve the threat or use of force against Nicaragua, this was not necessarily so in respect of all the assistance given by the US Government (para 228). According to the Court, the mere supply of funds to the contras, while undoubtedly an act of intervention in Nicaragua’s internal affairs, did not in itself amount to a use of force (ibid). See also Armed Activities on the Territory of the Congo (Democratic Republic of the Congo v Uganda) (judgment) [2005] ICJ Rep 168. In this judgment, the ICJ reaffirmed that acts which breached the principle of non-intervention ‘will also, if they directly or indirectly involve the use of force, constitute a breach of the principle of non-use of force in international relations’ (para 164).

10 Under the first principle of the Friendly Relations Declaration (UNGA Res 2625 (XXV) (n 8) annex), which expresses the prohibition of the use of force, it is stated that ‘[e]very State has the duty to refrain from organizing or encouraging the organization of irregular forces or armed bands, including mercenaries, for incursion into the territory of another State’ (first principle, para 8). Moreover, it is clarified that states have ‘the duty to refrain from organizing, instigating, assisting or participating in acts of civil strife or terrorist acts in another State … when the acts … involve a threat or use of force’ (ibid, para 9). The same provision also mentions the duty of every state to refrain from ‘acquiescing in organized activities within its territory directed towards the commission of such acts’ (ibid).

11 Among the acts that shall qualify as an act of aggression, as defined by the General Assembly in 1974 (UNGA Res 3314 (XXIX) (n 2) annex), is ‘[t]he sending by or on behalf of a State of armed bands, groups, irregulars or mercenaries, which carry out acts of armed force against another State’ of such gravity as to amount to other acts listed in the Definition, ‘or its substantial involvement therein’ (Article 3(g)).

12 Nicaragua (merits) (n 9) para 228. See also Armed Activities (judgment) (n 9) para 161.

13 See Oliver Dörr, ‘Use of Force, Prohibition of’ in Anne Peters (ed), Max Planck Encyclopedia of Public International Law (Oxford University Press, 2023) para 15 (noting that the principle of the non-use of force not only prohibits the direct use of force ‘but also the participation of a State in the use of force, by another State or by private individuals, against or in the territory of another State’).

14 Michael N Schmitt and W Casey Biggerstaff, ‘Aid and Assistance as a “Use of Force” under the Jus Ad Bellum’ (2023) 100 International Law Studies 186, 204. See also Michael N Schmitt, ‘Providing arms and materiel to Ukraine: Neutrality, co-belligerency, and the use of force’, Articles of War (7 March 2022) https://lieber.westpoint.edu/ukraine-neutrality-co-belligerency-use-of-force/; Kevin Jon Heller and Lena Trabucco, ‘The Legality of Weapons Transfers to Ukraine under International Law’ (2022) 13 Journal of International Humanitarian Legal Studies 251, 254.

15 See Schmitt and Biggerstaff (n 14) 203–4.

16 Ibid, 204.

17 The prohibition of aggression is a peremptory norm of international law. But it is questionable whether other components of the prohibition of the use of force are also part of jus cogens. For an overview of the discussion, see Olivier Corten, The Law Against War: The Prohibition on the Use of Force in Contemporary International Law (Hart Publishing, 2nd edn 2021) 206–17.

18 According to Article 41 ASR, states shall cooperate to bring to an end through lawful means any serious breach of an obligation arising under a peremptory norm of general international law. And no state shall recognise as lawful a situation created by such breach, nor render aid or assistance in maintaining that situation.

19 Claus Kreß, ‘The Ukraine War and the Prohibition of the Use of Force in International Law’ (Torkel Opsahl Academic EPublisher, Occasional Paper Series No 13, 2022) 15. See also James A Green, ‘The Provision of Weapons and Logistical Support to Ukraine and the Jus ad Bellum’ (2023) 10 Journal on the Use of Force and International Law 3, 7.

20 See Corten (n 17) 204–46.

21 Deutscher Bundestag, Drucksache 20/1918 (20 May 2022) 39 (Antwort der Staatssekretärin Susanne Baumann vom 18. Mai 2022 zu Frage 56).

22 Ibid.

23 See Green (n 19) 11–5.

24 See Verlinden (n 4) 257–62; Schmitt and Biggerstaff (n 14) 186. See also Benjamin K Nußberger, Interstate Assistance to the Use of Force (Nomos, 2023) (this work had not yet been published when the present article was finalised).

25 UNGA Res 498 (V), UN Doc A/RES/498(V) (1 February 1951) para 1.

26 But see Verlinden (n 4) 258–9 (pointing out that the ‘direct aid and assistance’ provided by China also involved direct intervention by Chinese troops, and that the resolution could therefore not be understood as an indication of a general rule according to which providing aid and assistance to aggression was equated to an act of aggression).

27 See Thomas Bruha, ‘The General Assembly’s Definition of the Act of Aggression’, in Claus Kreß and Stefan Barriga (eds) The Crime of Aggression: A Commentary (Cambridge University Press, 2017) 142, 163–4.

28 Erin Kimberley Pobjie, ‘“Use of Force” and Article 2(4) of the UN Charter: The Meaning of a Prohibited “Use of Force” between States under International Law’ (Universität zu Köln, Doctoral Thesis, 2019) https://kups.ub.uni-koeln.de/10875/, 190.

29 See Jackson (n 3) 137–9.

30 See Helmut Alexy, ‘Völkerrechtliche Praxis der Bundesrepublik Deutschland im Jahre 1958’ (1959/1960) 20 Heidelberg Journal of International Law 636, 663–4.

31 Rosalyn Higgins, The Development of International Law through the Political Organs of the United Nations (Oxford University Press, 1963) 189.

32 ILC, ‘Report of the International Law Commission on the work of its 30th session (8 May–28 July 1978)’ (1978) 33 UN GAOR Supp No 10, UN Doc A/33/10, 254.

33 Ibid.

34 See, e.g. Tom Ruys, ‘The Meaning of “Force” and the Boundaries of the Jus ad Bellum: Are “Minimal” Uses of Force Excluded from UN Charter Article 2(4)?’ (2014) 108 American Journal of International Law 159, 164.

35 See, e.g. UNGA, ‘Report of the Special Committee on Principles of International Law concerning Friendly Relations and Co-Operation among States’ (1969) 24 UN GAOR Supp No 19, UN Doc A/7619, 12–47.

36 UNGA Res 42/22 (n 8) para 4.

37 ‘Draft World Treaty on the Non-Use of Force in International Relations’, submitted by the Union of Soviet Socialist Republics, annexed to the ‘Report of the Special Committee on Enhancing the Effectiveness of the Principle of Non-Use of Force in International Relations’ (1979) 34 UN GAOR Supp No 41, UN Doc A/34/41, 64–6 (previously issued as UN Doc A/AC.193/L.3).

38 Ibid, Article I(2).

39 ‘Report of the Special Committee on Enhancing the Effectiveness of the Principle of Non-Use of Force in International Relations’ (n 37) 32–3.

40 Working Paper submitted by Belgium, France, the Federal Republic of Germany, Italy, and the United Kingdom, circulated as UN Doc A/AC.193/HG/R.1, reprinted in ‘Report of the Special Committee on Enhancing the Effectiveness of the Principle of Non-Use of Force in International Relations’ (n 37) 51, 54.

41 See, e.g. Council of the European Union, ‘Council Common Position 2008/944/CFSP of 8 December 2008 Defining Common Rules Governing Control of Exports of Military Technology and Equipment’, preambular para 12, 335 Official Journal of the European Union 99 (13 December 2008).

42 The Repertoire of the Practice of the UN Security Council concerning Article 2(4) of the Charter, as well as concerning actions with respect to threats to the peace, breaches of the peace, and acts of aggression, does not contain any references to cases in which states argued over the possible classification of such activities as a use of force within the meaning of Article 2(4).

43 See Wolff Heintschel von Heinegg, ‘“Benevolent” Third States in International Armed Conflicts: The Myth of the Irrelevance of the Law of Neutrality’ in Michael N Schmitt and Jelena Pejic (eds), International Law and Armed Conflict: Exploring the Faultlines (Martinus Nijhoff Publishers, 2007) 543, 544–56.

44 Heintschel von Heinegg (n 43) 553. On post-1945 state practice, see Stefan Oeter, Neutralität und Waffenhandel (Springer, 1992) 87–128.

45 UNSC Verbatim Record, UN Doc S/PV.1745 (11 October 1973) 15, para 159. See also UNSC Verbatim Record, UN Doc S/PV.1746 (12 October 1973) 4–5.

46 William B Quandt, Soviet Policy in the October 1973 War, A Report Prepared for Office of the Assistant Secretary of Defense/International Security Affairs (Rand, 1976) 23.

47 Ibid, 5.

48 Ibid, vii.

49 Ibid, iii, 21, 38.

50 See Jean Charpentier, ‘Pratique Française du droit international’ (1981) 27 Annuaire français de droit international 855, 859.

51 See Jean Charpentier, ‘Pratique Française du droit international: Année 1983’ (1983) 29 Annuaire français de droit international 850, 909.

52 Ibid, 854.

53 Oil Platforms (Islamic Republic of Iran v United States of America) (Further response to the United States’ counter-claim, submitted by the Islamic Republic of Iran) [24 September 2001] vol I, 15–20.

54 Ibid, para 7.51.

55 Ibid, para 7.50.

56 Michael Getler, ‘U.S. aid to Britain in Falklands War is detailed’, The Washington Post (7 March 1984) www.washingtonpost.com/archive/politics/1984/03/07/us-aid-to-britain-in-falklands-war-is-detailed/6e50e92e-3f4b-4768-97fb-57b5593994e6/.

57 Margaret Thatcher Foundation, ‘The US and the Falklands War (1): the US “Tilt” towards Britain’ (30 April 1982) www.margaretthatcher.org/archive/us-falklands.

58 Ibid.

59 Ibid.

60 US Senate, ‘A Resolution Stating United States Policy Regarding the Falkland Islands’ (1982) S.Res.382, 97th Congress. See also John F Barton, ‘Averting a Threatened Filibuster by Sen. Jesse Helms R-N.C., … ’, UPI (29 April 1982) www.upi.com/Archives/1982/04/29/Averting-a-threatened-filibuster-by-Sen-Jesse-Helms-R-NC/6407388900800/.

61 Letter dated 20 March 2003 from the Permanent Representative of the United Kingdom of Great Britain and Northern Ireland to the United Nations addressed to the President of the Security Council, UN Doc S/2003/350 (21 March 2003); Letter dated 20 March 2003 from the Permanent Representative of the United States of America to the United Nations addressed to the President of the Security Council, UN Doc S/2003/351 (21 March 2003); Letter dated 20 March 2003 from the Permanent Representative of Australia to the United Nations addressed to the President of the Security Council, UN Doc S/2003/352 (20 March 2003).

62 See Georg Nolte and Helmut Philipp Aust, ‘Equivocal Helpers – Complicit States, Mixed Messages and International Law (2009) 58 International and Comparative Law Quarterly 1, 2–5.

63 See, e.g. Deutscher Bundestag, Drucksache 15/288 (10 January 2003) 42 (Antwort des Parlamentarischen Staatssekretärs Walter Kolbow vom 23. Dezember 2002 zu Frage 72); Deutscher Bundestag, Plenarprotokoll 15/34 (19 March 2003) 2728 (Bundeskanzler Gerhard Schröder).

64 Government of the Netherlands, ‘Rapport Commissie van Onderzoek Besluitvorming Irak’ (Report, 2010) https://actorenregister.nationaalarchief.nl/actor-organisatie/commissie-van-onderzoek-besluitvorming-irak, 258. For an English translation of the relevant passages, see ‘Report of the Dutch Committee of Inquiry on the War in Iraq – Chapter 8: The Basis in International Law for the Military Intervention in Iraq’ (2010) 57 Netherlands International Law Review 81, 121.

65 See the English translation of the Report (n 64) 135.

66 Antwort der Staatssekretärin Susanne Baumann (n 21).

67 Ibid.

68 Notably, no state has yet accused Iran or North Korea of indirectly using force. Both countries are reportedly supplying Russia with arms and ammunition.

69 Ian Brownlie, System of the Law of Nations: State Responsibility Part 1 (Oxford University Press, 1983) 191.

70 See Quigley (n 3) 106.

71 Ibid.

72 Christian Dominicé, ‘Attribution of Conduct to Multiple States and the Implication of a State in the Act of Another State’ in James Crawford, Alain Pellet, and Simon Olleson (eds), The Law of International Responsibility (Oxford University Press, 2010) 281, 283.

73 Helmut Philipp Aust, Complicity and the Law of State Responsibility (Cambridge University Press, 2013) 219–20.

74 Harriet Moynihan, ‘Aiding and Assisting: Challenges in Armed Conflict and Counterterrorism’ (Chatham House Research Paper, November 2016) 9.

75 UNGA Res 2625 (XXV) (n 8) annex, first principle, para 9.

76 Ibid.

77 Nicaragua (merits) (n 9) paras 195, 228.

78 Christian Henderson, The Use of Force and International Law (Cambridge University Press, 2018) 61.

79 Ibid.

80 See n 53 – n 55 and accompanying text.

81 Nicaragua (merits) (n 9) para 228.

82 Christopher Greenwood, ‘Scope of Application of Humanitarian Law’ in Dieter Fleck (ed), The Handbook of International Humanitarian Law (Oxford University Press, 2nd edn 2008) 45, 58.

83 On the factors of timing and directness, see Schmitt and Biggerstaff (n 14) 210–6.

84 It is already controversial whether Article 2(4) of the UN Charter is per se subject to a de minimis gravity threshold: see, e.g. Ruys (n 34) 159; Corten (n 17) 77–85.

85 A different question is whether a state that suffers an armed attack could be entitled to exercise its right to self-defence against a state that assisted the attacking state in the attack: see Benjamin Nussberger and Paula Fischer, ‘Justifying self-defense against assisting states: Conceptualizing legal consequences of inter-state assistance’, EJIL:Talk! (23 May 2019); Verlinden (n 4) 223–71.

86 The authors of the International Law Commission’s Commentary on Article 16 ASR clarified that there was no requirement that the aid or assistance was essential to the performance of the internationally wrongful act. In their view, it was sufficient if the aid or assistance ‘contributed significantly’ to that act. See ‘Draft Articles on Responsibility of States for Internationally Wrongful Acts’ (n 1), Commentary on Article 16, para 5. But the Commentary also notes that ‘the assistance may have been only an incidental factor in the commission of the primary act, and may have contributed only to a minor degree, if at all, to the injury suffered’: ibid, para 10. These passages created considerable confusion. Many scholars, nevertheless, argued that the contribution, in order to constitute aid or assistance in the sense of Article 16 ASR, must indeed have been significant (other formulations commonly used are ‘substantial’ or ‘material’) or must have made it materially easier for the other state to commit the internationally wrongful act: see, e.g. Quigley (n 3) 120–1; Jackson (n 3) 158; Moynihan (n 74) 8–9.

87 Vladyslav Lanovoy, Complicity and its Limits in the Law of International Responsibility (Hart Publishing, 2016) 149–50.

88 Ibid, 150.

89 Schmitt and Biggerstaff (n 14) 220–1.

90 See n 86.

91 See Schmitt and Biggerstaff (n 14) 221.

92 ‘Draft Articles on Responsibility of States for Internationally Wrongful Acts’ (n 1), Commentary on Article 17, para 7.

93 Schmitt and Biggerstaff (n 14) 206.

94 Ibid.

95 Ibid, 209.

96 Ibid.

97 Ibid.

98 Marko Milanovic, ‘Intelligence Sharing in Multinational Military Operations and Complicity under International Law’ (2021) 97 International Law Studies 1269, 1304–23. According to Milanovic, it is questionable whether constructive knowledge would be sufficient for qualifying an act as aid or assistance within the meaning of Article 16 ASR: ibid, 1316–20.

99 See, e.g. UNGA, ‘Report of the Special Committee on the Question of Defining Aggression (24 February–3 April 1969)’ (1969) 24 UN GAOR Supp No 20, UN Doc A/7620, 19, 27–8. For an overview, see Julius Stone, ‘Hopes and Loopholes in the 1974 Definition of Aggression’ (1977) 71 American Journal of International Law 224, 228–30.

100 See UNGA, ‘Report of the Special Committee on the Question of Defining Aggression (11 March–12 April 1974)’ (1974) 29 UN GAOR Supp No 19, UN Doc A/9619, 16.

101 Ibid, 23.

102 Ibid, 35.

103 Ibid, 36.

104 See Ruys (n 34) 172 (see especially n 86 on that page).

105 For an overview of the debate, see Pobjie (n 28) 139–50.

106 See Henderson (n 78) 77; see Corten (n 17) 76, 85–7.

107 See Corten (n 17) 87.

108 See Ruys (n 34) 189.

109 Ibid, 191.

110 Independent International Fact-Finding Mission on the Conflict in Georgia (Report, September 2009) www.mpil.de/en/pub/publications/archive/independent_international_fact.cfm, vol II, 232.

111 Ibid.

112 See Henderson (n 78) 77; see Corten (n 17) 85–7.

113 Henderson (n 78) 50–2. For an in-depth study of the element of coercion, see Marko Milanovic, ‘Revisiting Coercion as an Element of Prohibited Intervention in International Law’ (forthcoming 2023) American Journal of International Law, https://papers.ssrn.com/sol3/papers.cfm?abstract_id=4504816.

114 See n 93 – n 97.

115 On the geopolitical context as a possible factor of its own, see Schmitt and Biggerstaff (n 14) 218–20.

116 See Kiel Institute for the World Economy (IfW), ‘Ukraine Support Tracker: A Database of Military, Financial and Humanitarian Aid to Ukraine’ (6 July 2023) www.ifw-kiel.de/topics/war-against-ukraine/ukraine-support-tracker/?cookieLevel=not-set.

117 Julia Monn and Andreas Rüesch, ‘How do Western weapons reach Ukraine? A visit to the secret site that coordinates the deliveries’, Neue Zürcher Zeitung (3 January 2023) www.nzz.ch/english/how-western-weapons-reach-ukraine-ld.1719248.

118 Ibid.

119 Julian E Barnes, Helene Cooper, and Eric Schmitt, ‘U.S. intelligence is helping Ukraine kill Russian generals, officials say’, The New York Times (4 May 2022) www.nytimes.com/2022/05/04/us/politics/russia-generals-killed-ukraine.html; Ken Dilanian, Courtney Kube, and Carol E Lee, ‘U.S. intel helped Ukraine sink Russian flagship Moskva, officials say’, NBC (5 May 2022) www.nbcnews.com/politics/national-security/us-intel-helped-ukraine-sink-russian-flagship-moskva-officials-say-rcna27559.

120 Isabelle Khurshudyan and others, ‘Ukraine’s rocket campaign reliant on U.S. precision targeting, officials say’, The Washington Post (9 February 2023) www.washingtonpost.com/world/2023/02/09/ukraine-himars-rocket-artillery-russia/.

121 Ibid.

122 See, e.g. Jahara Matisek, Will Reno, and Sam Rosenberg, ‘The good, the bad and the ugly: Assessing a year of military aid to Ukraine’, RUSI Commentary (22 February 2023) www.rusi.org/explore-our-research/publications/commentary/good-bad-and-ugly-assessing-year-military-aid-ukraine.

123 Council of Europe, Parliamentary Assembly, ‘Address by Ms Annalena Baerbock, Minister of Foreign Affairs of Germany’ (24 January 2023) https://pace.coe.int/en/verbatim/2023-01-24/am/en#speech-22025.

124 See also Michael N Schmitt and William Casey Biggerstaff, ‘Are states aiding and assisting Ukraine and Russia using force?’, Articles of War (7 April 2023) https://lieber.westpoint.edu/are-states-aid-assisting-ukraine-russia-using-force/. For a different view, see Kreß, ‘The Ukraine War and the Prohibition of the Use of Force’ (n 19) 15; Green (n 19) 15–6.

125 ‘Brussels Diplomatic Summits: French President Speaks ahead of EU Summit’, France 24 (24 March 2022) www.youtube.com/watch?v=_yV-VISe_NE.

126 Stefan Talmon suggested that one reason why states had not invoked the right to collective self-defence might be that it would have made them ‘co-belligerents’ of Ukraine in the armed conflict with Russia: Stefan Talmon, ‘The Provision of Arms to the Victim of Armed Aggression: The Case of Ukraine’ (Bonn Research Papers on Public International Law No 20/2022, 6 April 2022) 6.

127 See Tristan Ferraro, ‘The Applicability and Application of International Humanitarian Law to Multinational Forces’ (2013) 95 International Review of the Red Cross 561, 583–7; Tristan Ferraro, ‘The ICRC’s Legal Position on the Notion of Armed Conflict Involving Foreign Intervention and on Determining the IHL Applicable to this Type of Conflict’ (2015) 97 International Review of the Red Cross 1227; ICRC, ‘International Humanitarian Law and the Challenges of Contemporary Armed Conflicts’ (32nd International Conference of the Red Cross and Red Crescent, 8–10 December 2015) www.icrc.org/en/document/international-humanitarian-law-and-challenges-contemporary-armed-conflicts, 22–3.