Are States Aiding and Assisting Ukraine and Russia Using Force?


| Apr 7, 2023

Aid Ukraine

The provision of military aid and assistance from States supporting Ukraine and Russia has been a salient theme in their international armed conflict following Russia’s renewed invasion in February 2022. As of late March 2023, for instance, over fifty States have contributed more than $45 billion in security-related support to Ukraine alone. In addition to non-lethal equipment, major aid packages have included such lethal items as Javelin anti-armor and Stinger anti-air missiles, High Mobility Artillery Rocket Systems (HIMARS), integrated air defense systems, main battle tanks, and even MiG-29 fighter jets. Non-materiel assistance has also been considerable. For example, the United States has provided a “revolutionary” amount of real-time intelligence to Ukraine, enabling its forces to sink Russia’s flagship of the Black Sea Fleet and target senior Russian military leaders on the battlefield.

Other States are supplying similar military support to Russia, albeit on a smaller scale. Iran, for example, has provided Russia with drones used to infamously attack Ukraine’s energy infrastructure (for analysis on the legality of these attacks, see here and here). Media reports indicate that North Korea has also provided Russia with vital ammunition for its artillery batteries. And China is currently considering whether to supply Russia with similar support.

As observed in a previous Articles of War post, the provision of such aid and assistance to the parties to the Russia-Ukraine conflict implicates several international law issues. For instance, it has renewed the long-running debate over whether it violates the law of neutrality (see, e.g., here, here, here, here, and here). Another pertinent question is when the provision of arms and other assistance makes a supporting State a party to the conflict or, in other words, when the law of armed conflict applies between the supporting State and the supported State’s adversary. Finally, as examined in a piece in Just Security’s Stockton Center Russia-Ukraine Q&A Series, it raises the critical issue of whether, and if so, when, aid or assistance that contributes to another State’s use of force constitutes a separate threat or use of force under Article 2(4) of the U.N. Charter and its customary international law analog.

That challenging question lies at the heart of an article we recently published in International Law Studies. Concluding that aid or assistance can, under certain conditions, qualify as a use of force, we propose several non-exclusive factors that States are likely to consider when assessing whether aid or assistance from a supporting State crosses the prohibition’s threshold. This post summarizes our approach and conclusions. It begins with a review of the prohibition of the use of force before turning to whether, and if so, when, aid or assistance might constitute the use of indirect force.

The Prohibition of the Use of Force

Article 2(4), which is widely recognized as reflective of customary international law, states, “All Members shall refrain in their international relations from the threat or use of force against the territorial integrity or political independence of any State, or in any other manner inconsistent with the Purposes of the United Nations.” In the absence of one of two universally recognized “circumstances precluding wrongfulness”—a Security Council authorization to use force under Chapter VII of the U.N. Charter or individual or collective self-defense under the Charter’s Article 51 and its customary law counterpart—a State’s violation of this obligation constitutes an “internationally wrongful act” triggering various international legal consequences (see International Law Commission, Articles of State Responsibility, art. 2, Ch. V). These may include reparations (art. 31), countermeasures (art. 22), or, if the force rises to the level of an “armed attack,” the necessary and proportionate use of force in response pursuant to the individual or collective self-defense of the injured State (art. 21).

With respect to States supporting Ukraine and Russia, the relevant question is whether their military aid or assistance constitutes a “threat or use of force.” The issue looms especially large for those States that are militarily supporting Russia as the aggressor in the conflict (e.g., Iran) and those deliberating whether to do so in the future (e.g., China). Unlike the States aiding and assisting Ukraine, those supporting Russia would not enjoy a claim of collective self-defense to preclude the wrongfulness of any activities that may qualify as a use of force in violation of the prohibition.

Unfortunately, there is no authoritative definition of what constitutes “force” under international law with which to guide States’ analyses. And to complicate matters, States’ understanding of what constitutes the use of force continues to evolve. When the Charter was adopted, there was a general consensus that the rule only included armed force, such as that being used by Russian and Ukrainian troops engaged in conventional combat within Ukraine, in contrast to other forms of coercion, such as political and economic pressure. If this standard still prevailed, there would be little question that States supporting Ukraine or Russia without engaging in military hostilities directly would not be using force.

Can Military Support Qualify as an Indirect Use of Force?

Later interpretations of the Charter, however, indicate that “force” is broader than just armed force. As an example, modern cyber operations are increasingly testing the premise that force must be physical, kinetic, or involve conventionally “armed” forces at all. Other contexts likewise counsel in favor of a flexible interpretation of force. The U.N. General Assembly’s 1970 Friendly Relations Declaration, for instance, asserts that States have a duty to refrain from the use of force by “organizing, instigating, assisting, or participating in acts of civil strife or terrorist acts in another State… when [such acts] involve a threat or use of force.” Similarly, with respect to the related concept of State aggression, the U.N. General Assembly’s 1974 Definition of Aggression indicates that the use of force includes, for example, “[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 the acts [of armed force], or its substantial involvement therein” (art. 3(g)). For the purposes of our analysis, these latter employments of force are necessarily indirect.

Authoritative, albeit non-binding, judgments of the International Court of Justice also endorse an expansive interpretation of force that includes indirect force. For instance, in its 1986 Paramilitary Activities judgment, the Court concluded that the United States used unlawful force in contravention of customary international law by arming and training the contras engaged in an insurgency against the government of Nicaragua (para. 228). Similarly, in its 2005 Armed Activities judgment, the Court determined that Uganda violated the prohibition by supporting a non-State armed group militarily in its fight against the Democratic Republic of the Congo (paras. 161-63). While these judgments may have been criticized on other grounds, the Court’s reasoning as to the use of indirect force has garnered little, if any, State criticism. And although the cases involved non-international armed conflicts, not applying the same logic to the Russia-Ukraine conflict, in our assessment, would be counterintuitive and run counter to the object and purpose of the prohibition.

Thus, there should be little controversy that force includes the notion of indirect force. What is less clear, however, are the conditions under which aid or assistance, such as the military support States are providing Ukraine and Russia, crosses the prohibition’s threshold. Indeed, not all aid and assistance are equal in terms of their quantitative or qualitative contributions to Ukraine’s and Russia’s uses of force. Military support from some States, for example, includes uniform items, body armor, or similarly innocuous items. On the other end of the spectrum, other States are providing highly lethal arms, such as HIMARS or Iranian drones, or potent assistance, such as intelligence-sharing, wargaming, and real-time consulting. And the support from some States falls somewhere in between. China, for instance, is contemplating expanding its support to Russia to include the transfer of some lethal weapons. Thus, it is critical to determine the precise threshold at which aid or assistance falls within the prohibition, especially considering that any qualifying support provided to Russia would not fall within any recognized circumstance precluding wrongfulness under the law of State responsibility—and therefore constitute a violation of a peremptory norm of international law.

A Factor-Based Approach

The challenges illustrated above beg the question of how States should determine when the use-of-force threshold is met. Yet, despite this urgency, States have failed to engage in meaningful dialogue to clarify this essential question. Therefore, with the hope of stimulating discussion amongst States, we propose in our article that,

the most pragmatic approach to operating in the face of this ambiguity is to identify factors that States are likely to consider when deciding whether to characterize support as indirect force. While States may consider a range of factors, the most compelling, we suggest, are based in the nature of the relationship between the aid or assistance provided and the ultimate application of force by the supported State (p. 205).

We emphasize that the factors below are not exhaustive; as use-of-force determinations are highly contextual, States will undoubtedly consider others. Nor do we think they should be treated as a checklist. Rather, we believe they are simply unique considerations likely to be influential when States assess whether aid or assistance constitutes a use of force. Finally, we note that we have not made our approach “out of whole cloth;” it is similar to, and inspired by, that taken by the International Group of Experts in the Tallinn Manuals 1.0 and 2.0 in proposing a set of non-exclusive factors States would likely consider in the cyber context—an approach that has generally found favor with a gaining number of States.

Drawing on their approach, we suggest looking to the following factors, which are summarized here but examined in more detail in our article, to assess when aid or assistance that contributes to another State’s use of force qualifies as its own use of force:

– Intent—The clearer the intention to contribute to a supported State’s use of force, the more likely it is that aid or assistance will be considered a distinct use of force. By intent, we mean the purposeful or conscious design to enhance the supported State’s use of force.

– Timing—With respect to the timing of the aid or assistance, the more immediate the impact on the supported State’s use of force, the greater the weight it will likely have.

– Directness—When considering the causal nexus between the aid or assistance and the supported State’s use of force, the more direct the connection, the more likely States will consider it an indirect use of force.

– NatureStates’ assessments will likely focus on the nature of the support provided. Aid or assistance of a nature that directly contributes to a supported State’s use of force is likely to weigh more heavily than that which, for instance, only broadly sustains the war effort or simply increases a State’s general military capacity.

– Geopolitical context—The geopolitical context, which includes those variables that impact relations between States (e.g., geography, economics, demographics, etc.), in which the conflict and the relevant support occur, will likely influence how States characterize the actions of their fellow States.

– Impact—The extent to which the aid or assistance meaningfully impacts the supported State’s use of force will probably prove the most significant factor when assessing whether military support crosses the applicable threshold.

We further note that in applying these and other factors, States are likely to find—as we do—that the qualifying threshold for the use of indirect force is quite high. As described above, many States are providing a diverse array and range of aid and assistance to the parties in the conflict. And as the judgments of the International Court of Justice indicate, the use of force requires more than the mere provision of support to qualify.

Still, after reflecting on the factors above and the aid and assistance that States have provided Ukraine and Russia, we believe that some States are probably engaging in the use of force due to both the quantity and quality of their military support. But, more importantly, we emphasize that in the event States are providing qualifying support to Russia, their aid and/or assistance is clearly unlawful, in contrast to those that are providing support in the collective—and unequivocally lawful—self-defense of Ukraine.


States are providing an unprecedented degree of aid and assistance to the war in Ukraine. Russia, for one, has implicitly alleged that much of the support, especially that provided by the United States and its NATO allies, constitutes the use of force (see, e.g., herehere, and here). Yet, to date, States have not fully grappled with these accusations. Nor have States engaged in any meaningful debate to clarify what the use of indirect force entails, much less flesh out its precise requirements. Accordingly, we are optimistic that current events will spark a dialogue amongst States and reflection within academia to clarify when aid or assistance qualifies as a use of force. In our eyes, the best approach to doing so is through the examination of the factors described in our article, the bulk of which pertain to the relationship between the aid or assistance in question and the supported State’s application of force.


Michael N. Schmitt is the G. Norman Lieber Distinguished Scholar at the United States Military Academy at West Point. He is also Professor of Public International Law at the University of Reading and Professor Emeritus and Charles H. Stockton Distinguished Scholar-in-Residence at the United States Naval War College.

Lieutenant Colonel William C. Biggerstaff is a military professor at the Stockton Center for International Law at the U.S. Naval War College, where he co-teaches a course on the Law of Armed Conflict.


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