Editor’s Note: This Q&A is the first in a series following the 4th Annual Stockton Center Law of Armed Conflict Conference, Russia-Ukraine: Full-Spectrum Conflict and the Law, held at the U.S. Naval War College in Newport, Rhode Island over December 13-15, 2022 and co-sponsored by the Norwegian Defence University College and the Swedish Defence University. While the conference was held under the Chatham House Rule, several of the expert participants are taking part in a public question and answer series to illuminate some of the pressing issues addressed during the conference sessions that merit further analysis. The second Q&A in this series is available here and the third is available here.

Q:  Are States providing Ukraine with aid and assistance engaging in the “use of force” against Russia, as that term is understood in international law?

Major W. Casey Biggerstaff:  Article 2(4) of the U.N. Charter, which is reflective of customary international law, provides that “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.” The provision has been widely interpreted to mean that a State’s use of force is a breach of international law in the absence of one of two exceptions:  authorization from the Security Council under Chapter VII of the Charter or self-defense (individual or collective) under Article 51 and customary international law. Unless one applies, the use of force is an “internationally wrongful act” that opens the door to “countermeasures,” reparations, or if it rises to the level of an “armed attack,” the proportionate and necessary use of force by the injured State in response (see Draft Articles of State Responsibility, arts. 2, 21, 22, and 31). Thus, assessing whether States supporting Ukraine are using force against Russia is a crucial preliminary step in this inquiry.

There is no authoritative definition, however, of what constitutes a “use of force.” There used to be little question that the rule only encompassed the direct application of armed force, such as that being used by Russian soldiers invading Ukraine and by the Ukrainian forces repelling them in self-defense. By this standard, the United States and others supporting Ukraine would not be using force, as their armed forces are not engaging Russian soldiers directly.

But subsequent State interpretations of the Charter indicate that the term also includes so-called indirect force, whereby a State engages in coercive force by aiding or assisting another. For instance, in 1970, the U.N. General Assembly adopted the Friendly Relations Declaration, which stated that States have a duty to refrain from the use of force by, inter alia, “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.” The International Court of Justice (ICJ) subsequently looked to this resolution as evidence in finding that the United States violated the customary prohibition on the use of force by arming and training the contras in support of their insurgency against the Nicaraguan government (Paramilitary Activities, para. 228). The Court affirmed this reasoning when it similarly found that Uganda violated the prohibition by providing military support to a non-State armed group in the Democratic Republic of the Congo (Armed Activities, paras. 161-63). The notion that States can engage in the use of force by aiding or assisting others (assuming the supported conduct would qualify as a use of force if engaged in by a State) should be, therefore, relatively uncontroversial.

What is less clear are the precise conditions in which such support crosses the use of force threshold. Russia has implicitly alleged that supporting States are using force by providing Ukraine with, for example, long-range rocket systems, training, intelligence, and tanks (see, e.g., here, here, and here). Given that the United States and others have provided over $40 billion in military aid to Ukraine, which has notably enabled its forces to effectively counter Russia’s evolving strategies, this is not an irrational conclusion. Nevertheless, States continue to seemingly deny Russian accusations without elaborating on their basis for doing so (see, e.g., here). These competing positions, and the lack of meaningful State dialogue on the use of indirect force in light of the ICJ’s precedents, make it is difficult to say that there is any international consensus as to whether States supporting Ukraine are using force or not.

Professor Michael Schmitt and I are currently working on a forthcoming article for International Law Studies that proposes a framework for the factors States are likely to consider when assessing whether a State is using indirect force. We believe the best approach is a holistic examination of relevant factors, much as States are beginning to approach the use of force in the cyber context. The factors we identify, which are not exclusive, are assessed in terms of degree and predominantly pertain to the relationship between the aid or assistance in question and the supported State’s application of force. They include, for example, the intent underlying the support, its directness, and the impact the aid or assistance has on the supported State’s forceful operations.

After reflecting on these factors in the Ukraine aid and assistance case, and even though I consider the applicable threshold for the use of indirect force to be quite high, my view is that some States are likely engaging in force by virtue of the quantity and quality of the support they are providing. It will be interesting to see how States and others respond. My thoughts aside, what is abundantly clear is that even if some supporting States are engaging in the use of force, their activities are lawful as actions taken in the collective self-defense of Ukraine under Article 51 of the U.N. Charter.

Note: W. Casey Biggerstaff is a judge advocate in the U.S. Army currently assigned as a military professor at the U.S. Naval War College’s Stockton Center for International Law. His views, and those of Professor Schmitt referenced in the article, are their own and do not necessarily represent those of the U.S. Government or any of its components.

IMAGE: Ukrainian recruits and their British Armed Forces trainers pose for a photograph on Driver Tank Trainer (DTT) armoured vehicles at a military facility, on February 23, 2023 in Southern England. The UK will send 14 Challenger 2 tanks to Ukraine, marking the first time a Western country has sent its most powerful heavy armour to Kyiv to aid Ukrainians in the fight against the Russian Invasion. (Photo by Leon Neal/Getty Images)