Providing Arms and Materiel to Ukraine: Neutrality, Co-belligerency, and the Use of Force

Since the outbreak of renewed hostilities on February 24, numerous States have provided Ukraine significant military assistance in its ongoing armed conflict with Russia. NATO members that have approved or transferred lethal weapons to Ukraine include Belgium, Canada, the Czech Republic, Estonia, France, Germany, Greece, Latvia, Lithuania, Netherlands, Poland, Portugal, Slovakia, Slovenia, Turkey, the United Kingdom, and the United States. The scale of the effort is impressive. For instance, the European Union is providing a 450 million Euro lethal support package through the European Peace Facility, while the U.S. security assistance program for Ukraine over the last year stands at one billion dollars.

The outgunned Ukrainian armed forces badly need the assistance. But the effort has also sparked a flurry of legal questions about its significance, especially concerning neutrality, co-belligerency, and the use of force. Unfortunately, some of the discussion is creating confusion, particularly with respect to the relationship between the three issues.  In this post, I will tease them loose in the context of assistance to Ukraine and, where possible, provide my views on their applicability.


The treaty law of neutrality is found in two 1907 Hague Conventions – Convention (V) Respecting the Rights and Duties of Neutral Powers in Case of War on Land and Convention (XIII) Concerning the Rights and Duties of Neutral Powers in Naval War. It is well-accepted that a neutral State must not, as Yoram Dinstein has observed, “assist the war effort of one of the belligerents against its adversary through military supplies furnished on an inter-governmental basis” (see, e.g., DoD Law of War Manual, § This is so even though Hague V has no provision directly on point, as Hague XIII does for naval warfare (art. 6). However, under Hague V and XIII, States may allow the export and transport of war materiel by the private sector. Should they impose restrictions, they must do so on both parties to the conflict (Article 7 in both; see also Law of War Manual, §

Like Russia and Ukraine, some of the States providing military equipment are Parties to these instruments, but not all. For instance, the United States is a Party to both, while the United Kingdom is a Party to neither. This raises the question of whether these neutrality rules are customary in character. Although most experts agree they are, there are three views on their content.

By the first, the rules survive intact in both treaty and customary law. For advocates of this view, States providing military materiel to Ukraine are violating their obligations under neutrality law. According to the second view, neutrality law has fallen into desuetude (or obsolescence) and been replaced by the U.N. Charter collective security system and the rules regarding the use of force that came into force in the aftermath of the Second World War. This view would allow for deliveries of arms and other assistance unless the Security Council bars assistance.

In my estimation, the first view ignores post-Second World War developments in customary international law, as in the case of the establishment of a collective security regime that allows a victim State to receive assistance in collective self-defense. It would mean that every exercise of collective self-defense breaches the assisting State’s neutrality obligations.

But the second throws the baby out with the bathwater. There are situations where U.N. Charter mechanisms fail to maintain international peace and security. To suggest that the rules of neutrality no longer apply in these cases would open the door to expansions of hostilities and amount to a backward step in shielding non-belligerents from the effects of an armed conflict.

The third, and most defensible view, is that the law of neutrality must be interpreted to accommodate both State practice and the evolution of international law since 1907. In this regard, the DoD Law of War Manual (§ 15.2.2) notes,

[A]fter treaties outlawed war as a matter of national policy, it was argued that neutral States could discriminate in favor of States that were victims of wars of aggression. Thus, before its entry into World War II, the United States adopted a position of “qualified neutrality” in which neutral States had the right to support belligerent States that had been the victim of flagrant and illegal wars of aggression.

Variants of this argument stretch back to Justice Robert Jackson, then U.S. Attorney-General and later prosecutor at the Nuremberg Tribunal. He suggested in 1941 that in light of the 1928 Kellogg-Briand Treaty, in which the Parties renounced war, it was now permissible to distinguish among States. Since belligerents were no longer “equal” in the eyes of the law, the premise of equivalence underlying neutrality law had to give way a bit to the emerging prohibition on the use of force, in which there are wrongdoers and victims.

Today, the view is known as “qualified neutrality.” If the Security Council has authorized an enforcement action under Chapter VII of the Charter, neutrality rules necessarily yield to the obligation under Charter Article 2(5) to “give the United Nations every assistance in any action it takes in accordance with the present Charter, and … refrain from giving assistance to any State against which the United Nations is taking preventive or enforcement action.” For example, the Security Council could require States to prevent private entities under their jurisdiction from selling arms to one of the belligerents while allowing sale to its adversaries, a clear violation of traditional neutrality law. And the Security Council could likewise allow States to directly provide military equipment or other assistance to a belligerent, also normally a violation of neutrality law.

The more challenging question is whether qualified neutrality applies when no U.N. authorization or mandate for an enforcement operation exists. The DoD Law of War Manual states, “Regional and collective self-defense agreements may affect States’ rights to maintain a neutral status because the States may be required to provide assistance or take military action in collective self-defense until the U.N. Security Council has taken measures necessary to maintain international peace and security” (§ 15.2.4). In other words, U.N. go-ahead is not always necessary to provide a State assistance consistent with qualified neutrality.

Presumably, the same legal logic would apply in bilateral or ad hoc collective self-defense cases. Although generally skeptical of “qualified immunity,” Wolff Heintschel von Heinegg moved in this direction in his recent Articles of War post. He points out that Russia, the aggressor State, prevented the Chapter VII enforcement mechanism from functioning by exercising its veto, that the Russian operations are acts of aggression that cannot be justified on the grounds it advances, and that an overwhelming number of States have condemned the Russian attacks as international law violations. In such cases, von Heinegg argues, “neutral States can no longer be bound by an obligation of strict impartiality and a prohibition to supply the victim of aggression with the means necessary to defend itself against an aggressor State that is obviously determined to ignore core principles and rules of international law.” I find his analysis convincing.

Thus, some forms of military assistance may violate the law of neutrality, as in the case of Belarus hosting Russian forces to provide access to Kyiv. Others, in my view and von Heinegg’s, would not. The latter would be “internationally wrongful acts,” subject to the remedies available in the law of State responsibility, like countermeasures and reparations (Articles on State Responsibility, arts. 22, 31).

But do breaches of neutrality law obligations lead to co-belligerency status for the States supporting Ukraine? Jack Goldsmith and Curtis Bradley proposed looking to neutrality law to determine co-belligerency in a well-known 2005 Harvard Law Review article. They suggest, “One way that a State can become a co-belligerent is through systematic or significant violations of its duties under the law of neutrality.” Rebecca Ingber, by contrast, concluded, correctly in my opinion, that the Goldsmith and Bradley view “rests on flawed doctrinal grounds, both in its application of largely obsolete neutrality law principles designed for states to a modern conflict with a non-State terrorist group, and in its overstating of the consequences of a neutrality breach under that historic body of law.”

The critical point is that it is not the fact of violation that bears on co-belligerent status (or, more accurately, “party status,” as explained below), but rather the underlying actions. For instance, conducting attacks on behalf of a belligerent would bring a supporting State into the conflict as a party. But repeatedly failing to intern belligerent soldiers who cross into one’s territory for sanctuary would not. The point is that violations of a neutral’s obligations by providing military materiel does not per se trigger co-belligerency status.

Nor does breach of a neutral’s obligation through arms transfer or other assistance to a belligerent as such acts necessarily amount to an unlawful use of force or give the opposing belligerent a right of self-defense.  Those matters are likewise subject to a separate legal regime, the jus ad bellum, discussed below.


 Co-belligerency is a different legal question than compliance with neutrality law and is determined by a different body of law, international humanitarian law (IHL). Although the same action by a State may bear on both questions, it is essential not to conflate the two. As noted in the DoD Law of War Manual, “Acts that are incompatible with the relationship between the neutral State and a belligerent State under the law of neutrality need not end the neutral State’s neutrality and bring that State into the conflict as a belligerent” (§ 15.4.1).

The term “co-belligerent” refers to allies or other States engaged in an international armed conflict (IAC) with a common enemy. It is sometimes an important IHL concept, for instance, with respect to protected persons or POW status (see, e.g., Geneva Convention IV, art. 4; DoD Law of War Manual, §§,

But the pertinent question here is when does a State that provides arms or other assistance itself become a “party to the ongoing IAC.” It is a threshold question that determines whether IHL applies between the State providing the aid and the enemy of the State to whom it is delivered. In other words, it is simply a choice of law issue.

The nature and extent of support that renders the supporting State a party to a conflict is an unsettled issue of IHL. It has been analyzed extensively in the context of non-international armed conflicts, where one State gives support to either insurgents or the State fighting them (see Ingber and Ferraro). However, as the International Committee of the Red Cross (ICRC) has correctly observed, “foreign intervention in support of a party to an international armed conflict (IAC) … raise(s) no specific legal issues concerning the determination of the applicable IHL rules. In such cases, the application of IHL is clear: all the different relationships between belligerents are governed by the law of IAC.”

Therefore, it is appropriate to look to the threshold for initiation of an armed conflict. Common Article 2 of the four 1949 Geneva Conventions sets forth the accepted definition of an IAC – “declared war or of any other armed conflict which may arise between two or more [States], even if the state of war is not recognized by one of them.” The 1960 ICRC Commentary to Article 2 explains,

any difference arising between two States and leading to the intervention of members of the armed forces is an armed conflict within the meaning of Article 2, even if one of the Parties denies the existence of a state of war. It makes no difference how long the conflict lasts, how much slaughter takes place, or how numerous are the participating forces.

A debate has long raged over the requisite intensity of the violence between States that qualifies an action as triggering an international armed conflict. In the current situation, however, there can be no question that 1) some hostilities must take place between the States in question to qualify as an IAC between them, and 2) the level of violence between Russia and Ukraine has sufficed since 2014.

This leads to two conclusions.  First, the mere provision of arms and other materiel support to Ukraine’s armed forces does not, standing alone, mean the supporting States are parties to an IAC with Russia; there are no hostilities between the States concerned. This view is analogously supported by the International Criminal Tribunal for the former Yugoslavia’s Appeals Chamber judgment in Tadić, where the Chamber held that “equipping and financing” a non-State group does not alone “internationalize” a conflict. If materiel assistance to a non-State group in a non-international armed conflict does not initiate IAC between the supporting State and the State against which the arms and equipment will be employed, there is no rationale for saying it would do so in an international armed conflict.

However, at a certain point, support to a belligerent will make the supporting State a party to the conflict. This is a complex matter beyond the scope of this post. Still, some situations are obvious, such as when a supporting State is involved in joint planning of, and provides assistance essential to, another State’s combat operation that would trigger an IAC if conducted alone by the supporting State. Additionally, if the supporting State alone engages in operations that qualify, it is a party to the conflict. For instance, if States, perhaps for humanitarian purposes, declare a no-fly zone in sectors of Ukrainian airspace and subsequently use force against Russian aircraft and helicopters to enforce it, an IAC will exist between them and Russia.

Use of Force

 A third, and distinct, issue raised by the assistance being provided to Ukraine is whether its provision amounts to an unlawful threat or use of force under Article 2(4) of the U.N. Charter and customary international law. If so, it will constitute an internationally wrongful act and, as with a breach of neutrality obligations, open the door to countermeasures and reparations. This is an entirely separate determination from the neutrality and co-belligerent questions. For example, an action may violate neutrality obligations, qualify as a use of force, and trigger an armed conflict. Alternatively, it could qualify as a use of force, but not breach neutrality obligations or initiate an IAC.

That might be the case here. As noted, the better view is that the provision of materiel assistance to Ukraine is not a breach of neutrality, nor does it initiate an IAC with Russia. However, as the International Court of Justice observed in its Paramilitary Activities judgment, “arming and training” guerilla forces “can certainly be said to involve the threat or use of force against Nicaragua” (¶ 228).

Of course, that was a non-international armed conflict. Plus, ICJ judgments are only persuasive authority for States that are not parties in the case (ICJ Statute, art. 59). Therefore, its position on arming and training is not definitively settled. But the logic of the court’s holding arguably applies equally to IAC, for if arming and training a non-State group fighting a State is a use of force (as distinct from unlawful intervention, a separate legal question), why would it not also be a use of force to provide arms to another State engaging in hostilities against that State? After all, the harm to the State could be much more severe, thereby meriting equal protection by international law.

But even if the Court’s conclusion is accepted, it is clear that providing a State assistance when it is engaged in self-defense according to Article 51 of the U.N. Charter and customary international law is lawful. Ukraine has enjoyed the right of self-defense, and therefore the right to request even forcible assistance in collective self-defense from other States, since the original Russian “armed attack” in 2014. Since States may lawfully engage in hostilities against Russia in support of Ukraine, they surely may directly provide it with arms and materiel assistance without violating the use of force prohibition. After all, self-defense is a circumstance precluding wrongfulness in the law of state responsibility (Articles on State Responsibility, art. 21). This being so, any “wrongfulness” of their support on the basis of the use of force prohibition (or any other basis) would be precluded by the support’s qualification as collective self-defense. (See my earlier analysis of Russia’s jus ad bellum claims here).


 In assessments of the provision of materiel and other assistance to Ukraine, it is essential to carefully distinguish between breaches of neutrality, becoming a party to the conflict, and assistance as a use of force. They are separate issues, none of which depends on characterization under the others. In my estimation, the assistance to date does not violate the law of neutrality, has not triggered an international armed conflict between the supporting States and Russia, and does not qualify as an unlawful use of force.


Michael N. Schmitt is the G. Norman Lieber Distinguished Scholar at the United States Military Academy. He is also Professor of Public International Law at the University of Reading, Strauss Center Distinguished Scholar and Visiting Professor of Law at the University of Texas, and Professor Emeritus at the United States Naval War College.