U.S. and Allied Involvement in the Russo-Ukrainian War: The Belligerent Status of NATO States and Its Implications

[Peter S. Konchak is a lawyer and independent researcher whose work focuses on legal and policy issues in the fields of national security, foreign affairs, and economics.]

As the Russo-Ukrainian War enters a phase likely to be defined by protracted attrition warfare, the United States and many of its NATO allies are expanding their efforts to supply Ukraine with vital military assistance. Nevertheless, many of those states remain fearful that the provision of such support will drag them into the conflict as “co-belligerents” of Ukraine.

Those concerns are well-founded. As this article explains, a state becomes a party to an ongoing interstate armed conflict when it provides significant operational, logistical, or intelligence support to an existing, active belligerent. States that are currently providing Ukraine with such assistance have thus already entered the armed conflict against Russia. That conclusion should not, however, lead those states to curtail their support of Ukraine’s war effort. Instead, they should redouble their efforts to support the collective defense of Ukraine in full recognition of their participation in the war.

The Onset of Belligerency—Significant Participation in Hostilities

Properly defined, the aforementioned concerns surrounding “co-belligerent” status relate to the issue of whether a state can become a party to an ongoing armed conflict—a belligerent—by providing material assistance to an existing belligerent party. Often, that question is framed in terms of whether an otherwise-neutral state can become a belligerent by violating its neutral duties. As many scholars have correctly noted, violations of neutrality never per se terminate neutral status. Nevertheless, a state’s abrogation of its neutral duties will cause it to become a belligerent when those violations involve its participation in hostilities.

Neutrality is the status of a state that refrains from participation in the hostilities of an ongoing interstate armed conflict and therefore is not a party to the conflict. Accordingly, an otherwise-neutral state loses its neutral status and becomes a belligerent when it participates in hostilities to a significant extent. That clearly occurs when a state “directly” intervenes in the conflict by deploying its military forces in combat or supplying troops to an existing belligerent. However, a state may also participate in hostilities “indirectly.”

A state participates in hostilities on that basis when it renders “direct military support” to an active belligerent or otherwise provides an active belligerent with material assistance that has a “direct nexus with belligerent or hostile activities.” Accordingly, whether an otherwise-neutral state becomes a belligerent as a result of its significant, indirect involvement in an ongoing armed conflict depends upon whether its “participation is closely related to ‘hostilities.’”

“Hostilities…are acts of force performed for the purpose of attacking a belligerent” that are carried out through myriad “methods of warfare”—various operational modes of utilizing armed force. Those concepts are primarily centered on “attacks,” or acts of violence carried out against an adversary. However, the operational means by which hostilities are conducted also involve a variety of non-violent, ancillary efforts that enable the effective conduct of attacks, including critical logistical and intelligence activities.

A state therefore participates in hostilities when it carries out operational, logistical, or intelligence functions that are integrated into a military operation conducted by an existing belligerent. A state renders such support when, for example, it provides a belligerent with intelligence that is immediately used to conduct attacks or helps a belligerent coordinate its military operations.

Moreover, a state participates in hostilities when it supplies an active belligerent with operational, logistical, or intelligence assistance that directly contributes to the supported party’s overall military effort in the conflict. Critically, that includes circumstances in which a state supplies military matériel—arms, ammunition, and other military equipment—to an active belligerent. As Judge John Bassett Moore explained, the “supply of arms and munitions” to an active belligerent constitutes “a direct contribution to its military resources, and as such is a participation in the war.”

Accordingly, a state enters an ongoing interstate armed conflict as a belligerent when it supplies an active belligerent with significant operational, logistical, or intelligence assistance that either is integrated into a specific military operation carried out by the supported party or otherwise directly contributes to the supported party’s overall military effort in that conflict.

Some scholars, however, claim that the foregoing rules are no longer relevant in light of the “non-belligerency” doctrine. According to that theory, the illegalization of aggressive war altered the content of neutrality law. Whereas historically, an otherwise-neutral state that provided material assistance to a belligerent would thereby become a party to the conflict, in the modern era—so the argument goes—a state may extend such support to a victim of aggression while remaining uninvolved in the conflict.

However, state practice during the Second World War and since 1945 demonstrates that the “non-belligerency” doctrine is not reflective of established customary international law, and that the traditional jus in bello rules regarding the onset of belligerency remain operative.

The Belligerent Status of NATO States and its Significance

The foregoing analysis indicates that the United States and many of its NATO allies have entered the Russo-Ukrainian War as belligerents due to their significant indirect participation in hostilities. The United States, for example, has supplied approximately $3.4 billion of military matériel since to Ukraine since the war began, and will likely soon provide it with another $20.4 billion in security assistance. Washington is also sharing increasingly extensive amounts of intelligence—apparently including detailed, real-time targeting information—with Kyiv to help it effectively conduct operations against Russian forces. That assistance is significant both in absolute terms and because it has been integral to Ukraine’s war effort against Russia. It has enabled Kyiv to reverse the initial Russian offensive of the war, and will remain decisive with respect to Ukraine’s capacity to effectively resist Russian aggression going forward.

Critically, moreover, those U.S. efforts constitute the kinds of significant direct military support for an active belligerent’s war effort that have previously led states and scholars to recognize a supporting state as a belligerent in an ongoing armed conflict. For example, early in the Second World War prior to Pearl Harbor, many scholars concluded that the United States was engaged in an armed conflict against Nazi Germany via its efforts to supply the United Kingdom with substantial amounts of military matériel. The German government similarly identified the United States as an enemy belligerent following the onset of those initiatives.

State practice regarding the Russo-Ukrainian War also supports the foregoing legal conclusions. Four days after the Russian invasion began, Hungary announced that it would not transfer arms to Ukraine in order to avoid becoming “involved in [the] war.” Similarly, China responded to U.S. warnings against “directly assisting Russia with military equipment to use in Ukraine,” in part by stating that it “is not a party to the [Russia-Ukraine] crisis” (the term China uses for the war).

France and Poland also appear to recognize that a state participates in hostilities when it provides some types of military matériel to an active belligerent. French President Emmanuel Macron has stated that NATO will provide “defensive and lethal” weapons to Ukraine, but will avoid crossing the “red line” of becoming a co-belligerent—a threshold that would “very clearly” be crossed by supplying Ukraine with tanks or aircraft. The Polish prime minister has similarly explained that Warsaw will not unilaterally transfer “offensive weapons” to Ukraine because “[Poland is] not a party to [the Russo-Ukrainian] war.”

Legally, the belligerent status of several NATO states is significant because participation in an armed conflict alters a state’s legal relations with enemy belligerents and neutrals. Most importantly, the parties to an armed conflict possess expansive authorities to employ armed force against the military forces and infrastructure of enemy belligerents. This is not to suggest that it would be lawful for Russia to attack a NATO member state. As a matter of the jus ad bellum, Russia is currently waging an illegal war of aggression against Ukraine, and any escalation of that conflict against an ally of Ukraine would be similarly unlawful on that basis. Nevertheless, in the context of an ongoing armed conflict between Russia and certain NATO countries, such an attack would not necessarily violate other peacetime obligations that Russia otherwise owes to those states.

Moreover, since the onset of belligerency involves a state’s participation in hostilities, that change in status often leads opposing belligerents to treat that state—in strategic terms—as a necessary object of comprehensive armed force. For example, during the Soviet-Afghan War, the Soviet Union launched covert attacks against Pakistan, because Islamabad was providing extensive logistical support to the Mujahadeen. Moscow’s recent claim that “NATO, in essence, is engaged in a war with Russia through a proxy” because it is providing military support to Ukraine suggests that Russia has adopted a similar strategic logic with respect to retaliation against NATO in this case. As European diplomats reportedly noted earlier in the war, Russia likely sees NATO’s “supply of weapons and other support to Ukraine as direct intervention of a kind that requires retaliation.”

Remaining the Arsenal of Democracy

Those implications have no doubt driven NATO member states’ concerns about their participation in the Russo-Ukrainian War. However, such risks should not deter those states from continuing to engage in the collective defense of Ukraine.

While as discussed, Russia likely sees a strategic need to attack NATO, Russia is nevertheless highly unlikely to engage in the kind of large-scale escalation against the alliance feared by some commentators. Historically, states that directly participate in an armed conflict have exercised caution in targeting enemy belligerents that limit their participation in hostilities to indirect means. The direct participant must weigh the advantages of targeting the military-industrial center of gravity of the enemy coalition against the risks of engaging in direct, large-scale hostilities with the enemy supporting state.

Accordingly, the direct participant is usually deterred from engaging in large-scale retaliation when it is militarily weaker than the enemy supporting state. Critically, that is the case here—NATO’s overall conventional power far outstrips Russia’s, and the reality of mutually assured destruction makes deliberate nuclear escalation unviable. Therefore, Russia faces significant strategic pressures to refrain from initiating direct, large-scale conventional hostilities against NATO and indeed almost certainly seeks to avoid engaging in such hostilities. This should incentivize NATO to provide Ukraine with the heavy arms it is requesting, irrespective of whether that causes Russia to formally label its members as belligerents.

Indeed, NATO states should affirmatively recognize their belligerent status, as that would bolster the legitimacy of forcible responses to indirect armed aggression perpetrated by great powers. As the U.S.-Soviet proxy wars in Korea and Vietnam demonstrate, such aggression constitutes a serious threat to international peace and security, as great powers can effectively conduct major wars of aggression through state proxies by providing essential material support to those states’ war efforts. Indeed, that reality animates Washington’s concerns about potential Chinese support for Russia’s invasion of Ukraine. Such assistance would dramatically expand the military-industrial resources available to Russia and thus substantially improve its prospects for defeating Ukraine.

Effective self-defense against indirect aggression may therefore require targeting the aggressor coalition’s military-industrial center of gravity by employing armed force against the indirect aggressor. For example, in 1972 the United States interdicted Soviet-North Vietnamese sea lines of communication by mining North Vietnamese harbors against Soviet shipping. Critically, such actions are far more likely to be viewed as lawful elements of a “war of self-defense” if it is recognized that a state which provides significant material assistance to a full-scale war of aggression is a belligerent in that armed conflict due to its significant indirect participation in hostilities. That makes it crucial for NATO states to uphold established jus in bello norms regarding belligerency by recognizing that their indirect participation in the Russo-Ukrainian War makes them parties to that conflict.

Far from curtailing their assistance, then, NATO states should further expand their role in the Russo-Ukrainian War as the collective “arsenal of democracy” in a new era of authoritarian aggression.