The United States’ evolving response to Russia’s war against Ukraine – including the deployment of additional U.S. troops to eastern Europe and potentially additional arms transfers – has raised questions about the legal framework for the United States’ involvement in NATO, the obligations it creates, and the implications for presidential war powers. This article seeks to address some of these legal questions which bear on the allocation of war-making authority within the U.S. government.

Question 1: Does the deployment of additional U.S. forces to NATO members in eastern Europe constitute the introduction of U.S. forces “into hostilities or into situations where imminent involvement in hostilities is clearly indicated by the circumstances” – and thus trigger the provisions of the 1973 War Powers Resolution (WPR) that require the President to withdraw troops within 60 days of such an introduction absent congressional authorization?

Answer: The reported troop movements have not thus far pulled the hostilities or imminent hostilities triggers of the WPR. The WPR seeks to prevent the President from taking the country to war on his own, and also encourages transparency with respect to troop movements that could be preparations for the use of military force. To this end, Section 4(a) of the WPR establishes reporting requirements, under which the president must report certain activities of U.S. armed forces within 48 hours to Congress if they have not already been authorized:

  • First, the president must report when U.S. military forces are introduced into “hostilities” or “situations where imminent involvement in hostilities is clearly indicated by the circumstances.” Although not defined in the WPR, the executive branch has narrowly interpreted hostilities to mean “a situation in which units of the U.S. armed forces are actively engaged in exchanges of fire with opposing units of hostile forces” and imminent hostilities to mean “a situation in which there is a serious risk from hostile fire to the safety of United States forces.” (4(a)(1))
  • Second, even if U.S. forces are not introduced into hostilities (or situations where they face the imminent risk of hostilities), the president must report the introduction of “combat equipped” forces into a country — which the executive branch reads as forces equipped with crew-served weapons such as machine guns requiring more than one person to operate and mortars. (4(a)(2))
  • Third, the president must also report a substantial enlargement of such combat equipped forces in a country where such forces are already present. (4(a)(3))

Notably, under Section 5(b) of the WPR, the submission of a report under the first of these scenarios – introduction of U.S. forces into hostilities or situations of imminent risk thereof – also starts a 60-day clock for the withdrawal of U.S. armed forces from such hostilities (extendable to 90 days under certain circumstances) unless Congress declares war or otherwise enacts specific statutory authorization for the use of force.

Based on publicly available information, President Biden’s actions to date do not trigger Section 4(a)(1), or in turn Section 5(b), of the WPR. The administration has repeatedly stated that no U.S. armed forces will be deployed to Ukraine.  Indeed, President Biden has gone so far as to state that U.S. troops would not be sent to Ukraine even to rescue U.S. citizens because of the risk of a “world war” between the United States and Russia. Instead, additional U.S. forces are being deployed to NATO member states in eastern Europe, including Poland and Romania. Based on the current facts, it therefore does not appear that U.S. forces deployed to or deploying to eastern Europe will be at “serious risk of hostile fire” or would imminently face such a risk.

Whether such deployments might implicate other WPR requirements is less clear. Depending on the exact details of the deployments to specific countries, they might, for example, constitute substantial enlargements under the WPR and thus require reports to Congress, though such reports would not implicate the 60-day clock for withdrawal.

Question 2: Would the United States become a “co-combatant” with Ukraine in its conflict with Russia by virtue of providing arms or other military assistance to Ukraine?

Answer: Recent stories in the media (here and here) report that in the course of deliberations within the Biden administration over whether to provide additional arms and other military assistance to Ukraine, lawyers have raised legal concerns over whether the provision of such assistance could render the United States a “co-combatant.”[1] Although it is difficult to be certain, the reference to “co-combatant” in one recent story appears to be an allusion to the concept of “co-belligerence” under the law of neutrality.

Under treaty and customary international law prior to the adoption of the U.N. Charter, the law of neutrality regulated the relationship between states not at war (neutrals) and warring states (belligerents). States seeking to avoid involvement in a war could adopt “neutral” status. Neutral states were generally obligated to comply with requirements of non-participation in the conflict and impartiality as between the belligerent parties. Neutrality law also governed the responses by belligerents when a neutral state violated its obligations of neutrality. In the case of serious and systematic violations of neutral duties, such retaliation could include a declaration of war against the neutral. States fighting alongside one another were considered to be “co-belligerents.”

Eighty years ago, the United States took the position that neutrality law distinguishes between military assistance by a neutral to a victim state as opposed to an aggressor.  In a 1941 speech delivered before the United States entered World War II, Attorney General Robert Jackson defended the transfer of U.S. destroyers to the United Kingdom, then under attack by Nazi Germany.  Jackson argued that international law, including the law of neutrality, had developed such that neutral states were no longer under an obligation of “rigid impartiality” toward both aggressor and victim states.  The United States could “extend to England all aid ‘short of war’” while at the same time avoiding “entry into the war as a belligerent.”  Thus, consistent with the U.S. interpretation (whether or not it was the best view at the time) and U.S. practice in 1941, the United States could provide military support to Ukraine without becoming a co-belligerent.

Whatever the continuing relevance of neutrality law generally (a topic of some debate), it is widely understood that any recourse to force is now governed by the U.N. Charter’s framework. (Read Oona Hathaway and Scott Shapiro for an account of how the U.N. Charter altered and supplanted the law of neutrality in these respects.) States now justify recourse to force through reference to the U.N. Charter (even when their legal arguments are wholly unpersuasive as with Russia’s declaration of war on Ukraine), not neutrality law.

As a matter of international law, the U.N. Charter prohibits the use of force, except when authorized by the U.N. Security Council or in individual or collective self-defense following an armed attack. Russia would have no more legal basis to attack the United States because of arms transfers to Ukraine than Russia had to attack Ukraine in the first place. (Under U.S. domestic law, the U.S. executive branch has adapted the notion of co-belligerency – with roots in neutrality law – in order to stretch the scope of the 2001 Authorization for the Use of Military Force to groups not described in the text of the authorization, as thoroughly explicated by Rebecca Ingber, but this application of “co-belligerency” has no legal implications for any U.S. arms transfers to Ukraine.)

That said, Moscow’s analysis may be different and obviously any decisions relating to arms transfers to Ukraine or other military assistance are not only matters of law but of policy, and thus must be looked at through both a legal and a prudential lens.

Question 3: Does the North Atlantic Treaty require the United States to automatically go to war if another member, such as Poland is attacked?

Answer: The North Atlantic treaty is a security pact that treats an attack on one member as an attack on all.  To this end, Article V of the treaty legally obligates NATO members under international law to come to the collective defense of one another in the event of an attack.  President Biden has referred to the U.S. commitment to Article V of the treaty as “ironclad.” Making these kinds of pronouncements and being prepared to stand behind them is essential to the alliance’s capacity to deter attacks on its members. It is important, however, to understand how the treaty intersects with U.S. law in three key respects.

1. Article V of the treaty does not function as a standing authorization for the use of military force under U.S. law. Article 11 of the treaty states that the provisions of the instrument “shall…be carried out by the Parties in accordance with their respective constitutional processes.” This envisions that each NATO member’s domestic law processes will govern its actions should Article V be invoked.

During a 2 March hearing before the House Foreign Affairs Committee (HFAC), Rich Visek, the acting Legal Adviser of the Department of State, reemphasized this point, making specific reference to Art 11. By the same token, the treaty presumes that each member’s domestic politics will be sufficiently functional to generate the approvals that are required in a timely way.

Prior to the enactment of the WPR in 1973, Presidents had repeatedly invoked treaties as a source of authority under domestic law to use force even in the absence of prior congressional authorization, on the grounds that the Constitution’s Take Care Clause obligated them faithfully execute laws, including treaties.  The most significant such episode was President Truman’s reliance on the U.N. Charter to launch a “police action” in Korea without prior congressional authorization.  But the WPR foreclosed further reliance on such legal justifications. Section 8(a)(2) provides in pertinent part that the authority to introduce United States Armed Forces into hostilities shall not be inferred “from any treaty heretofore or hereafter ratified unless such treaty is implemented by legislation specifically authorizing the introduction.” In the case of NATO, there is no implementing legislation authorizing the use of force should Article V of the North Atlantic Treaty be invoked. Congress would therefore have to pass new legislation to authorize conflict in defense of another NATO member.

2. As a legal matter, the North Atlantic Treaty allows states to calibrate their response to an armed attack. Article V of the treaty provides in pertinent part that:

The Parties agree that an armed attack against one or more of them in Europe or North America shall be considered an attack against them all and consequently they agree that, if such an armed attack occurs, each of them, in exercise of the right of individual or collective self-defence recognised by Article 51 of the Charter of the United Nations, will assist the Party or Parties so attacked by taking forthwith, individually and in concert with the other Parties, such action as it deems necessary, including the use of armed force, to restore and maintain the security of the North Atlantic area. (emphasis added)

This provision creates some useful flexibility for parties to manage escalation risk in the context of a potentially very dangerous situation – the facts at hand will matter as the alliance, and its members, determine what actions are necessary in any particular situation (as Acting Legal Adviser Visek also emphasized in the HFAC hearing). With respect to a major attack, in practice the alliance has tended to signal that such uses of force will be met in kind. For purposes of the present crisis, President Biden has stated that the United States will “defend every inch of NATO territory with the full force of American power.”

3. Although executive branch lawyers have over the past several decades found that the president has some unilateral authority to use force to advance sufficiently important national interests, they have also taken the view that such actions may not by their “anticipated nature, scope and duration” amount to “war in the constitutional sense” and thus impinge on Congress’s authority under Article I. In assessing the “nature, scope, and duration” of anticipated military action, the risk of escalation is a particularly important factor.

Administration witnesses repeatedly cited this constraint on the President’s war powers during the recent HFAC hearing and noted the relevance of escalatory risk in taking military action with respect to the Ukraine conflict, including the anticipated response from Russia.  Acting Legal Adviser Visek stated that “if the assessment [by the executive branch] is that it would be escalatory to war, then absolutely, we would need to come to the Congress” for authorization.  This caution seems consistent with past executive branch practice: if “war in the constitutional sense” means anything, it must mean war with Russia. Yet, it should be noted that the mere request by the President to Congress for a use of force authorization could be viewed as provocative by Russia and itself be escalatory.

All of this said, as a practical matter, the President is not likely to feel legally inhibited from taking those steps he deems necessary to defend U.S. forces stationed in a NATO member state should they come under direct attack, even without congressional approval. There are many variations on this hypothetical, and whether self-defense exigencies present a compelling legal justification for the use of force without prior congressional authorization would depend on facts and circumstances that fortunately have not yet developed. Thus far, President Biden and his administration have made clear that they understand the risks of escalating to this point, and are clearly working to avoid a nightmare scenario.


[1] White House Spokesperson Jen Psaki confirmed that the United States was also sharing intelligence with Ukraine regarding Russian military “plans and activities.”


Photo credit: White House press secretary Jen Psaki speaks during the daily press briefing in the James Brady Press Briefing Room on March 3, 2022 (Anna Moneymaker/Getty Images)