(Editors’ note: This post also appears on Lawfare.)

Debate has been swirling in Washington and among NATO allies about whether states can legally provide Ukraine military assistance in its ongoing armed conflict with Russia. Several NATO members have sent or promised lethal weaponry to the Ukrainian government. The United States alone has promised a billion dollars of security assistance. Germany broke its long-standing policy of banning all exports of lethal weapons to a conflict zone when it agreed to deliver 1,000 rocket launchers and 500 Stinger surface-to-air missiles to Ukraine. France, Belgium, the Netherlands and many other states have joined the effort, providing weapons and protective equipment to the Ukrainian government.

But some have begun to worry that this support violates the law of neutrality–that supplying arms might be an act of war by the United States. The Biden Administration reportedly debated the legality of arming the Ukrainians. Some officials warned that arming the Ukrainians could make the United States a “co-combatant” or party to the conflict. More recently the Biden Administration refused a Polish offer to transfer MiG-29 fighter jets to Ukraine via a U.S. airbase in Germany. While there were practical and political concerns, some also worried that supplying the jets would cross a legal line into direct participation in the war.  

These fears have been stoked by Russia. Vladimir Putin has tried, in all sorts of contexts, to fudge the legal line of participation in war. He declared, for example, that Russia would treat economic sanctions against Russia “akin to an act of war.” The Russian ministry of defense issued a statement warning that if countries allowed their bases to be used as a safe haven for Ukrainian planes, “subsequent use against the Russian armed forces can be regarded as the involvement of these states in an armed conflict.” The Russian legal argument, however, is based on laws of neutrality that no longer hold. The United States and other states supporting Ukraine should refuse to accept these outdated arguments.

The Law of Neutrality

The law of neutrality has a long history. Scholars have debated, for example, whether classical Greece and Rome recognized neutrality in war. Whether or not ancient international law recognized neutrality, the legal regime that dominated in Europe beginning in the seventeenth century—which, in other writing, we have labeled the “Old World Order”—certainly did. Indeed, by the eighteenth century, states had developed a rich and detailed set of rules to regulate states that wished to stay out of hostilities. 

This law of neutrality granted several important rights to states that were not involved in a war. The most important right was that no belligerent state could force a neutral one to fight alongside it, unless the two states had agreed to the alliance beforehand. The territory of a neutral state was also inviolable. Thus, international law prohibited fighting and recruiting soldiers on neutral ground. Neutrals also had the right to conduct business with belligerents. For example, during the wars of the French Revolution, the United States traded with France. Great Britain, however, did not complain that the United States was supplying its enemy, even though France and its colonies would have starved without American produce. By ensuring the right of neutrals to trade with belligerents, the Old World Order minimized the economic disruption of war. Even when hostilities broke out, the world would still remain open for business. 

With rights, however, came responsibilities. Neutral states were expected to be strictly impartial: They were prohibited from discriminating between warring sides unless there was an explicit agreement otherwise. As Vattel stated:

As long as a neutral nation wishes securely to enjoy the advantages of her neutrality, she must in all things show a strict impartiality towards the belligerent powers: for, should she favor one of the parties to the prejudice of the other, she cannot complain of being treated by him as an adherent and confederate of his enemy.

Jefferson quoted this precise passage in a letter (June 17, 1793) to Ambassador Genêt of France to rebuff his attempts to get the United States to favor his country over Great Britain in their war. 

The duty of impartiality was not an isolated legal rule. It was a consequence of states having the right to wage war to repair wrongs done to them. Trading with one side to the exclusion of the other transformed a neutral—“a common friend of both parties”—into a co-belligerent, an ally of the trading partner, because partial treatment interfered with the disfavored party’s right of war. Vattel was explicit about the forfeiture of neutrality. “But to refuse any of those things to one of the parties purely because he is at war with the other, and because she wishes to favor the latter, would be departing from the line of strict neutrality.” Discrimination in trade was an act of war that licensed the other side to attack the discriminator even though it had fired no shots.

The Hague Convention of 1907 included the strict duty of impartiality in its formulation of the laws of neutrality. Article 9 states: “Every measure of restriction or prohibition taken by a neutral Power in regard to the matters referred to in Articles 7 and 8 must be impartially applied by it to both belligerents.”

The End of Impartiality

But the law of neutrality changed dramatically in the early Twentieth Century. As we explain in our book, The Internationalists, the 1928 Kellogg-Briand Pact, which was at the time the most ratified treaty in the world, outlawed war. That treaty is commonly treated as a laughing stock, but we argue in our book that, by eliminating a state’s right of war, it set in motion a legal transformation that gave us our modern legal order. What’s more, the Kellogg-Briand Pact led directly to the UN Charter’s prohibition on wars of aggression and recognition of the right of self-defense. The modern system of international law, accordingly, affirms the transformation of neutrality initiated by the Pact.

The transformation in the law of neutrality can be seen in then-Attorney General Jackson’s speech at the First Conference of the Inter-American Bar Association in 1941, where he defended President Franklin Roosevelt’s Lend Lease Program under which the United States would provide significant weapons and other equipment to assist in the fight against Germany. The United States had not yet entered the war, and some critics of the plan worried that providing arms to the allies would violate the United States’ duty of neutrality and make the United States a co-belligerent. The American public was at the time opposed to entry into the war and hence if the plan was tantamount to entering the war, it would have been dead in the water.

Jackson explained: “The Kellogg-Briand Pact of 1928, in which Germany, Italy, and Japan covenanted with us, as well as with other nations, to renounce war as an instrument of policy, made definite the outlawry of war and of necessity altered the dependent concept of neutral obligations.” He went on to explain that 

[i]in the light of the flagrancy of current aggressions…the United States and other states are entitled to assert a right of discriminatory action by reason of the fact that, since 1928 so far as it is concerned, the place of war and with it the place of neutrality in the international legal system have no longer been the same as they were prior to that date. 

The decision to outlaw war does not impose on states a duty to discriminate against the aggressor, he argued, but “it conferred upon them the right to act in that manner.” He concluded: 

These events have ushered into international law a basis upon which the United States, may legally give aid to the Allies in the present situation. No longer can it be argued that the civilized world must behave with rigid impartiality toward both an aggressor in violation of the Treaty and the victims of unprovoked attack. . . such an interpretation of international law is not only proper but necessary if it is not to be a boon to the lawless and the aggressive.

Jackson’s position wasn’t a rogue one. As we document in our book, Jackson had met with the premiere international lawyer of his generation, the Lviv-born Hersch Lauterpacht. Lauterpacht had already noted the change in the law of neutrality in 1935 when revising the premiere international law treatise of its day, Lassa Oppenheim’s International Law (1935, 5th edition). In a memo he wrote for Jackson, Lauterpacht explained that the Kellogg-Briand Pact, by outlawing war, had changed the fundamental presupposition of the classical law of neutrality. 

Discriminating against aggressors, either through economic sanctions or by transferring arms to their opponent, did not violate their right of war, because belligerents no longer had that right.

Lauterpacht’s view has become state practice. Take, for example, economic relations. Financial sanctions, and more generally what we have called “outcasting,” has become the standard tool for enforcing international law in the postwar world. Instead of responding with war, states now enforce international law, including against states waging illegal wars, by withdrawing the benefits of cooperation. Outcasting has become so unremarkable that legal challenges to them are themselves remarkable.

What This Means for Ukraine

If this war were taking place in 1922 rather than 2022, Putin would have a good legal argument and a basis for claiming that the United States and others can become parties to the conflict by supplying arms to Ukraine. In the Old World Order that existed before 1928, economic sanctions against a belligerent and supplying arms to one side and not another were violations of the duty of neutrality. But when the states joined together to outlaw war in 1928 and reaffirm that commitment in the UN Charter in 1945, they created a New World Order in which might is no longer right and in which states can provide weapons and other support to a state unjustly attacked so that it can defend itself. 

The end of impartiality means that states are permitted to supply weapons or other support to Ukraine. Doing so violates no legal duty of neutrality. States would become parties to the international armed conflict between Russia and Ukraine if, and only if, they resort to armed force against Russia. Indeed, if anything, providing assistance to Ukraine supports the international legal order by allowing Ukraine to defend itself against a war of aggression.


Image credit: Members of the Ukrainian Territorial Defence Forces examine new armament, including NLAW anti-tank systems and other portable anti-tank grenade launchers, in Kyiv on March 9, 2022 (Genya SavilovAFP via Getty Images)