In a post at Just Security Oona Hathaway and Scott Shapiro argue that supplying the Ukrainians with weapons is not a breach of U. S. neutrality:
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.
Their basic argument is that the Kellogg-Briand Pact and the United Nations Charter superceded the previous understanding of neutrality articulated by Vattel:
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.
In honesty I would be more comfortable with their assessment if I didn’t think that the UN Charter is largely dead. You only have a rule of law when you don’t do something you want to do because it’s against the law. We’ve been engaging in acts of war in a dozen countries over the last 20 years without Security Council authorization which, too, is a violation of the charter. You can’t pick and choose and maintain a rule of law.
An “act of war” is entirely subjective. It’s important to understand that war is inherently a political activity – the use of organized violence to achieve political ends.
And that’s why the various “rules” about war are, in reality, more like guidelines or idealized norms.
In the right set of circumstances, the US would definitely nuke Russia out of existence and we wouldn’t give one whit about UN authorization or anything else. The same goes for any country. Norms will be followed until countries decide the costs of doing so are too great.
And this explains US behavior. Breaking norms on attacking other countries is easy because we don’t pay any price except our international reputation.
And undermining the basis of the liberal world order. Other than that we don’t pay much of a price.