This article analyses the legality of Western states providing weapons to Ukraine. It focuses on five areas of international law: (1) the jus ad bellum; (2) the law of neutrality; (3) international humanitarian law; (4) state responsibility for complicity in internationally wrongful acts; and (5) international criminal law. It concludes that weapons transfers likely violate the law of neutrality, entitling Russia to respond with countermeasures; that Russia can lawfully target transferred weapons under ihl; and that weapons transfers could lead to state and individual responsibility if evidence comes to light that the Ukrainian military is using weapons previously supplied by the West to commit war crimes. By contrast, providing weapons to Ukraine does not violate the jus ad bellum because they are in service of Ukraine's right of self-defence against Russia and does not make the supplying states co-belligerents in Russia's international armed conflict with Ukraine.