The Supply of Weapons to a Victim of Aggression: The Law of Neutrality in Light of the Conflict in Ukraine

Abstract The relationship between the customary law of neutrality and the contemporary rules of the jus ad bellum is notoriously unclear, especially when an international armed conflict stems from an act of aggression, but the United Nations (UN) Security Council has not mandated or authorized any measure pursuant to Chapter VII of the UN Charter. The issue has emerged once again during the recent international armed conflict in Ukraine, in which states not participating in the conflict have disregarded, among other things, the customary prohibition to supply weapons to either belligerent by supplying weapons to Ukraine – that is, the state victim of armed aggression – despite the deadlock in the Security Council. These acts of unilateral unneutral support have not been characterized by the supplying states as being pursuant to exceptions to the law of neutrality or by Russia or other states as being violations of it. This raises the question whether the law of neutrality still bears relevance whenever an act of aggression occurs. In the uncertainty as to the legal regime applicable in this case, the Ukrainian conflict offers an important instance of state practice that might help shed light on the applicability of the customary prohibition to supply weapons, as well as of the law of neutrality in general, when the international conflict is initiated by an act of aggression.