When it comes to the legality of the U.S. and Israel’s war against Iran, there are several conversations and subconversations taking place. In the U.S., one conversation is about domestic law and the extent to which the president can initiate or declare war unilaterally without Congress. Internationally, the questions are whether this war is lawful under existing treaties and if it is being conducted lawfully. Another overarching question is whether this even matters when powerful states seem able to do as they please with little consequence. On all three fronts, the picture is more complicated than many observers allow.
With regard to the domestic legality of the war, the debate over legislative approval certainly matters greatly in any democracy, but the set of questions about U.S. treaty obligations are arguably more pressing. Regardless of congressional authorizations for specific wars, Article 6 of the U.S. Constitution states that treaties to which the U.S. is a party are the “supreme law of the land.” As such, any wars, whether congressionally authorized or not, rely for their constitutionality on adherence to international treaty norms in the absence of an affirmative statutory act by Congress that supersedes those treaties.
At present, the relevant treaties include the U.N. Charter, which governs the use of force between states, and the Geneva and Hague Conventions, which govern the conduct of states when using force.