What are just causes for war? For orthodox just-war theorists, these are confined to national defence and humanitarian intervention; for radically revisionist theorists, the enforcement of subsistence rights also qualifies as a just cause for war. I argue that both outlooks are unsatisfactory, and propose what I call a ‘constitutional approach’ to the just cause. On this approach, only the enforcement of constitutional principles of justice may provide a just cause for war. Constitutional principles, in turn, set out demands of justice that are not susceptible to reasonable disagreement, and thus ought to be included in the ‘constitution’ of the political order at hand—whether domestic or international. I conclude by showing that the constitutional approach leads to a plausible picture of the just cause for war, steering a middle course between orthodox and radical stances.