This article asks whether humanitarian intervention is legal and reviews contemporary legal arguments on both sides. It finds that both views are sustainable by conventional accounts of the sources of international law; humanitarian intervention is at once legal and illegal. The article then considers the implications of this for the idea of the rule of law in world politics. The power of international law in this case comes from its utility as a resource for justifying state policies, not as a means for distinguishing compliance from non-compliance. Law remains important to world politics, but in a different way than usually understood.