Military action for humanitarian purposes or to counter acts of terrorism is not easily accommodated within existing legal rules on the use of force. The implications of a rigid application of those rules have prompted renewed interest in the plea of necessity as a defense against violations of the law. In this article, I build on the work of Oscar Schachter to examine that controversial doctrine in light of recent military actions in Kosovo, Afghanistan and Iraq. My central argument is that the necessity plea has a role to play in context of humanitarian intervention, but not counter-terrorism. The policy underlying the doctrine – namely, that it is a useful ‘safety valve’ to escape the harmful consequences of strict adherence to the law – provides a persuasive rationale for its application in the face of dire humanitarian need. The evolving law of self-defense, on the other hand, is a more appropriate framework for assessing the legality of military action against terrorists and their sponsors. In making this argument, I claim that the domestic law distinction between an “excuse” and “justification” is important and that, in the international context at least, necessity is better viewed as the former. Moreover, I argue that international legal discourse, structured by an interpretive community, is robust enough to draw that distinction in a meaningful way. The doctrine of necessity leaves existing law on the use of force relatively intact, while acknowledging that in extreme humanitarian circumstances, the interpretive community will turn a blind eye to its violation.