It is generally agreed that using lethal or otherwise serious force in self-defense is justified only when three conditions are satisfied: first, there are some grounds for the defender to give priority to his own interests over those of the attacker (whether because the attacker has lost the protection of his right to life, for example, or because of the defender’s prerogative to prefer himself to others); second, the harm used is proportionate to the threat thereby averted; third, the harm is necessary to avert that threat. The first and second conditions have been exhaustively discussed, but the third has been oddly neglected. Meanwhile a prominent school of thought has arisen, in the ethics of war, which seeks to ground the justification of killing in war in principles of individual self-defense. They too have failed to offer any substantive analysis of necessity, while at the same time appealing to it when it suits them to do so. In this paper, I attempt a detailed analysis of the necessity constraint on defensive force, and explore the implications of that analysis for the attempt to transpose principles of individual self-defense into the context of warfare.