When the Bush and Blair administrations justified the 2003 war on Iraq as an act of preemptive self-defense, this was greeted in many quarters with understandable skepticism. How can the right of self-defense be legitimately invoked when no prior aggressive attack has occurred and there is no evidence that one is imminent? This question, much debated in the months leading up to the war, invites us to reflect critically on the content of the right of self-defense. Yet there is a deeper question to be asked about the idea of a war of self-defense; namely, how is it that war can be considered an act of self-defense at all? How exactly is it that the concept of self-defense can provide a justification for war? It is this question that I ask in War and Self-Defense and the answer I arrive at is a surprising one.