Most of us take it for granted that wars in defence of one's political community are the quintessential just wars. Indeed, while in recent years philosophers have subjected all of our other assumptions about just war theory to radical revision, this principle has emerged largely unscathed. But what underpins the morality of defensive war? In this book, leading moral and political philosophers both show the profoundly challenging nature of that question, and advance novel answers to it. The first part exposes the deep tension between the individualist foundations of much contemporary philosophy and plausible conclusions about the morality of defensive war. The second part offers an individualist attempt to resolve that tension, while the third seeks to justify defensive war by appeal to more collectivist values.