 War can be necessary and proportionate only if it serves an end worth all this death and destruction. Hence the importance of having a just cause. And hence too the widespread belief that just causes are few and far between. Indeed, traditional just war theory recognizes only two kinds of justification for war: national defence (of one’s own state or of an ally) and humanitarian intervention. What’s more, humanitarian intervention is permissible only to avert the very gravest of tragedies—“crimes that shock the moral conscience of mankind” (Walzer 2006: 107).