Perhaps the most disputed issue in the vast literature on armed humanitarian intervention is its rationale: are nations sometimes justified to use military force to rescue victims of tyranny? If so, why? Answers to this question vary. The two extreme positions are absolute non-interventionism, according to which armed humanitarian interventions are never justified, and proactive interventionism, the view that military interventions are justified to remedy any situation of injustice, no matter how slight (though I do not know anyone who holds this view).

I will confine the scope of this chapter in three ways. First, I assume that the truth lies between those extremes, that is, I assume that a military humanitarian intervention is sometimes permissible. Still, war is presumptively prohibited for obvious reasons, and the burden of proof always lies on those who argue that a particular war can be justified on humanitarian grounds. Second, I confine the discussion to the permissibility of military intervention to protect persons against their own governments – humanitarian intervention, for short. Important as they are, I will not discuss here non-military diplomatic measures that governments may adopt to respond to humanitarian crises, although the same basic principles I propose here apply, mutatis mutandis, to those as well. And finally, I will address only the moral status of humanitarian intervention, and not its status in international law. I do this for several reasons, but the most important is that it is not clear at all that the legal status of humanitarian intervention should track the philosophical analysis. International legal rules create their own incentives that have to be considered in their design, and sometimes these considerations recommend a legal rule that will be, from the standpoint of philosophy, a second-best solution.