Over the centuries, societies have gradually developed constraints on the use of armed force in the conduct of foreign relations. The crowning achievement of these efforts occurred in the midtwentieth century with the general acceptance among the states of the world that the use of military force for territorial expansion was unacceptable. A central challenge for the twenty-first century rests in reconciling these constraints with the increasing desire to protect innocent persons from human rights deprivations that often take place during civil war or result from persecution by autocratic...