For many centuries, western European attitudes toward the legality of war were dominated by the teachings of the Roman Catholic Church. War was regarded as a means of obtaining reparation for a prior illegal act, and was sometimes regarded as being commanded by God. In this way much of the debate centered on the distinction between just and unjust wars, a distinction that began to break down in the late sixteenth century. In time, leaders justified wars if they were undertaken for the defense of certain vital interests, although there were no accepted objective criteria for determining what those vital interests were. In the twenty-first century, international lawyers and states rarely use the term war. This is because "war" has a technical and somewhat imprecise meaning under international law, and states engaged in hostilities often deny there is a state of war. The difference between war and hostilities falling short of war may...