This large and lucid book grew out of a course taught for a number of years at Yale University. The book looks at the outbreak of the Peloponnesian War, World War I, the Second Punic War, the Second World War, and a war that was not -- the Cuban missile crisis. Kagan, a blunt and effective defender of the Western canon, is an eminent ancient historian and has written an outstanding four-volume history of the Peloponnesian War. He has prepared five engrossing narratives, each of which could stand on its own, that address common questions: What was the nature of the broken peace, and how did each country decide to go to war? Some chapters (not surprisingly those on the classical period) are better than others, but overall this work is powerful and important. Like many historians, Kagan is perhaps too skittish about making general observations, but a theme that repeatedly emerges is the importance of a failure of willpower on the part of a world power to deter a rising challenger. Not a fashionable or even a novel proposition, perhaps, but one advanced with rare force and conviction.