This compelling examination of the causes of World War I deserves to become the new standard one-volume account of that contentious subject. Clark, a history professor at Cambridge University, concedes the importance of basic structural causes, such as rigid alliance commitments; the temptations of preventive war on a rapidly growing, militarized continent; and the peculiarities of authoritarian decision-making. Yet he believes that such forces alone cannot explain the war and might just as likely have led to peace. He argues that war emerged from a complex conjunction of factors, each of which was far from inevitable and in many cases even improbable, often because it involved decision-makers who behaved less than fully rationally. They indulged in illusions of power, stereotypes about their enemies, and outmoded conceptions of sovereignty; they succumbed to the demands of transient domestic coalitions; and they misperceived their surroundings, sometimes for no good reason. In all of this, such leaders were sleepwalkers,generally unaware of the horrific consequences of the war they were about to unleash. This interpretation not only captures trends in modern historiography on the Great War but also highlights striking similarities with (and a few differences from) the decision-making in contemporary conflicts.