Theoretically ambitious and historically rich, this book synthesizes realist thinking about the origins of great-power wars. Copeland argues that such wars tend to be preventive actions initiated by leading states fearful of decline. Major war is extraordinarily risky, he asserts, but a dominant state may see militarized conflict as a strategic tool for self-preservation -- if it perceives its decline to be severe, and if it faces only one challenger rather than many. Other realist theories that look at the balance of power or hegemonic stability may capture part of the logic of war, but only the understanding of shifting power disparities can make sense of the entire history of great war. In this view, it was not Germany's rise that provoked World War I but its fears of falling behind a potentially powerful Russia. More provocative is Copeland's argument that the same strategic worries led the United States -- not the Soviet Union -- to initiate the Cold War. The book acknowledges that the rise of nuclear weapons has altered the rational calculation of security-seeking states, but it points out that the Berlin crisis and the Cuban missile crisis stemmed from respective Soviet and American worries over shifts in each country's strategic position. Copeland ably shows how great-power leaders think about and respond to changing power disparities. What is less clear is whether strategic decline means the same thing today as it did in the past, or whether preventive war can ever again be a tool of great-power politics.