In this chapter, I turn from war in the past to war in the future. I ask if the future will resemble the past. Will interstate war plague us in this century as it has in the past? Is it conceivable that interstate war could diminish and even disappear as peaceful means for resolving competition among states become more widely practiced?

There is a general tendency by social scientists to use their findings about the past to understand the future. Linear projection flies in the face of history: the future rarely resembles the past – in any domain – but especially politics and international relations. Sharp discontinuities dramatically transform the dynamics of social interactions. The limited, dynastic warfare that characterized eighteenth-century Europe was rendered obsolete by the French Revolution and its concept of the nation under arms. The peace of Europe was restored in 1815, and by the end of the century many thoughtful observers considered the likelihood of great-power war increasingly remote. World War I shattered this illusion and the optimism of European civilization. After several decades, the Cold War appeared to many policymakers and scholars as remarkably stable, and hardly anyone predicted its demise or the subsequent breakup of the Soviet Union. The post-Cold War world has evolved in ways that defy the expectations of liberals and realists alike.