Debates about the causes of war have tended to focus on conflict among the great powers and the global balance of power. This important study asserts that the real puzzles of war and peace exist on a lesser scale within regions. Most of the wars in the last two centuries have been between small to medium-size states, neighbors struggling over prosaic matters of boundaries and politics. But regions vary widely in the incidence of war. In the current era, Europe and North America have been quite peaceful, whereas East Asia, South Asia, the Middle East, and the Balkans have repeatedly experienced war or the threat of war. To explain these variations, the book offers an intriguing theory about the "fit" between states and nations. Marshaling a mountain of statistical and historical evidence, Miller argues that peace is most likely to exist where there is "congruence" between the underlying national aspirations and political identifications and the formal political-territorial borders. Where states and nations are not aligned, conflict lurks. Miller shows convincingly that conditions of anarchy and power competition alone are rarely a trigger for war. Rather, it is contested boundaries and territories that create a sense that the regional order is not stable or legitimate, and this unsettled situation turns mere political disputes into dangerous spirals of insecurity and threats of violence. In an interesting chapter, Miller looks at the nineteenth-century colonial wars in Latin America and the ways in which nation building and regional territorial settlements removed the sources of war. Although highly theoretical, the book is full of useful insights about potential pathways toward regional peacemaking, particularly in regard to the Middle East.