The idea that countries balance against powerful and threatening states is perhaps the oldest and most celebrated insight in the study of international relations. In recent years, the puzzling absence of a counterbalancing response to U.S. preeminence has stimulated fresh thinking by scholars about the theory and practice of the balance of power. In this groundbreaking book, Schweller observes that history is in fact full of what he calls "underbalancing" -- the failure of states to form alliances or build arms in the face of threatening accumulations of power. Only the United Kingdom consistently balanced Napoleonic France, and none of the other great powers balanced Nazi Germany in the 1930s, at least not with any sense of urgency. Schweller's explanation for this focuses on the complex dynamics of domestic politics: balancing may be a rational strategy, but states only infrequently approximate the rational and unitary actor that drives this theory. States with leaders that are weak or preside over divided societies, for example, are not able to undertake the costly and risky actions needed to mobilize the country for geopolitical action. Schweller develops a sophisticated framework that allows him to identify variations in the mobilization capacities of states and trace the implications for balancing and nonbalancing power politics in pre-World War I and interwar Europe and in Latin America in the 1860s. He cleverly shows the limits of theories that ignore the internal characteristics of states and societies, but he ends the book without exploring his framework's implications for a contemporary world of nuclear weapons and democracies.