How do foreign policy-makers learn from history? When do states enter alliances? Why have some small powers chosen to enter alliances whereas others have stayed neutral? In Crucible of Beliefs , Dan Reiter uses work in social psychology and organization theory to build a formative-events model of learning in international politics. History does inform the decisions of policy-makers, he suggests, but it is history of a specific sort, based on firsthand experience in major events such as wars. Tested against balance-of-threat theory, the leading realist explanation of alliance behavior, Reiter's formative-events model of learning emerges as a far better predictor of states' decisions. Crucible of Beliefs shows that, contrary to balance-of-threat theory, state leaders ignore the level of international threat and focus instead on avoiding past mistakes and repeating past successes. A serious blow to realism, these findings demonstrate that to understand the dynamics of world politics, it is essential to know how leaders learn from history.