What makes some multiethnic states integrate and others descend into civil war? Ishiyama and Breuning extend traditional explanations centered on socioeconomic, cultural, and historical factors to argue that the actions of leaders of ethnic segments—too often ignored—are also critical determinants of policy outcomes. Applying a framework derived from comparative politics and IR theory, the authors explore two sets of empirical cases: the emergence of new nationalisms in old European democracies (the United Kingdom and Belgium) and the reemergence of old nationalisms in several new democracies (Bulgaria, Slovakia, Estonia, and Latvia). Their work sheds crucially important light on an issue that is one of the most prominent in world politics today.