What drives a state's choice to assimilate, accommodate, or exclude ethnic groups within its territory? In this pathbreaking work on the international politics of nation-building, Harris Mylonas argues that a state's nation-building policies toward non-core groups - any aggregation of individuals perceived as an unassimilated ethnic group by the ruling elite of a state - are influenced by both its foreign policy goals and its relations with the external patrons of these groups. Through a detailed study of the Balkans, Mylonas shows that the way a state treats a non-core group within its own borders is determined largely by whether the state's foreign policy is revisionist or cleaves to the international status quo, and whether it is allied or in rivalry with that group's external patrons. Mylonas explores the effects of external involvement on the salience of cultural differences and the planning of nation-building policies. The Politics of Nation-Building injects international politics into the study of nation-building, building a bridge between international relations and the comparative politics of ethnicity and nationalism. This is the first book to explain systematically how the politics of ethnicity in the international arena determine which groups are assimilated, accommodated, or annihilated by their host states.