One can define ethnic cleansing as a mass-scale, violent, and permanent removal of an ethnically defined group from one territory to a perceived external homeland. Deportations within a state were special in this regard because there was no vision of an external territory to which the cleansed population would be sent. It still needs to be explored why some states treated deported minorities worse than other states treated their supposed external enemies. This article examines the origins and three preconditions of ethnic cleansing: modern nationalism, the concept of the modern nation-state, and the development of population policy. It also discusses four major periods of ethnic cleansing: 1912–1925, ethnic cleansing under the hegemony of Nazi Germany (1938–1944), ethnic cleansing and the postwar order in Europe (1944–1948), and ethnic cleansing in Yugoslavia (1991–1995).