In the nineteenth century, refugee generation and other forms of ethnic cleansing were a new and central feature in the dismantling of European empires and nationalists’ efforts to territorialize popular sovereignty based on demographic homogeneity. With the 1878 Treaty of Berlin, Europe’s Great Powers sanctioned the territorial principle, but included minority protection clauses intended to maintain mixed populations. This article argues that these protection clauses enabled states to make sovereign claims based not only on population distribution as such, but on the ability to control population movement itself. In its effort to win international sanction—and even Ottoman support—to occupy and administer the Ottoman provinces of Bosnia and Herzegovina, the Habsburg government based its arguments almost exclusively on its promised ability to repatriate refugees and manage long-term migration in the provinces. The article shows that states’ claims of power over refugee movement were an essential element of nineteenth-century European diplomacy and an indispensable tool of domestic policy. In the face of nation-state formation and an emerging ideal of demographic homogeneity, the ability to re-establish mixed populations asserted not only state power, but the legitimacy of an ‘imperial’ model of demographic heterogeneity.