This paper describes the evolution of Ottoman military and defensive strategies in the Balkans from 1600 to 1800. It argues that three major imperial crises, engendered by sustained warfare, forced a transition from a standing army to state commissioned militias. To do so, it sites the Ottoman imperial context in a discussion of multiethnic eastern European empires, comparing Ottoman options and limitations with those of the Habsburgs and the Romanovs for the same period. The geopolitics of Danubian and Black Sea frontier territories, and the relationship between imperial center and native elites serve as two points of comparison, emphasizing the interplay between sovereignty, religious affiliation, and assimilation. By the end of the eighteenth century, Ottoman contraction and the movement of large numbers of Muslim refugees from surrendered territories, meant the increased nomadization of central Ottoman lands, and the almost total reliance on undisciplined, volunteer militias as a fighting force, whose acculturation to "Ottomanism" was never desired nor attempted by the ruling elite.