The Black Sea, Russia, and eastern Europe exported slaves throughout the medieval period. Most had been born free but were enslaved through capture or occasionally through sale by relatives. During the eighth through tenth centuries, slaves were traded from eastern Europe and the Baltic to elite households in Byzantium and the Islamic world via the Dniepr and Volga river systems, the Carolingian empire, and Venice. In the thirteenth century, the structure of this slave trade changed as a result of the Mongol invasion of eastern Europe, Italian colonization of the Black Sea, the success of the Mamluk state, and the crusading activities of the Teutonic Knights in the Baltic. People enslaved in the Baltic now tended to be traded westward rather than eastward; people enslaved in eastern Europe and the Caucasus tended to pass through the Black Sea into Italian, Mamluk, or Ottoman hands; and people enslaved in the Balkans were trafficked primarily by Venetians or Ottomans. Many aspects of this trade deserve further study, however, such as political marginality and decentralization as factors that enabled slaving; violations of the principle that slaves should come from a different religious background than their owners; and the logistics of local slave trades.