The situation became worse for Byzantium during the civil wars after Andronikos III died. A six-year-long civil war devastated the empire, allowing the Serbian ruler Stefan Dušan () to overrun most of the Empire's remaining territory and establish a Serbian Empire. In 1354, an earthquake at Gallipoli devastated the fort, allowing the Ottomans (who were hired as mercenaries during the civil war by John VI Kantakouzenos) to establish themselves in Europe. By the time the Byzantine civil wars had ended, the Ottomans had defeated the Serbians and subjugated them as vassals. Following the Battle of Kosovo, much of the Balkans became dominated by the Ottomans.