The Battle of Bapheus was fought on 17 July 1302 between the armies of the Greek Byzantine Empire and the Turkish Ottoman Beylik. The battle was an Ottoman victory, resulting in the Ottoman conquest of Bithynia.
By 1301, the Ottomans had encircled the former Byzantine capital of Nicaea and were raiding the area around Nicomedia, roaming the countryside and prohibiting the collection of the harvest in order to starve the city into surrender. In 1302, when Emperor Michael IX of Byzantium marched out to confront the Ottomans with a large army, the Turks surrounded him at Magnesia, where his men deserted him and forced him to withdraw by sea along with a large wave of Greek Christian refugees.
Michael's father, Emperor Andronicus II of Byzantium, responded by sending an army of 2,000 men (half of whom were hired Alans) under George Mouzalon to cross the Bosporus and relieve Nicomedia. At the plain of Bapheus, located to the east of the city, the Byzantine army was confronted by Bey Osman's 5,000-strong army. The Turkish light cavalry - which included the Turkmen tribes of Paphlagonia and the Meander River area - charged the Byzantines, whose Alan contingent did not participate in the battle. When the Turks broke the Byzantine line, Mouzalon withdrew into Nicomedia under the cover of the Alan force. The Byzantines were forced to withdraw from the countryside and into their forts, which fell to the Ottomans one-by-one. The Greek Christians of the region fled to Europe, leading to the area being repopulated by Turks. However, it was not until 1337 that Nicomedia fell to the Ottomans, heralding the Byzantines' permanent loss of Asia Minor.