Osman I had succeeded in the leadership of his clan in c. 1282, and over the next two decades launched a series of ever-deeper raids into the Byzantine borderlands of Bithynia. By 1301, the Ottomans were besieging Nicaea, the former imperial capital, and harassing Prussa. The Turkish raids also threatened the port city of Nicomedia with famine, as they roamed the countryside and prohibited the collection of the harvest. In the spring of 1302, Emperor Michael IX launched a campaign which reached south to Magnesia. The Turks, awed by his large army, avoided battle. Michael sought to confront them, but was dissuaded by his generals. The Turks, encouraged, resumed their raids, virtually isolating him at Magnesia. His army dissolved without battle, as the local troops left to defend their homes, and the Alans, too, left to rejoin their families in Thrace. Michael was forced to withdraw by the sea, followed by another wave of refugees.
Battle
To counter the threat to Nicomedia, Michael's father, Andronikos II Palaiologos, sent a Byzantine force of some 2,000 men, under the megas hetaireiarches, George Mouzalon, to cross over the Bosporus and relieve the city. At the plain of Bapheus, an unidentified site on 27 July 1302, the Byzantines met a Turkish army of some 5,000 light cavalry under Osman himself, composed of his own troops as well as allies from the Turkish tribes of Paphlagonia and the Maeander River area. The Turkish cavalry charged the Byzantines, whose Alan contingent did not participate in the battle. The Turks broke the Byzantine line, forcing Mouzalon to withdraw into Nicomedia under the cover of the Alan force.
Aftermath
Bapheus was the first major victory for the nascent Ottoman Beylik, and of major significance for its future expansion: the Byzantines effectively lost control of the countryside of Bithynia, withdrawing to their forts, which, isolated, fell one by one. The Byzantine defeat also sparked a mass exodus of the Christian population from the area into the European parts of the empire, further altering the region's demographic balance. Coupled with the defeat at Magnesia, which allowed the Turks to reach and establish themselves on the coasts of the Aegean Sea, Bapheus thus heralded the final loss of Asia Minor for Byzantium. According to Halil İnalcık, the battle allowed the Ottomans to achieve the characteristics and qualities of a state. The Ottoman conquest of Bithynia was nonetheless gradual, and the last Byzantine outpost there, Nicomedia, fell only in 1337.
