Osman I had succeeded in the leadership of his clan in c. 1281, 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 Prusa. 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. To counter the threat to Nicomedia, Michael's father, Andronikos II Palaiologos, sent a Byzantine force of some 2,000 men (half of whom were recently hired Alan mercenaries), under the megas hetaireiarches, George Mouzalon, to cross over the Bosporus and relieve the city. At the plain of Bapheus, 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. 

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.