After defeating Bilecik, Yar-Hisar, İnegöl, and Yenişehir, and seeking independence from Seljuks in 1299, Osman Bey now besieged the Byzantine capital of Nicaea (İznik). Consequently, in the spring of 1302, Byzantine Emperor Andronikos II Palaiologos sent his son and (co-emperor) Michael IX to liberate the city. But, he is blocked by Osman and some of his heroes. Escape was the only option left for Michael IX.
And in July 1302, Andronikos II sent an army under George Mouzalon to attack Osman. Eventually, both sides met on the plains of Bapheus near Nicomedia (Izmit). Osman won a decisive battle to force the Byzantines to withdraw from Nicomedia. After this defeat, the Byzantines succeeded in controlling the rural areas of Bithynia and allowed the Ottomans to gradually weaken them.
After this victory, Osman could not ignore it. Byzantine historian Pachymeres describes how the news of Osman’s victory spread and attracted Turks from other parts of western Anatolia to join his followers, and how powerful his power was in defeating the Byzantine army near Nicomedia (Izmit).
According to Halil İnalcık, at a time when Mehmed’s claim to the Conqueror of the world empire has become a monarchy in the Middle East and the West, the Ottomans used Osman’s conquest at Baphues as a legitimacy of the monarchy as the founding emperor.
That is to say, this victory allowed the Ottomans to fulfill the characteristics and qualities of the empire. The Byzantine emperor, Andronikos II, refused to accept the loss of his territory by the Ottomans. Thus began a long period of Ottoman-Byzantine warfare, when the Byzantines gradually lost all their territory in Anatolia to the Ottomans.