The Turkic Muslim frontier warriors called ghazis…

The Turkic Muslim frontier warriors called ghazis had fought first against the Empire, then the Mongols, who had invaded Anatolia following the establishment of the Il-Khanid empire in Iran and Mesopotamia in the last half of the thirteenth century.

With the disintegration of Seljuq power and its replacement by Mongol suzerainty, enforced by direct military occupation of much of eastern Anatolia, independent Turkmen principalities—one of which is led by Osman, descended from the Kayi branch of the Oghuz Turks—emerge in the remainder of Anatolia.

Following the Mongol defeat of the Seljuq army in 1293, Osman, whose father, Ertugrul, had established a border principality centered at Sügüt, emerges as prince (bey).

He gradually extends his control over several former imperial fortresses, including Yenisehir, which provides the Ottomans, as Osman's dynasty and their later empire will be called, with a strong base to lay siege to Bursa and Nicaea (now Iznik), in northwestern Anatolia.