Osman was descended from the Kayı branch of the Oğuz Turkmen. His father, Ertugrul, had established a principality centred at Sögüt. With Sögüt as their base, Osman and the Muslim frontier warriors (Ghazis) under his command waged a slow and stubborn conflict against the Byzantines, who sought to defend their territories in the hinterland of the Asiatic shore opposite Constantinople (now Istanbul). Osman gradually extended his control over several former Byzantine fortresses, including Yenişehir, which provided the Ottomans with a strong base to lay siege to Bursa and Nicaea (İznik) (now İznik), in northwestern Anatolia. The greatest success of Osman's reign was the conquest of Bursa shortly before his death.