The first century of Ottoman existence was their heroic age. Living largely in the saddle as leaders of the gazis, the founders of the dynasty were no more than commanders of Turkish tribes organized primarily to raid and conquer the infidel territory around them. With the collapse of Byzantine resistance, the Ottomans found it far easier to expand in western Anatolia and across the Dardanelles into Southeastern Europe than to move against their more powerful Muslim and Turkish neighbors. Advancing rapidly through Thrace and Macedonia, the Ottomans took much of Bulgaria, northern Greece, Bosnia, and Serbia as far as the Danube, developing a system of rule by which the native Christian princes retained their positions and lands in return for acknowledging Ottoman suzerainty and providing soldiers and money. The Ottomans were successful at this time because as gazis fighting the infidel they attracted into their service thousands of nomads fleeing into Anatolia from the Mongols. The early Ottoman leaders also were members and sometimes leaders of the urban ahi brotherhoods that were organized so as to bring help and relief to the people when the defenses of the centralized state failed. While there were other Turkoman gazi leaders in Anatolia, the Ottomans were in direct contact with the Byzantines and could best exploit the latter's weakness and thus attract the manpower that enabled them to conquer and rule the Christian lands across the Straits in Europe.

The first Ottoman Empire was, then, based on both religious and economic motives. Its followers sought to extend the dominion of Islam and to secure booty.