Oghuz expansion by means of military campaigns went at least as far as the Volga River and Ural Mountains, but the geographic limits of their dominance fluctuated in the steppe areas extending north and west from the Aral Sea. Accounts of Arab geographers and travelers portray the Oghuz ethnic group as lacking centralized authority and being governed by a number of "kings" and "chieftains." Because of their disparate nature as a polity and the vastness of their domains, Oghuz tribes rarely acted in concert. Hence, by the late 10th century, the bonds of their confederation began to loosen. At that time, a clan leader named Seljuk founded a dynasty and the empire that bore his name on the basis of those Oghuz elements that had migrated southward into present-day Turkmenistan and Iran. The Seljuk Empire was centered in Persia, from which Oghuz groups spread into Azerbaijan and Anatolia.