The Ottomans were able to take advantage of the decay of the Byzantine frontier defense system and the rise of economic, religious, and social discontent in the Byzantine Empire and, beginning under Osman and continuing under his successors Orhan (Orkhan, ruled 1324–60) and Murad I (1360–89), took over Byzantine territories, first in western Anatolia and then in southeastern Europe. It was only under Bayezid I (1389–1402) that the wealth and power gained by that initial expansion were used to assimilate the Anatolian Turkish principalities to the east.
