At various points in its history, the region has been inhabited by diverse civilizations including the Assyrians, Greeks, Thracians, Phrygians, Urartians, and Armenians. Hellenization started during the era of Alexander the Great and continued into the Byzantine era. The Seljuk Turks began migrating into the area in the 11th century, and their victory over the Byzantines at the Battle of Manzikert in 1071 symbolizes the start and foundation of Turkey. The Seljuk Sultanate of Rûm ruled Anatolia until the Mongol invasion in 1243, when it disintegrated into small Turkish principalities. Beginning in the late 13th-century, the Ottomans started uniting these Turkish principalities. After Mehmed II conquered Constantinople in 1453, Ottoman expansion continued under Selim I. During the reign of Suleiman the Magnificent the Ottoman Empire encompassed much of Southeast Europe, West Asia and North Africa and became a world power. In the following centuries the state entered a period of decline with a gradual loss of territories and wars. In an effort to consolidate the weakening social and political foundations of the empire, Mahmut II started a period of modernisation in the early 19th century, bringing reforms in all areas of the state including the military and bureaucracy along with the emancipation of all citizens.