Early in the eleventh century the bulk of the Turkish nation, under its leader Tongrol Bek, moving out from Turkestan, swept down upon Persia. The Khaliphate at Bagdad was, however, permitted to remain, and not only so, but Tongrol Bek and all his tribes embraced the Mohammedan religion. The invaders then marched west in vast numbers to make an attack upon Christendom, and in the course of time subdued Armenia and most of Asia Minor. Europe became alarmed, and the Byzantine Emperors eagerly sought the assistance of the nations of the west. Hence arose the Crusades, which had as their chief object the deliverance of Palestine from both Saracens and Turks, and which served to retard, though not to prevent, the advance of the Turkish power in Europe. The Turks Enter Europe. Early in the thirteenth century a mighty movement of Mongols south-west from Central Asia, involving the immediate destruction of the Khaliphate at Bagdad, exerted an important influence upon the Turks, in driving those Turkish tribes which had remained east of Armenia westward into Asia Minor. This resulted in the establishment of various Turkish dynasties in that country. At the close of the thirteenth century the paramount power over these was exercised by Osman (or Othman, whence the name Ottoman), who seized all that remained of the ancient Roman world in Asia, and thus practically founded the Ottoman Empire. In the middle of the fourteenth century the way was opened for the Ottomans to advance into Europe. They were invited by one of the rival factions at Constantinople to undertake their cause. The Turks accordingly crossed the Hellespont and seized Gallipoli and the territory in the vicinity of the capital. Constantinople itself was left unattacked for the time. Under Murad I., the grandson of Osman, Roumania and several kingdoms south of the Danube, including Bulgaria, were subdued. The kings of Hungary, Bosnia and Serbia rose against the invader, but were severely defeated, and by the decisive victory of Kosovo, in 1389, Serbia and Bosnia were annexed. Constantinople Taken. Constantinople was temporarily saved by another advance of the Mongol Tartars upon the Turkish dominions in Asia, where, in 1402, the Ottomans suffered a severe defeat. From this check they recovered, and during the first part of the fifteenth century were at war with the Hungarians and neighbouring races, whom they eventually overthrew. In 1451 Mohammed II. ascended the Ottoman throne, and in 1453 led an immense army against Constantinople. The city was taken by storm, the last of the Roman Emperors of the east died fighting, and Mohammed II. rode in triumph to the cathedral of St. Sophia, where he established the Moslem worship.