The Ottomans accepted the invitation. They crossed into Europe. They established the usurper on the throne. They drove his enemies right up into the Balkans. And, for the time being, they returned to their own land. But they had learnt that this corner of Europe was a desirable territory and that it was undefended by any effective force. Bulgarians, {220} Serbians, Bosnians, and Albanians held the lands, or nearly those same lands, that you will see marked under their names on any map of Europe made before the Great War. By the end of the fourteenth century the Ottomans had overrun all these countries and had organised them under Turkish rule. They had taken Adrianople, the city of second importance in the Eastern Empire. They had spread terror westerly in Europe by a great victory won over a Christian army of twice the number of the Ottoman force at Kossovo. and again by a victory, in which many crusading knights were killed, at Nicopolis. At the very end of the century they were besieging Constantinople itself: but for a while the capital of the Empire was delivered from their hands. Partly by the stubborn courage of the besieged forces in the city, partly by bribery, and partly by a new danger appearing on the eastern border of their own kingdom in Asia, they were induced to raise the siege. The new danger came, as ever, from the east. It was really Timur, or Tamerlane, with his Tartar hordes, who saved Constantinople, the capital city of Eastern Christendom, for another half-century from the Turks. [Sidenote: Turks take Constantinople] The Tartars came in irresistible numbers. They swept over nearly all Asia Minor and down into Egypt where the Caliph, the religious head of Mohammedanism, ruled. And then, as always before, they went back again. The ravaged countries were left to recover as best they could, and the Ottomans resumed their campaign in Europe. Constantinople, again besieged in 1422, was again saved for a while by the appearance of a rival claimant to the Sultanship of Turkey. But the Turks pushed northwards into Hungary, where the Hungarians opposed them with a resolute resistance. Battles were fought with varying result, until, again on the fatal battlefield of Kossovo, the Moslem won another great {221} victory. The siege of Constantinople was recommenced with more vigour than ever. In 1453 the long-deferred end came. The city was taken by assault. The Christian Church of St. Sophia became the Moslem mosque.