Already, however, a new enemy had appeared who threatened not only Venice but all Europe. This was the Ottoman Turk. The Turks were not like the Arabs, members of the Indo-European family, but a race from the eastern borders of the Caspian Sea, a branch of the Mongolian stock. As these peoples moved south and west they came in contact with Mohammedanism and became ardent converts. Eventually they swept over Asia Minor, crossed the Dardanelles, took Adrianople, and pushed into Serbia. Thus, when Constantinople fell in 1453 it had been for some time a mere island of Christianity surrounded by Moslems. Indeed it was only the civil wars among the Turks themselves that held them back so long from the brilliant career of conquest that characterized the 15th and early 16th centuries, for these later followers of Mohammed had all the fanaticism of the Saracens. Before the fall of Constantinople and the transfer of the Turkish seat of government to that city, a corps of infantry was organized that became the terror of the Christian world--the Janissaries. By a grim irony of the Sultan, who created this body of troops, these men were exclusively of Christian parentage, taken as children either in the form of a human tribute levied on the Christian population of Constantinople, or as captives in the various expeditions in Christian territory. The Janissaries were brought up wholly to a military life, they were not permitted to marry, and their lives were devoted to fighting for the Crescent. For a long time they were invincible in the open field. The first half of the 16th century saw the Turks in Persia, in the east, and at the gates of Vienna in the west. For a time they got a foothold in Italy by seizing Otranto. They had conquered Egypt and Syria, penetrated Persia, and in Arabia gained the support of the Arabs for the Turkish sultan as the successor to the Caliphs. Constantinople, therefore, became not only the political capital for the Turkish empire but the religious center of the whole Moslem world. Moreover, the Arab states on the southern borders of the Mediterranean acknowledged the suzerainty of the Turkish ruler.