In 1169 the forces of Islam were rallied under a Kurdish adventurer named Saladin who had made himself master of Egypt. He preached a Holy War against the Christians, recaptured Jerusalem in 1187, and so provoked the Third Crusade. This failed to recover Jerusalem. In the Fourth Crusade (1202-4) the Latin Church turned frankly upon the Greek Empire, and there was not even a pretence of fighting the Turks. It started from Venice and in 1204 it stormed Constantinople. The great rising trading city of Venice was the leader in this adventure, and most of the coasts and islands of the Byzantine Empire were annexed by the Venetians. A “Latin” emperor (Baldwin of Flanders) was set up in Constantinople and the Latin and Greek Church were declared to be reunited. The Latin emperors ruled in Constantinople from 1204 to 1261 when the Greek world shook itself free again from Roman predominance.
The twelfth century then and the opening of the thirteenth was the age of papal ascendancy just as the eleventh was the age of the ascendancy of the Seljuk Turks and the tenth the age of the Northmen. A united Christendom under the rule of the Pope came nearer to being a working reality than it ever was before or after that time.