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Fall of Constantinople in 1204 and the exile in Nicaea

The Fourth Crusade in exchange for promised funds attempted to help the deposed emperor Alexius IV regain his throne. After taking Constantinople, returning Alexius IV to the throne, the revolt against and murder of Alexius IV left the Crusaders without payment. On 12 April 1204, the crusaders inflicted a severe sacking on Constantinople for three days, during which many ancient and medieval Roman and Greek works were either stolen or destroyed. Despite their oaths and the threat of excommunication, the Crusaders ruthlessly and systematically violated the city's holy sanctuaries, destroying, defiling, or stealing all they could lay hands on; nothing was spared. It was said that the total amount looted from Constantinople was about 900,000 silver marks. The Venetians received 150,000 silver marks that was their due, while the Crusaders received 50,000 silver marks. A further 100,000 silver marks were divided evenly up between the Crusaders and Venetians. The remaining 500,000 silver marks were secretly kept back by many Crusader knights.

Nicetas Choniates gives a vivid account of the sack of Constantinople by the Frankish and Venetian Crusaders of the Fourth Crusade:

The Latin soldiery subjected the greatest city in Europe to an indescribable sack. For three days they murdered, raped, looted and destroyed on a scale which even the ancient Vandals and Goths would have found unbelievable. Constantinople had become a veritable museum of ancient and Byzantine art, an emporium of such incredible wealth that the Latins were astounded at the riches they found. Though the Venetians had an appreciation for the art which they discovered (they were themselves semi-Byzantines) and saved much of it, the French and others destroyed indiscriminately, halting to refresh themselves with wine, violation of nuns, and murder of Orthodox clerics. The Crusaders vented their hatred for the Greeks most spectacularly in the desecration of the greatest Church in Christendom. They smashed the silver iconostasis, the icons and the holy books of Hagia Sophia, and seated upon the patriarchal throne a whore who sang coarse songs as they drank wine from the Church's holy vessels. The estrangement of East and West, which had proceeded over the centuries, culminated in the horrible massacre that accompanied the conquest of Constantinople. The Greeks were convinced that even the Turks, had they taken the city, would not have been as cruel as the Latin Christians. The defeat of Byzantium, already in a state of decline, accelerated political degeneration so that the Byzantines eventually became an easy prey to the Turks. The Crusading movement thus resulted, ultimately, in the victory of Islam, a result which was of course the exact opposite of its original intention.

Meanwhile, the Latin Empire of Constantinople was established, and Byzantine refugees founded their own successor states, the most notable of these being the Empire of Nicaea under Theodore Lascaris (a relative of Alexius III), the Empire of Trebizond, and the Despotate of Epirus.

The new seat of the Patriarchate was established in the city of Nicaea until in 1261, when Constantinople was reconquered by the Byzantine Emperor Michael VIII Palaiologos.