The Emperor Alexius rejoiced at the coming of the first crusaders, hoping to regain lands lost in Malazgirt. The agreement was for the Byzantines to support the crusaders and occupy lands taken from the Muslims. The crusaders didn’t go along with the plan and founded the Eastern Catholic Kingdom in Jordan in 1099. The crusaders were despised by the residents of Istanbul who openly showed their disfavour. Meanwhile, the crusades continued and the fourth crusade ended in the invasion and dividing up Istanbul.

During that period there was a great deal of dissension as to the succession to the throne. The crusaders, realising their opportunity, entered the Golden Horn with the aid of the Venetian. The attack began on 9 April and the city fell on 13 April. For three days, in an unprecedented example of barbarism, Istanbul was looted and the inhabitants were murdered. Many important structures, including Hagia Sophia were damaged and the books that were hundred years old were burned. Important Byzantine works of art were taken to Europe. The looting became routine and the crusaders joined with the Venetian to divide Byzantium among themselves, founding the Eastern Catholic Empire.