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[[File: Cuman_warrior.jpg|300px|thumbnail|left|Modern Reenactors of Cuman Warriors]]
The Fourth Crusade (1202-1204) was unlike any crusade before or after it because instead of pitting Christian crusaders against Muslim armies, it was a war between the forces of Western Christianity and Orthodox Christianity. Although the Roman Catholic Church and the Greek Orthodox Church officially split in 1054, they maintained somewhat cordial relations: Pope Urban II (in power 1088-1099) even called for the First Crusade (1095-1099) partly at the behest of the Byzantine Emperor Alexius I Comnenos (ruled 1081-1118), whose lands were being encroached on by the Muslim Seljuk Turks. But as the Crusades continued, Westerners began turning their eyes toward the Byzantine Empire.
The Germans, Franks, Normans, and other Western crusaders saw how wealthy the Byzantine Empire was and began coveting some of that wealth for themselves. The Republic of Venice was particularly interested in Byzantium’s wealth, as it was a merchant/trade based state with many Venetians already living in the Byzantine capital of Constantinople. The Catholic
pope of the time, Innocent III (in power 1198-1216) also saw potential in the Byzantine Empire. Although Innocent recognized Byzantium as a legitimate Christian kingdom and initially threatened any Western crusaders with excommunication if they attacked Orthodox Christians, he hoped that at least peripheral lands in the Byzantine Empire could be peeled off for the Roman Church. Once the Western crusaders entered Byzantine territory, though, things immediately became very complicated.
The crusaders were actually invited to Constantinople by the short-lived Emperor Alexius IV (reigned 1203-1204)
, before he was killed by his successor, Alexius IV (ruled 1204). The death led to the Western coalition declaring war on Alexius IV, taking Constantinople, and installing their own “Latin” dynasty that lasted until 1216. As this all was taking place, Kaloyan (reigned 1170-1207), the Tsar of Bulgaria, embarked on his own campaign to conquer the Byzantine Empire. Among the backbone of Kaloyan’s army were the nomadic steppe people, the Cumans.
The Cumans had an immediate impact on the course of the Fourth Crusade. They were highly effective on the battlefield against the Western crusaders, helping Kaloyan conquer much of the Balkans. As the war endured, though, Kaloyan became cruel and used the Cumans as a cudgel against the Greeks and other people of the Balkans. The wanton destruction of the Cumans eventually pushed the Greeks to the side of the Western crusaders and when there was little left for the Cumans to loot, they simply returned to their lands north of the Danube River. In the end, the Cumans were much more harmful to Kalyon’s efforts than they were beneficial.