Medieval History
-The decline of the Byzantine Empire began when Constantinople was sacked in 1204 during:

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The Holy Roman Empire sacked the capital city of the Byzantine Empire in 1204 during the Fourth Crusade. Though Byzantine forces were eventually able to overthrow the Crusaders and reclaim the city, the event is still considered the onset of the Byzantium's decline. It was the penultimate act in the Great Schism, which permanently divided the Eastern Orthodox Christians from the Roman Catholic Church. The original intention of the Fourth Crusade was to capture Jerusalem, but the western Christian forces were ultimately redirected.

Medieval History -The Decline of the Byzantine Empire Began When Constantinople Was