What was the crisis of the Byzantine Empire?
The major crisis the Byzantine Empire faced--the crisis which led to its downfall--was the rise and expansion of Islam. During the Middle Ages, expansionist Muslim armies had conquered much of Eastern Christendom--including the Judeo-Christian holy city of Jerusalem--and repulsed European attempts (e.g. the Crusades) to reclaim this land. In 750, the Abbassid Muslims moved their capital to Baghdad, which shifted their focus toward Asia--and the Byzantine Empire. Power changed hands several times in the Muslim world, but eventually the Seljuk Turks seized power and assaulted the Byzantines. They effectively defeated the Byzantines at the Battle of Manzikert. The Byzantine Empire never again reclaimed its glory, and the Seljuk and Ottoman Turks chipped away at Byzantium until the Byzantine capital--Constantinople (modern-day Istanbul)--fell in 1453.