The Crusades were a series of military campaigns by Christians in the 11th to 13th centuries, to reclaim the Holy Land from the Muslims. Today many people think of the Crusades as an dark period of church history, due to atrocities committed by the Crusaders, and the animosity it has caused with Muslims.
During the time of Jesus, the Holy Land was under the control of the Roman Empire. After Emperor Constantine converted to Christianity, the church grew into the Roman Catholic Church and the Eastern Orthodox Church, also called Byzantine.
In 636 AD, Muslim Arabs defeated the Roman/Byzantines at the Battle of Yarmouk. The Muslims still allowed Christians to make pilgrimages, but there were times of persecution. When the Seljuk Turks took control, the Christians began to feel threatened in the Holy Land and in the Byzantine empire.
Pope Urban II launched the first crusade to reclaim the Holy Land, gaining support and rallying fighters by promising immediate entry into heaven for those who died in the cause, and also suggested material rewards, power and prestige. (These promises were not Biblical)
The Crusaders gained control of Jerusalem in 1099, and set up four Crusader states from the land they conquered. Over the next two centuries however, most of the land was regained by the Muslims. There were a total of nine crusades, most of the later ones ended by treaties between the Christians and occupying Muslims.
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