Final answer:
The First Crusade marked the beginning of the Crusades when Christians launched military campaigns to reclaim Jerusalem from the Muslims, resulting in its capture in 1099.
Step-by-step explanation:
The historical event that marked the beginning of the Crusades was The First Crusade. It started in 1096 when Pope Urban II called for Christians to take up arms and reclaim Jerusalem from the Muslims. The Crusaders launched military campaigns in the Holy Land, resulting in the capture of Jerusalem in 1099.
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