In a bid to take back land the Europeans attempted to throw Muslims out of Jerusalem. These attempts were called the Crusades and done under a religious flag. The first crusade occurred in the 11th century on the behest of Pope Urban II.
Pope Urban II arose to the papacy in a time of controversy. He wanted to create unity among factions and strengthen the church. He also wanted the papacy and stop lay clergy from swearing fealty to secular leaders. During the height of his success in the papacy, Pope Urban II received a call for help from the Byzantine Emperor.
In response to this call, Pope Urban II preached to hundreds of clerics about the need to take back Jerusalem from the Muslims. Thousands of people responded. Some went to battle for God, while others went for riches. Regardless, they went and so started a history of the Crusades. Though Pope Urban II called for the first crusade he did not live to hear of the victory that occurred in 1099.