The document provides background information on the Crusades. It discusses how the Crusades began when Pope Urban II agreed to help the Byzantine Emperor Alexius I against invading Muslim armies. Jerusalem was an important holy city for Christians and Muslims, and was under Muslim control in the late 11th century. Urban helped organize the First Crusade to retake the city. The Crusaders expected to receive elite mercenaries but Alexius received untrained peasants instead. The Crusades continued for over 200 years and fought against various groups in the holy lands.