The origin of the Crusading movement can be traced to the beginnings of Islam in the seventh century, specifically to the teachings of Muhammad. It was the duty of all Muslims, Muhammad taught, to bring non-believers into Islam by means of violent struggle. This led to an imperialistic and expansionist mindset. By the 11th century Islamic armies had conquered all of North Africa, most of Spain, part of Turkey, Sicily and were raiding Europe.
Muslim persecution of indigenous Christians in its conquered lands and of pilgrims in the Holy Land increased throughout the 11th century culminating in the 1065 Good Friday massacre of 12.000 pilgrims who were journeying to Jerusalem. In 1071, they defeated a large Byzantine Army captured Emperor Romanus IV, and proceeded to conquer Byzantine territory. This led the later emperor Alexius I to seek help from the pope in raising troops to defend their fellow Christians.
Blessed Pope Urban II responded to the Byzantine emperor’s call for help when he preached the Frist Crusade at the local council of Clermont in 1095. Urban knew how great a sacrifice it would be for Catholic warriors to leave home, family, and friends, and to risk death defending the Byzantines, so he focused the Crusade on the liberation of Jerusalem and the Church of the Holy Sepulcher. This exalted aim, plus the promised spiritual benefit of the indulgence given to those who fought to protect persecuted Christians, motivated tens of thousands of warriors to take up the cross and travel to the Holy Land. The Crusades were not offensive wars intended for the conquest and slaughter of Muslims; rather they were defensive just wars aimed at recovering occupied Christian territory and defending indigenous Christians and pilgrims from Muslim attacks.
Acknowledgement: Steve Weidenkopf - The Real Story of Catholic History