As with today, religion was a driving force of much of the conflict that was seen in the Middle Ages.
The Crusades were a series of religious wars fought during the period between the 11th and 16th centuries that were not only sanctioned by the Churches of Christianity, but also in part funded and armed by the very same.
The aim of the Crusades was to liberate the holy lands from the hand of the Islamic ruled caliphates that had successfully conquered them in the proceeding centuries.
The very first foray into what is now known as the First Crusade began after Catholic Pope Urban II issued a call to arms in 1095, and urged the formation of a Christian army to help the beleaguered Byzantine Empire and its Emperor Alexios I, who was embroiled in conflict with the Turks in Anatolia who were migrating west.
Under the guise of protecting pilgrims who were attempting to access the holy lands currently under the control of the Caliphate, the accepted historical aim was to join the Western and Eastern branches of Christendom who had suffered a split during the East-West Schism in 1054 and nominate himself as the leader of all Christians.
Urban II offered indulgences to all Christians who joined the army and headed into battle. What this amounted to was absolution for past sins by agreeing to fight the Muslims in control of the Holy Land of Jerusalem.
The First Crusade began in 1096 and officially ended in 1099, but this event started subsequent Crusades that continued in one form or another for the next 400 years.
The ramifications of these religious wars continued to influence world events for much longer that time, however and still resonate today with the conflict in the current Middle East still ongoing over what is considered Holy Land by a variety of different religions.