Crusades belong to that most viscous class of warfare-those undertaken in the name of faith. Medieval battles fought from the close of the 11th century through the 13th were engaged for a variety of motives, but they were, for the most part, wars over religion. The Catholic Church sought to harness both the knights' energy and martial skills-and found a role for them in the church's structure of society as protectors. Rules were formulated under which a "just war" could be fought and enlisted the knights to fight for them in the Crusades.
Eight crusades were fought; the first, called by Pope Urban II, taking place between 1096-1100 under the command of Godfrey de Bouillon, Raymond of Toulouse and Bohemund of Tantrum. More than 5,000 knights, joined by thousands of other soliders, peasants, men, women and children, marched eastward with the battle cry Deus Vult (It is God's Will). Few knew anything about the difficult climate awaiting them in the Holy land, and many died along the way from disease, thirst and hunger. The first Crusade was successful for the West, but they would only hold the territory for a century before being repelled by Muslim forces.
Subsequent crusades never had the success of the first, and thousands of people perished on both sides of the battlefields. The victory of the first Crusade may have been a fluke, as the Christian army arrived in the Middle East at a time when Muslim armies were at an all-time weak point.
Later crusades were fought against Turks who were trying to destroy what was left of the Byzantine Empire, but these failed as well. Other battles, fought in Spain, and Germany would prove to be more successful.
One of the positive effects of the crusades were the advances in medicine that knights saw in their Arab and Persian opponents and brought back to Western Europe.