Albigensianism appeared in the 12th cent. and soon had powerful protectors. Local bishops were ineffectual in dealing with the problem, and the pope sent St. Bernard of Clairvaux and other Cistercians to preach in Languedoc, the center of the movement. In 1167 the Albigenses held a council of their own at Toulouse. Pope Innocent III attacked the problem anew, and his action in sending (1205) St. Dominic to lead a band of poor preaching friars into the Albigensian cities was decisive. These missionaries were hampered by the war that soon broke out.
In 1208 the papal legate, a Cistercian, Peter de Castelnau, was murdered, probably by an aid of Raymond VI of Toulouse, one of the chief Albigensian nobles. The pope proclaimed (1208) the Albigensian Crusade. From the first, political interests in the war overshadowed others; behind Simon de Montfort, the Catholic leader, was France, and behind Raymond was Peter II of Aragón, irreproachably Catholic. Innocent attempted to make peace, but the prize of S France was tempting, and the crusaders continued to ransack the entire region.
In 1213 at Muret, Montfort was victor and Peter was killed. The war went on, with the son of Philip II (later Louis VIII) as one of the leaders. Simon's death in 1218 robbed him of victory and left his less competent son to continue the fight. Raymond's son, Raymond VII, joined the war, which was finally terminated with an honorable capitulation by Raymond. By the Peace of Paris (1229), Louis IX acquired the county of Toulouse. The religious result of the crusade was negligible.
In 1233, Pope Gregory IX established a system of legal investigation in Albigensian centers and put it into the hands of the Dominicans; this was the birth of the medieval Inquisition. After 100 years of the Inquisition, of tireless preaching by the friars, and of careful reform of the clergy, Albigensianism was dead.