In 1208 Pope Innocent III called for a Crusade--this time, against a country of fellow Christians. The new enemy: Raymond VI, Count of Toulouse, prince of all the territories in southern France where the langue d'oc was spoken. Thus began the Albigensian Crusade, which culminated in 1244 at the mountain fortress of Montsegur with the massacre of the Cathars, or "pure ones"--a faith more ancient than Roman Catholicism. This fascinating history captures all the drama of a conflict that threatened not only a religion but the very survival of the Languedoc itself as an autonomous region of...