In 1209 Simon of Montfort led a war against the Cathars of Languedoc after Pope Innocent III preached a crusade condemning them as heretics. The suppression of heresy became a pretext for a vicious war that remains largely unstudied as a military conflict. Laurence Marvin here examines the Albigensian Crusade as military and political history rather than religious history and traces these dimensions of the conflict through to Montfort’s death in 1218. He shows how Montfort experienced military success in spite of a hostile populace, impossible military targets, armies that dissolved every forty days, and a pope who often failed to support the crusade morally or financially. He also discusses the supposed brutality of the war, why the inhabitants were for so long unsuccessful at defending themselves against it, and its impact on Occitania. This original account will appeal to scholars of medieval France, the Crusades and medieval military history.