Beziers, Simon de Montfort, and the Peace of Paris

The crusader army's first target, Beziers, was stormed on 22 July 1209. The city held both Cathars and Catholics, and the massacre that followed killed a large part of the population regardless; Arnaud Amalric is credited, in a later and disputed account, with telling the crusaders to kill them all and let God sort out his own, a line historians treat as a later embellishment of the massacre's brutality rather than a verified order. Simon de Montfort, who took over the crusade's military command later in 1209, led it through two decades of campaigning marked by further sieges, mass burnings of captured Cathars including at Minerve and Lavaur, and his own death at the siege of Toulouse in 1218.

Fighting continued at reduced intensity after Simon's death until the French crown itself took over the campaign in the 1220s. The Treaty of Paris in 1229 ended the military phase, transferring most of the county of Toulouse to direct French royal control and marrying its heiress to the king's brother. Cathar belief survived the treaty in scattered rural communities, and rooting it out afterward became the task of a newly organized inquisitorial apparatus, licensed by the papacy in the years that followed, rather than of further armed campaigns.