The diplomatic attempts of Pope Innocent III to roll back Catharism meet with little success, and after the papal legate Pierre de Castelnau is murdered (allegedly by an agent serving the Cathar count of Toulouse) the Pope declares a crusade against Languedoc, offering the lands of the schismatics to any French nobleman willing to take up arms.

The violence leads to France's acquisition of lands with closer cultural and linguistic ties to Catalonia.

Innocent III, upon ascending the papal throne in 1198, had at first tried pacific conversion of the Cathar heretics of southern France, who have long denied the sacraments and the authority of the ecclesiastical hierarchy.

In 1207, he appoints Pierre de Castelnau, a prominent Cistercian, as apostolic legate and inquisitor to lead an expedition against the Cathari (better known as Albigenses after the town of Albi) and particularly to obtain the recantation of Count Raymond VI de Toulouse, who is allowing the heresy to spread throughout his domain.

A nephew of King Louis VII of France and brother-in-law of King Richard I of England, Raymond is tolerant toward the many heretics among his subjects.

Apparently, although the Cathari reject Judaism on theological grounds, many also reject the notion of Jesus as a god and accuse the Church of social and economical corruption.

Jews fare well in areas under their control, even attaining positions of prominence.

Having urged him from 1205 to stamp out the heretics, Peter now excommunicates Raymond, placing the Languedoc region under interdict, which arouses grave opposition.

French ecclesiastic Pierre de Castenau, born in the diocese of Montpellier, was in 1199 archdeacon of Maguelonne, and was appointed by Pope Innocent III as one of the legates for the suppression of the Cathar heresy in Languedoc.

In 1202, when a monk in the Cistercian abbey of Fontfroide, Narbonne, he had been designated to similar work, first in Toulouse, and afterwards at Viviers and Montpellier.

In 1207 he was in the Rhone valley and in Provence, where he had become involved in the strife between the count of Baux and the powerful count Raymond VI of Toulouse.

Raymond had refused to assist and was excommunicated in May 1207.

The Pope has called upon the French king, Philip II, to act against those nobles who permit Catharism, but Pierre has declined to act.

Count Raymond meets with the papal legate on January 13, 1208, and after an angry meeting, Castelnau is murdered the following day near Saint-Gilles, supposedly at Raymond's instigation.

Innocent reacts to Castelnau’s murder by issuing a bull declaring a crusade against the Waldenses and other groups (notably the Albigenses) in the Languedoc, branding them heretics and offering the land of the schismatics to any who will fight.

Although Philip II of France declines to lead the crusade, the papal offer of land draws the northern French nobility into conflict with the nobles of the south.