The ultimate responsibility for having called the crusade rests with the reigning Roman Pontiff. Whatever may have been the motives of the feudal nobles in joining, opposing, or merely using the crusade it is to Rome that one just look for the reasons why the knights of northern France were summoned to ride south. As we have seen, the papacy had labored for almost a half century to instruct the faithful more adequately in the truths of their faith, to protect them from the contagion of false doctrine, and to reconcile dissidents to the Roman Church. It had utilized all its traditional methods: preaching missions, papal legates making their rounds, local and provincial synods, special efforts by St. Bernard and other Cistercians and by St. Dominic. All this had been tried over and over again with but a modicum of success. Repeated appeals for the cooperation of the secular rulers had for one reason or another all come to naught. It was only when Innocent III became convinced of the paralysis of the public powers and of the local churches that he resorted to a crusade. The imminence of complete domination by the Albigensians, the inertia of the king, the impotence and bad will of the princes, the complicity of a great number of local nobles who had permitted the Cathars to flourish, the exposed position of the laity, the repulse of the papal missions, the lost prestige of the religious leaders, the ineffectiveness of spiritual methods swamped by the unrestrained devastation of brigands and mercenaries – and now the murder of the papal legate, Peter of Castelnau – all combined to force the conclusion on the mind of the pope that military constraint alone could restore public order and thus permit the peaceful pursuit of the people’s welfare.
Albert Shannon, The Medieval Inquisition (1991)