The Albigensian Crusade or Cathar Crusade, a twenty-year military campaign initiated by the Roman Catholic Church to eliminate the Cathar heresy in Languedoc, is prosecuted primarily by the French and promptly takes on a political flavor, resulting in not only a significant reduction in the number of practicing Cathars but also a realignment of southern France, bringing it into the sphere of the French crown and diminishing the distinct regional culture and high level of foreign influence.When Innocent III's diplomatic attempts 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), Innocent III 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 (see Occitan).

An estimated two hundred thousand to one million people die during the crusade.The Albigensian Crusade also has a role in the creation and institutionalization of both the Dominican Order and the Medieval Inquisition.