The Albigensian Crusade or Cathar Crusade (1209 - 1229) was a 20-year military campaign initiated by the Roman Catholic Church to eliminate the heresy of the Cathars of Languedoc.
When Innocent III's diplomatic attempts to roll back Catharism met with little success, he declared a crusade against Languedoc, offering the lands of the schismatics to any French nobleman willing to take up arms. The violence, extreme even by medieval standards, led to France's acquiring of lands with closer cultural and linguistic ties to Catalonia (see Occitan).
The Albigensian Crusade also had a role in the creation and institutionalization of both the Dominican Order and the Medieval Inquisition.