The Albigensian Crusade (1209–1229) was a military campaign initiated by the Catholic Church against a group of Christian heretics.

The heretics were known as the Cathars, or Albigensians, and were concentrated in the Languedoc region of southern France.

Catharism was a dualist religion, believing in a good, spiritual god and an evil, material god.

The Catholic Church viewed the Cathars as a dangerous threat to its authority and the souls of Christians.

After peaceful attempts at conversion failed, Pope Innocent III launched the crusade following the murder of a papal legate.

The crusade was led by northern French nobles, who were motivated by both religious zeal and the desire to seize the lands of the southern lords who protected the Cathars.

The conflict was known for its extreme brutality, exemplified by the massacre at the city of Béziers.

The war devastated the prosperous and culturally distinct region of Languedoc.

The crusade ultimately succeeded in crushing Catharism and bringing southern France under the direct control of the French crown.

It also led to the establishment of the Medieval Inquisition to root out any remaining heresy.