The Albigensian Crusade or Cathar Crusade (1209–1229) was a 20-year military campaign initiated by Pope Innocent III to eliminate Catharism in Languedoc, in the south of France. The Crusade was prosecuted primarily by the French crown and promptly took on a political flavour, resulting in not only a significant reduction in the number of practicing Cathars but also a realignment of Occitania, bringing it into the sphere of the French crown and diminishing the distinct regional culture and high level of Aragonese influence. The Cathars were a medieval Christian sect with a neo-manichaean philosophy.

It originated from a reform movement within the Bogomil churches of Dalmatia and Bulgaria calling for a return to the Christian message of perfection, poverty and preaching. They became known as the Albigensians as it gained many adherents in the city of Albi and surrounding area in the twelfth and thirteenth century.

When Innocent III's diplomatic attempts to roll back Catharism met with little success and after the murder of the papal legate, Pierre de Castelnau, Innocent III declared a crusade against Languedoc, offering the lands of the Cathar heretics to any French nobleman willing to take up arms. The violence led to France's acquisition of lands with closer linguistic, cultural, and political ties to Catalonia (see Occitan). The pope declared that all Albigenses "should be imprisoned and their property confiscated".
The Albigensian Crusade also had a role in the creation and institutionalization of both the Dominican Order and the Medieval Inquisition.

http://en.wikipedia.org/wiki/Albigensian_Crusade

This Albigensian Crusade continued from 1209 to 1229, for two decades the blood would spill by the 'Holy Pontiffs' decree. Though it was launched by the pope, the killing shifted to the local secular detachments. One example is Simon de Montfort. Repression formulas on the heresy of the Cathars granted new methods. Noted above most was the dreadful day of July 22, 1209. On this day the city of Beziers was sacked, plundered and ravaged. Relentless butchering, of which it was, was once again rife, with estimations of over 20,000 inhabitants, including children, murdered by so called holy crusaders.

During the attack, the Cistercian abbot Arnald-Amalric, when questioned if Catholics to should be 'liquidated' or perhaps absolved, retorted "Kill them all, for God knows His own". This is a man, commissioned by Papal authority, which is supposedly the authoritative representative of Christ on earth. The day after he wrote to Innocent III, "Nearly twenty thousand of these people were put to the sword, without regard for age or sex." Simon de Montfort play a major role in the massacre and was rewarded and selected as commander.