The Albigensian Crusade
An estimated 200,000 to 1,000,000 people were massacred in Languedoc Roussillon during the Albigensian Crusade of 1209–1229 in the name of religion! (‘Albigensian’ was another word for Cathar, so called because of the strong Cathar following in the town of Albi.)
This 20-year military campaign to eliminate Cathars was launched by Pope Innocent III and directed against the Count of Toulouse, who was wrongly blamed for the murder of a papal legate.
In fact, the real aim of the crusade was to force the count and other members of the Languedoc nobility to take action against the Cathar heretics in their lands.