What 1209–1229 crusade targeted heresy in southern France?

The story behind the answer

It was a papally sanctioned campaign (1209–1229) to eradicate the Cathar, or “Albigensian,” heresy in Languedoc, southern France. Preached by Pope Innocent III and led by northern French nobles like Simon de Montfort, it opened with the notorious sack of Béziers—where a papal legate reportedly said, “Kill them all; God will know his own”—and continued through sieges such as Carcassonne.

Beyond suppressing heresy, it transformed French politics. Capetian royal power expanded into the south, formalized by the Treaty of Paris (1229). The effort also helped institutionalize mechanisms that evolved into the medieval Inquisition and left deep cultural scars on Occitania, diminishing the region’s language and vibrant troubadour tradition.