Béziers, a strategically important city in southern France, became the site of one of the first major military actions of the Albigensian Crusade. The crusade was initiated by Pope Innocent III in response to the rise of the Cathar movement, a Christian sect deemed heretical by the Catholic Church. In July 1209, Crusader forces laid siege to the city after it was accused of harboring Cathars. The siege lasted a few days, during which the defenders prepared for a fierce resistance. However, the Crusaders, motivated by religious fervor and the promise of absolution, managed to breach the city's defenses. Once inside, the troops wreaked havoc on the population.

Arnaud Amalric, the papal legate in charge of the crusade, famously ordered the extermination of all inhabitants, reportedly stating, "Kill them all; God will know His own." This tragic quote highlights the brutal reality of medieval warfare, where the lines between combatants and non-combatants were often blurred. The end result was a massacre where the vast majority of the city's residents—estimates suggest as many as 20,000 people—were slain, regardless of their alignment to the heretical beliefs.

This shocking event established a precedent for the ferocity of the Albigensian Crusade, which aimed not only to reclaim geographic territory but also to eradicate the Cathar faith. It signified a shift in the approach to religious dissent in the region, as military action became a method of enforcing spiritual conformity. The massacre at Béziers has since been regarded as a notable example of the brutal techniques employed during the crusade, marking an indelible moment in the struggle between differing Christian ideologies in medieval Europe.