One of the great paradoxes of the medieval period is the Albigensian Crusade (1209-1225), in which monks of the Cistercian Order took an active and violent role in campaigning against the heretics of the Languedoc. Why, and how, did this order officially devoted to prayer and contemplation become one of the prime orchestrators of one of medieval Europe’s most gruesome affairs? This thesis seeks to answer that question, not by looking at the crusading Cistercians themselves, but at their predecessor Bernard of Clairvaux, who—I will argue—made the Albigensian Crusade possible by making it permissible for monks to intervene in the world outside the cloister. The logic of this thesis is as follows. Bernard of Clairvaux lived in a world in which monastics had a certain spiritual authority that granted them special privileges over ecclesiastics (Chapter II). The Cistercian Order itself, even before Bernard became their prime mover and shaker, used these privileges to cultivate contacts beyond monastic borders (Chapter IV), and once Bernard became a prominent abbot himself, his desire to do good and criticisms of the outside world (Chapter VI) led him to intervene in various endeavors (Chapter V). These interventions drew backlash from other monastics and ecclesiastics, which then required justification in order to reconcile the vita passiva and Bernard’s active lifestyle (Chapter VII). These justifications, along with Bernard’s justifications of violence (Chapter VIII), came to more broadly characterize the Cistercian Order as a whole (Chapters I, IV), and thus the ideological material to justify monastic holy war was all present in eloquently defended and rapturously accepted form by the time Henry of Clairvaux took a castle during his 1281 preaching mission turned mini-crusade (Chapter IX). With all of this built into the Cistercian DNA, Arnaud Amaury found it very easy to lead a crusade in 1212. Could he have done this without Bernard’s example paving the way and ingraining such lessons in Cistercian thought? It is my contention that he could not have.