Abstract

The crusades that began around AD 1100 are among the most staggering examples of how religious ideas can be reinterpreted to justify warfare. How can we explain this "Christianization" of war and how can we explain its resonance in medieval Europe? This paper argues that the eleventh-century church reform movement, made possible by the ninth- and tenth-century Carolingian state collapse, incentivized its lay supporters to go on crusade as part of the struggle to spread its religious program. This relationship was strongest for the First Crusade; it weakened for subsequent crusades as the new and revolutionary ideas about penitential warfare became widely accepted following the surprising (and bloody) conquest of Jerusalem in 1099. Using data on the location of crusaders between 1096 and 1192 — and instrumenting for proximity to the church reform movement using distance from its place of origin (Cluny Abbey in Bourgogne) — we provide evidence in support of this argument.