 Dating from the works of the 4th-century theologian Augustine a doctrine of holy war developed.Augustine wrote that an aggressive war was sinful, but a "just war" could be rationalised if proclaimed by a legitimate authority such as a king or bishop, it was defensive or for the recovery of lands, and a it did not involve excessive violence.The breakdown of the Carolingian Empire in Western Europe created a warriors caste who now had little to do but fight among themselves.