十字軍東征 wiki
The evolution of a Christian theology of war developed from the link of Roman citizenshipto Christianity, according to which citizens were required to fight the empire's enemies. This doctrine of holy war dated from the 4th-century theologian Saint Augustine. He maintained that aggressive war was sinful, but acknowledged a "just war" could be rationalised if it was proclaimed by a legitimate authority, was defensive or for the recovery of lands, and without an excessive degree of violence.
