If Christian teaching condemned all warfare, then the soldiers in the gospel who were seeking guidance about their security would have been told to throw away their weapons and withdraw entirely from the army. But what was said to them was: 'Do not intimidate anyone; do not bring false charges against anyone; be satisfied with your wages' [Lk 3:14]. He instructed them to be satisfied with their due wages, but he didn't prohibit military service in general.

augustine, letter to marcellinus, p.39