I was perusing Augustine's City of God, and this passage struck me as perhaps still relevant:
"Peace and War had a competition in cruelty; and Peace won the prize. For the men whom War cut down were bearing arms; Peace slaughtered the defenceless. The law of War was that the smitten should have the chance of smiting in return; the aim of Peace was to make sure not that the survivor should live, but that he should be killed without the chance of offering resistance."
Augustine, City of God, 3.28.