[anigbrowl]

> I'd recommend he look at Aquinas' just war theory

There have been whole books written about this - though I can't blame you for not having read them, many of them are turgid in the extreme. Christian ethics aim to break cycles of recrimination and escalation, but not in general to the point of helpless passivity. Of course the strictures of just war doctrine are considerably harder to square with the de facto imperial power of a country like the United States which is practically and to some extent aspirationally similar to the Roman empire in its heyday.

For a more modern take on the ethics of warfare you might find it interesting to examine the Lieber doctrine, which was instituted during the US civil war and (nominally) still in force in many respects.