In my late-vocations class were were informed that during late antiquity in the Eastern (very Christian influenced) Roman empire there was an operational just war theory. That theory was quite simple and was as follows.
War is never just.
Now this is an interesting theory of war to be held by a Empire which was almost continuously at war (mostly for defense) for 800 years or so. This merely points out that the conclusion that war is not just is not equivalent to the claim that war is at times necessary.
War not being just however, did not mean war was not practice or even should not be practiced. Those engaged in war, because of its inherent injustice, were excluded from Eucharist for a period of five years (if the war was not deemed defensive, in which case it was three years). I think there are some problems with this theory as presented about how the Eastern Roman Empire viewed justice vis a vis war, in that I’m pretty sure that clerical presence was found alongside the army. What was its purpose if these soldiers were all “out of communion” during wartime?