This is a poorly defined and inflammatory question.
It also seems to be a question with the obvious answer of "yes" from almost any perspective.
Once you decide on a moral system and a capacity for judgment--i.e., "vengeful general" or "concerned liberal citizen"--you could use standard brands of decision and probability theory to determine optimal actions--i.e., "go to war" or "vote against war"--but basically the only claim this entails is that "it is theoretically possible to think clearly when it comes to politics and wars" which is (hopefully) not a controversial (or, really, interesting) claim.
Could you use Bayes Theorem to figure out whether or not a given war is just?
If so, I was wondering how one would go about estimating the prior probability that a war is just.
Thanks for any help you can offer.