Cost/benefit on what scale?

The answer to “when is war justified” must be “when that war is just?” So the real question we must answer is “what constitutes justice?” What are the criteria involved in Jus ad Bellum?

Thomas Aquinas said that war required three conditions in order to be just: a regime with authority to wage war, a just cause, and rightful intention. While somewhat useful this rubric once again leads us into syllogistic circles never to emerge any better off than where we started. The Augustinian conception states that war is only justified in defense of the nation against serious injury. He would argue that going to war simply for vengeance or economic gain is unacceptable. Others, such as Vitoria, add in proportionality as a calculation in war. This is something akin to the idea that the punishment must be commensurate with the offence. This, however, leads us into Jus in Bello, which is probably off topic for right now.

One of our most significant problems with Just War Theory these days comes from Aquinas’ first condition. Under most theories of Just War, war must be contracted, so to speak, by a legitimate authority. In the days of Augustine or Aquinas this was the sovereign and under the Geneva Convention this became the ‘State.’ Now we face a world in which ‘non-legitimate’ parties are starting wars. That whole condition is creating new problems for us as we try to attack adversaries within ‘legitimate’ authorities that may or may not be involved in the cause for the war.

The Romans, despite their imperial drive, longed to see war justified, at least in their own eyes. They always sought to present it as defensive in nature. Interestingly, until the time of the Principate the temple of Mars was outside of the city. The thought ran that war should be kept at a distance. Even when Augustus built the Temple to Mars Ultor (the Avenger) we see the desire to depict war in a justified fashion. Mars brings justice, in this case on the murderers of Julius Caesar. He is not the god of wanton, unbridled destruction we see in early Greek mythology.

So, after all this wandering excursus I would say that war for defense is justified given right intention. Anything else allows room for wide speculation and untoward violence. I don’t think that this outlook rules out preemptive action either. It does, however, rule out war for solely economic gain.