The issue of just war isn't just on when Christians should fight, but on how they should fight. There is no justification for evil done in the name of good. Just war, according to Cole (see margin for book info), is an act of love. He starts by noting that acts of force can be loving by using the example of a parent punishing a child, even if the analogy is too simple. There is no place for evil in a just war. Augustine said, "...courage decides to endure evil rather than consent to evil" (p. 94), and yet Augustine believed in just war, not pacifism.
The two keys to fighting justly are Discrimination and Proportion. Discrimination is the granting of immunity to noncombatants. Noncombatants should not be killed intentionally. While such deaths are inevitable, intention should be determined by word and deed. If one can justify executing an operation whether or not there were civilians present or not, it seems likely that proper discrimination has taken place. And just because one can see that there will be civilian casualties does not mean it is one's intention to bring them about. One can see the challenges of determining "proper discrimination" in the Israeli-Hamas conflict in Gaza - at least from my limited knowledge of the conflict.
The second issue is Proportion. This asks, essentially, "Does the intended good outweith the potential consequences of the innocents killed?" This is the responsibility of every warrior.
Along these lines, Cole believes World War II to be just, but the saturation bombing and the atomic bomb unjust tactics. They crossed the line and were indiscriminate and actually targeted civilians. Cole goes on and recognizes the moral challenges of the Vietnam War, but does not fault US soldiers, apart from My Lai and like isolated events, for failing to discriminate their enemy from the civilians. The guerilla armies of the North Vietnamese made this task difficult and one can argue the soldiers did their best. He does question the justice of the war on the grounds that the South Vietnamese were not a country worth partnering with because they, too, were unjust. It may stand on the other points of just war. Finally, the 1991 Gulf War was just and the discrimination, while not perfect, was more perfect than any other modern war - by far.