We are often told, for various reasons, that "violence never solved anything." Well, certainly, at first impression, this is true, and we have seen that pointed or what you might call rhetorical or symbolic nonviolence can help along positive change in ways that an army or guerilla movement never could. Vendettas and wars of reprisal don't end and don't accomplish anything other than the moral hollowing-out of everybody involved. And yet there have been bloody wars in history that were followed by remarkable periods of peace. Is there any reason for this, other than people being sick of fighting ?
The fact is, most people, except a few psychopaths aren't just going to pick a gun and start a war. Wars might be the outgrowth of ideology, imperialism, the fixing of some historical slight, or opportunism, but whenever they pop up, even the "other" side has a pretty good idea of why it started on some immediate level. (There are "deeper" reasons that historians argue over, of course.) Clausewitz (who has been cited on this topic so many times that this first name wore out) said that "War is politics by other means," although that doesn't say anything about the morality of war or a war, especially when even an attacked country's self-defense will have problematic consequences. If (he said, dragging out the usual warhorse) WWII was the "good fight," then why did the good guys put innocent people in internment camps, firebomb some cities for nontactical reasons, and try out a new, nasty form of explosion on two others ? A "good" war, then, perhaps needs to meet a test that goes beyond immediate consequences and operates on a level that is "meta"-moral.
Perhaps the reason for a just war is that one side is actually trying to maintain the truth of the statement "violence never solves anything." After all, if violence, in itself, did solve anything (like a Lebensraum problem for the Aryans), and people actually believed that, the world would descend into bloody, Hobbesian nasty/brutish/short chaos, or a dictatorship of the strong. The unpleasant truth, if you like moral certainty, would be that we ought to believe in the uselessness of violence even though it's a conclusion you can not draw from scientific methods or even some by some kind of anthropological study. Getting back to history, then, leaving the Axis alone after their successes would not have avoided violence in the long term because violence would go unchallenged, and thus vindicated (not to mention the strategic benefit of no resistance to grow even stronger). And because the instigators of violence tend to respect only violence as a means of persuasion, the people who abhor violence must use it to defend both themselves and a principle that is good.
Nota bene, Even in this scenario, the good guys don't have to be very good or even somewhat good in other ways, just fighting against somebody else trying to push a more destructive set of principles over on them. A lot of empires were "bad" by today's standards but brought their subjects a certain amount of peace, prosperity, and dignity because they enforced a peace that allow people to just live, however constricted their freedom might have been. That peace was enforced by monopolizing force so that any potential attackers would have to consider the consequences of reaction from a strong defense.
Maybe someday instead using guns, bombs, and nasty substances, these kinds of misunderstandings can be solved in nonverbal ways that are at least less "technically" nasty. For example, instead of raining death on the bad guys, however accurate that might become, we would just immobilize them and whatever tools of death and destruction that they command in immense wads of sticky, but non-lethal goo. (I think I read somewhere that in fact the US DoD is looking into this sort of thing.) Then we'd have a nice schoolmarmish chat with the naughty boys, and if they don't like it, put them on trial at the UN or something.