Some wars are absolutely necessary - with WWII being a prime example. It was a case of true evil reaching out to destroy and subjugate the innocent. But at the same time - WWII was a direct result of the even-a-10-year-old-could-probably-figure-out-that-these-terms-will-never-work-in-the-long-term terms that WWI ended on. The victors forced such ridiculous, unsustainable, oppressive terms on Germany and other countries that the whole country became like a broiling pot, ready to spill over. Like slavery - it was always going to explode eventually. Hitler was just the match that lit the spark. And yeah, he made it worse and much more genocidal than it would have been otherwise - the man personally has millions of innocent deaths on his conscience that wouldn't have happened otherwise - but he still just filled a dark need for a people who felt a boot on their necks and wanted to rip off the metaphorical foot. So even necessary wars are both anticipatable and preventable, if we're not stupid and blind about it. But we're human, so most of us will always be stupid and blind about too many things. And large groups will always be much stupider and blinder than most individuals.
Anyway - that was a pretty wanders-around-a-lot rant. What I'm trying to get at is that most wars are unnecessary and preventable, but that we're too blind and stupid of a species to keep from bringing things to the point of war sometimes. And when that point comes, and real evil exists on the other side - evil that won't leave the innocent alone - then I do believe in fighting those wars. But I'll always err on the side of questioning whether a war is really necessary or not, because most of them aren't.