The post-hoc rationalizations are clearly defined, which makes people more comfortable paying homage to WWII despite it being incredibly dicey. I say post-hoc because the US actually had a rather large Nazi party forming. Most people joining the war were pretty aloof as to why. It was only when those troops encountered the the Nazi death camps that the state of things became clear. The US primarily joined the war effort not because we heard what was happening to Jews, not because there was a massive land grab occurring, and not because two countries had gone an absolutely psychotic blitzkrieg - but because the Japanese Empire dared to bomb our strategic island fleet. It was a major blow, but just enough to wake up a sleeping beast.

I say WWII was dicey because we literally burned cities, we dropped nuclear bombs, we leveled entire cities with bombing runs as show of force. You can argue these things were necessary - and I agree! But it's important to realize much of this occurred before we knew about the atrocities.

If you read enough Sun Tzu, nobody really knows (from a warriors perspective) why they go to war until you're there. The populace almost never knows the truth, even through time, because most of what they're told is a story that changes with time. It's true of most wars.