Well, that's the comfortable myth, sure, but there were evils on both sides. We allied with Stalin, remember, not to mention imprisoning our own Japanese-American citizens in concentration camps. Not to mention that both sides bombed each other's civilian populations into oblivion with equal brutality. The American wartime propaganda against the Japanese was as horrifically racist as any Nazi propaganda. Yes, Hitler had to be fought, but neither side's hands were clean.
I think it's easy for us to see WWII as "good" in retrospect than it was for the people who lived through the horror of it all. The attitude I've heard in period fiction produced shortly after the war's end was not "Wow, that all-out war against evil really worked, let's use the same tactics next time!" It was "This war was necessary, but it was awful and it took a horrific physical and moral toll on all of us, and if we ever let this happen again, it will doom the world. So we must make sure there is never another war like this, period." And they kinda succeeded. They didn't banish war altogether, as they hoped to do, but every war since then has been on a much smaller, less globally cataclysmic scale.
And they did so after WWII as well, as I've been saying. Sure, WWI was the start of it, but it was the dress rehearsal for WWII. They're both really just one long, continuous process with a lull in the middle.
