It was theoretically about Pearl Harbor, but beyond that, Hitler, Mussolini and Tojo had a vision to conquer the world. There was no length they wouldn't go to. That's when you can't just sit back and do nothing. When its a local dispute over whatever, keep out, but when a nation is trying to conquer the whole world, you have to stop them.
Stalin, for all the people he killed, wasn't trying to conquer the world at the time, and the Nazis were the immediate threat.
I would never have "Allied" with them, but I would have said "OK, we may not like you, but we've got to stop this Hitler guy." Which is basically what happened.
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I support involvement in WWII both because Japan attacked us, and more directly, Germany wanted to conquer the entire world. Had they stopped when they had restored their own country after Europe kicked it to the can in WWI, I wouldn't have gotten involved, heck, I wouldn't have gotten involved in WWI to begin with.
But WWII was special because Hitler wanted to conquer the world, and there wasn't anything that would stop him except defeat. Had he taken England and Russia, he would have come after us next. And so we had to get involved before we lost any assistance we would have had.