Just a reminder, this happened: 'In 1939 the Nazis Held a Rally at Madison Square Garden'

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10. Nazi ideas were quite popular in the Depression-era USA

There was a fairly large popular following and there were some very influential people, such as Henry Ford, who idolized Hitler and praised the Nazis. Kristallnacht notwithstanding, I don't think it's fair to say these people knew "exactly" what the Nazis were going to do. That was widely regarded as a targeted action against those Jews who had too much wealth and power. I don't think too many people on this side of the Atlantic realized they were seriously considering a massive technological solution to systematically murder every single Jew (and "defectives" of other types) in the world.

We got into the war not because there was popular dislike of the Nazis, but because they were Japan's allies and therefore complicit in the sneak attack on Pearl Harbor. For their part the Germans seemed to realize what a disaster this was for them, as even people who were sympathetic to their ideas were disgusted by the sneak attack and it gave those who didn't like what Germany was up to plenty of cause to pursue the European war aggressively. But Pearl Harbor was a surprise to Hitler too, and they dared not risk complaining about it now that they really needed the alliance to keep us distracted.

It was only after the war ended and the revelation of what had happened at the death camps that it became truly unfashionable to express sympathy with the Nazi cause. If they had not murdered so many bona fide innocents in such a horrible manner, it's likely that the Nazi cause would have been rehabilitated in the US within a decade. As it is, people are diligently trying to scrub our collective memory of that knowledge to this day.