It’s true that there were a bunch of Nazi scientists that were welcomed into the US. I don’t think that that’s really to blame for the US’s fascist tendencies, though. The US has always had fascist tendencies. If anything the US welcoming Nazi scientists is a reflection of the way that the US already was, rather than something that corrupted the US.

I don’t think that Einstein was a Nazi, from the information I have seen. I think that seeing every German as “the opposition” is itself a pretty fascist position to take.

The true opposition is not people of different nationalities, it is people of a different class. That is, the “owners” who exploit the working class by claiming ownership of the fruits of their labor.