It would be interesting to know where the divergence occurred. I don't think it's that Hitler won, because John's concerned at one point at seeming "dangerously unAmerican". (Though he may have won in Europe, since Radek is in Atlantis because he gypsy.) Instead, perhaps, it is that McCarthyism was never exposed for the sham it was, and instead thrived until America became a true fascist state, with re-education camps and political officers.