Nihilist German professors fleeing Hitler were allowed to embed in American universities bringing with them the intellectual virus that killed Europe.
This is a very important point. They corrupted an entire university generation (probably my teachers in NY were among them, although many of my teachers came from families that were very left-wing to begin with, having been part of the union movements in the 1930s).
But the European academics who came here had a profound effect on American universities. Deweyization had struck in the ED schools and they were a bit on the weak side anyway, and the European influence in the higher disciplines pushed them over the edge.
The confluence of European nihilism and American progressivism reached critical mass in my generation (growing up in the 50s-60s). If you lived outside of any of the so-called intellectual centers, it took a few years to extend itself. But with the communications of the time, it was literally only a few.