Published August 1941:

It is an interesting and somewhat macabre parlor game to play at a large gathering of one’s acquaintances: to speculate who in a showdown would go Nazi. By now, I think I know. I have gone through the experience many times—in Germany, in Austria, and in France. I have come to know the types: the born Nazis, the Nazis whom democracy itself has created, the certain-to-be fellow-travelers. And I also know those who never, under any conceivable circumstances, would become Nazis.

I’ve read and reread this a few times over the years.

https://archive.org/details/HarpersMagazine1941080020122/mode/1up archive link