A number of friends and colleagues have been sharing this brief essay from the NYRB.  The Nazi horror looms large over any discussion of fascism/authoritarianism for obvious reasons, and the author is right about the role exploitation of terroristic violence can play in authoritarian reversions, as happened in Nazi Germany.  The essay omits mention of two rather important points, however, about the Nazi experience.   First, the German Constitution had a clause specifically authorizing the suspension of all civil liberties, which was invoked after the Reichstag fire.  The American Constitution has no such general clause, though Congress can suspend the right of habeus corpus (in the face of "rebellion" or "invasion" to protect "public safety"), which would be very serious.  But Congress can not suspend freedom of speech, assembly and the press.  (Note, however, that courts can decide that the state has a compelling reason to violate those individual rights, though it is very rare for them to do so.  But this makes it more cumbersome for a would-be authoritarian to violate civil liberties.)  Second, and this is also crucial, one can not forget the complicity of the German President, Hindenburg, in all this.  For he alone had the stature with the military and the public at large to put a stop to Hitler, yet he aided and abetted Hitler's seizure of power at every turn.  There is no one with similar stature in the American system, and there is every indication that the leading military figures, including those serving under Trump, believe in democracy, unlike Trump. 

Two of my constitutional law colleagues, Tom Ginsburg and Aziz Huq, have examined how constitutional democracies slip into authoritarian regimes in comparative perspective (this is a shorter version of a longer academic paper to which they link in their essay).