"It Can Happen Here"--Sinclair Lewis' 1936 novel that imagines a fascist takeover of the USA- can be instructive here. As is the copious history of the Third Reich.  It was Lewis who's credited with the saying, "When fascism comes to the USA it will come wrapped in the flag and carrying a Bible."

Krugman here is talking about America's equivalent of the "Good Germans" of the 30's and 40's, people who went along with Hitler because they valued patriotism, law and order over their own humanity. 

I'd also recommend Timothy Snyder's great 2016 essay "On Tyranny: 20 Lessons from the 20th Century." This is a short book that distills lessons from Hitler, fascism, Stalinism, etc. and the resistance to them