I recommend "The Ominous Parallels" [1] for a detailed examination of the philosophical and historical causes of Nazi Germany, and how these causes have echoes resonating in the US today.

[1] https://www.amazon.com/Ominous-Parallels-End-Freedom-America...

(the now-flagged book review was https://www.nytimes.com/2018/06/14/books/review/benjamin-car...)

Apathy? A lot of smart people were active supporters of the National Socialists, at least before they started losing the war.

The Ominous Parallels[1] explores this, and the parallels with current American politics:

The Nazis did not gain power against the country’s wishes…The Nazi party was elected to office by the freely cast ballots of millions of German voters, including men on every social, economic, and educational level. In the national election of July 1932, the Nazis obtained 37 percent of the vote and a plurality of seats in the Reichstag. On January 30, 1933, in full accordance with the country’s legal and constitutional principles, Hitler was appointed Chancellor. Five weeks later, in the last (and semi-free) election of the pre-totalitarian period, the Nazis obtained 17 million votes, 44 percent of the total…

In 1933, when Hitler did establish the system he had promised, he did not find it necessary to forbid foreign travel. Until World War II, those Germans who wished to flee the country could do so. The overwhelming majority did not. They were satisfied to remain…

[1] https://www.amazon.com/Ominous-Parallels-End-Freedom-America...