Robert Paxton's five stages of fascism, a classic read: http://pryan2.kingsfaculty.ca/pryan/assets/File/Paxton's%205...
Recommended: Ian Kershaw's books on Hitler.
https://www.amazon.com/Hitler-Ian-Kershaw-ebook/dp/B00BJIMCA...
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...)
The short answer is, "the same way that Trump did."
To learn more, I recommend reading the excellent post link.
This is covered by the first episode of the World at War. https://www.youtube.com/watch?v=0b4g4ZZNC1E