The same mistake that millions, now dead, made thinking "It can't happen here, we're different."
This isn't based on culture, but on basic human nature. Human nature is the same here as it was in Soviet Russia, China, Cambodia, Nazi Germany, Turkey, etc. And if you think that the populations of NY, NJ, MA, CT, MD, IL, etc. don't have the vast majority who will support the tyrant, you're making another mistake. I reiterate history does not agree with you.
I know that when I was in uniform, either as a E1 or E8 I would not fire on American citizens.
The liberals are master shepherds. Read any social psych textbook and it is all explained.