That is exactly what is happening right now in Portland, Oregon. Police units without any identifying insignia or names have been plucking non-violent citizens off the street without probable cause -- people who have not broken a law, and are just exercising their constitutional right to address their government in a peaceful demonstration.
The Trump administration says these federal police officers are just in Portland to protect federal building. But they have been doing much more than that. They have gassed peaceful demonstrators and fired "nonlethal" weapons at them -- and they have left those federal building to cruise the streets in unmarked vehicles to arrest peaceful demonstrators. Their actions have not quelled the demonstrations, or the little violence committed by a few. It has done just the opposite -- by creating larger crowds and exacerbating crowd violence by directing violence toward them.
The authorities in Portland did not ask for these secret police to come, and do not want them there. This federal secret police force is Donald Trump's idea. He doesn't think the demonstrations should be allowed, and thinks local officials should have dealt with them much more harshly -- so he has sent in his own goons. And I think at least part of his purpose is to create more problems, so he can campaign on a law and order agenda (ignoring the fact that much of the disorder has been created by his own actions).
Now, it is learned that he wants to take his secret police to other cities (mostly governed by Democratic mayors). It has been learned that 150 of these secret police are being readied to go to Chicago -- even though Chicago does not need them or want them there. Other cities being considered are New York, Philadelphia, Detroit, Baltimore, and Oakland.
This is a. terrifying prospect. Secret police sent by the federal government to impose its will over citizens is a clear violation of the rule of law. And it is a giant step toward a fascists-style dictatorship. This gives us a clear picture of what America would look like if Trump is re-elected.