The point is the us has a very bad history with long occupations, they can put bombs on foreheads and "curb stomp" a standing force but when it comes to the hard part they have little success. And a home occupation is a lot worse then one on the other side of the world. The armed forces in the us are already dealing with some large morale issues currently and I can't think ordering them to repress a state/city/campus will help that situation much. The us is not immune to all the nasty things that can happen to nations and is not special, only think they are.