The way I approach this in my new novel is to split the military. During a terrible depression, the president's utopian brownshirts organize a constitutional convention, and the new constitution enshrines virtual communism. But half of the nation rejects the new constitution as false and invalid. This causes the loyalty of the military to be divided between the original and the new consitution. That's when the country comes completely unglued. Some regions will be loyal to the president and the new constitution, other regions will reject his authority. It's only fiction, but it's a way we could see the military fracture, and then all bets are off. There's a long excerpt here that details the bogus constitutional convention, and how this could come to pass.
