A mass of civilians aren't going to be able to stand up to a column of tanks easily, but just a hundred civilians armed with AR-15s are going to be able to stand up even to massed police forces. And if an insurgency is supported by the public at large then life is not going to be easy for the police and the military. This is the way that all guerrilla wars go, and there are many examples of successful insurgencies when they have widespread popular support, even against very well armed government forces.
I'm just not buying it. Even with their massively over-inflated budget and their guilt-elimination drones there is no way the DoD could maintain an armed occupation of America. They are having a hell of a time doing it in a country less than one-tenth the size with less than one-tenth the people, filled with people who have been thoroughly dehumanized by popular media, people with far fewer connections to the outside world, and people for whom the soldiers have no tribal connections.
Thinking they would do better occupying America than Afghanistan is a really strange form of patriotic hubris.
(Lest you get the wrong impression, I support gun control and think that worrying about the possibility of Americans having to fight the American army is incredibly silly.)