That last part is my question. What prevents a group from taking up arms and attacking a city in it's own interest? IN the case of the U.S., certainly not the U.S. Military (no such thing exists.
Wtf. The US military does indeed exist, even to quash internal rebellion. The national guard is the branch that would deal with such a threat. The number of armed citizens alone might suprise you as well. So all in all, I am not sure what you mean. Anyone could 'attack a city in their own interest', but there are quite a few organized parties very invested in resisting that with their lives.