You are forgetting you must have at least some level of the local populace willing to either fill in the ranks when you take loses, recruitment. And or at the very least feed and hide your group and either fear you or be loyal enough not to rat you out to the authorities. There are a few areas of the usa that might be primed somewhat for radical views. It takes alot of guts to not give up fighters when the authorities are bashing down your door and ordering your family outside by gunpoint and yelling at you to tell them where are the guerilla fighters.
Militias that go off half cocked and start attacking infrastructure for the sake of it will be more of a threat then if they did nothing. Militias and their members have a sort of rambo mentality that has creeped into the american psyche. Again I wont go over every single thing posted but this is a key, you must have a area where the local people will join your ranks or at the very least feed and hide you when the time comes. No one can be a one man army. It will take multiple strategies and multiple ways to take and hold the land against the roving gangs of people. The military on the other hand is a entirely different matter, if 13 southern states couldn't fight off the oppressors then you really have to look at how radical your people would be willing to go to fight and how long the struggle may actually take.