We talk about the "need to protect against an unjust government" (going back to the original reasoning why 2A was enshrined)...
You basically have two scenarios there, as we've seen play out around the world:
1. The military / soldiers revolt, 'remember the patriotism', whatever you want to call it. The government is toothless because their military isn't going to quell civil uprising.
2. A general or colonel decides it's his time, and has the backing of the military. A military with access to fighter jets, armored helicopters, bombers, not to mention the actual big guns, NBC weaponry. And then we have the 'militia' populace, limited to largely semi automatic small arms or less. Is the military going to "play fair", or are they, per current US doctrine, "shock and awe" a demonstrative show of force.
Either way, largely, we'd be fucked. The days of rebel insurgencies hanging out in the jungle/desert/ wherever providing credible resistance to a tyrannical government never really applied to the US.