The citizenry wouldn't have to fight the U.S. military. Some of the strongest supporters of the 2cd Amendment and the Constitution in general are in the military. I firmly believe the military would turn on the government before they'd turn wholesale against the American people.
"People say that marijuana smoking is going to get in the way of my career. I say to them that on the contrary, my fighting career is getting in the way of my marijuana smoking." -Nick Diaz