Civil war, a military coup attempt against the POTUS stupid enough to call for a total ban on firearms and other civil iberities, entire units (most of the military) diserting with their weapons, local police forces and some federal ones along with the those military units telling the special squads to get lost and opening fire on them, ect, ect...
They would need lots of foreign troops to enforce this, and guess where they would bring them in from- Communist and Islamofascist dictatorships!