Should an event trigger a cascading series of social events nationwide, yes, things could fall apart pretty rapidly. I just don't believe it is going to happen. But I do prepare for disruptions to a certain degree.
The US military would not be able to enforce any kind of martial law except perhaps in a few selected large cities. Do the math. Too many armed citizens, too much land mass, not enough military or military equipment.
It comes down to this and that is reality.
How much disruption would it take before the "UN" is calling for a new government here in the USA saying that the current govenment just doesn't work? The USA has done it in Lybia, Syria, Irag, Afghanastan, the old Soviet Union, and I believe in some of the smaller African nations. But I don't believe you will see a fleet of UN vehicles parading around the US ever. People would accept food, but that wouldn't change anything politically.
If the US falls, most of the world is going to crumble politically including China. Nobody can save the savior. That is actually the biggest selling point for martial law in the USA; short term stability in exchange for lost freedoms.