Funny thing is that China could end the US without firing a single shot.
They could make it hard for us untill we get our s*** together, but I don't think they could "end" us.
Thats one of the good things about having a large standing military, and big stockpiles of weapons; if someone we owe money to tries to collect, we can hold the country together untill we sort things out.
If several major exporters did it simultaneously, we'd most likely be screwed. But I don't think they would have the nessecary level of cooperation, especially with China.