China, even if they could, would not call in all our debt. One, we couldn't pay it, which means a default, and that means they probably won't get a dime. Two, it would crash the world economy since we are the #1 customer of a lot of nations. Three, we are the world's top food exporter by twice the amount of the second best country (Netherlands). China produces more food overall, but they consume the bulk of it. So countries other than China would likely step in to prop us up, so as to not have a worldwide famine.