If the government doesn't pay back/at least keep current on that debt, it's likely to fall, as in, the whole system of government, as happened the last time it screwed up so badly ("Not worth a Continental." although there were of course other factors). Secured by hard assets or not, a very great deal depends on people continuing to have a degree of faith in it as a whole, and it's hard to see how repudiating the debt would be survivable.

And it would still be just as sovereign as it is now, or are you claiming it's not sovereign over the Eastern part of the country???

Or course, there are many other gambits it could and likely will try, like the already mooted one of seizing people's tax system entangled retirement accounts.