Both of your links are pretty spooky. I would say we should be worried about sovereign debt defaults in the cases of BOTH America and Japan.

But, if it came to Japan suddenly 'wanting' their $600 billion back, at some inopportune time, I rather strongly suspect that America would be collectively thumbing a very large nose at them.

We seem to think that other countries should be grateful for the opportunity to give us their money. It is simply part of the American hubris, perhaps an inevitable symptom or characteristic of the late imperial stage of decline.

Perhaps a natural follow-up question is: how would China react to sovereign debt defaults by Japan and/or the United States?

And, if you ask me, that's when things get really scary.