What will happen if the United States defaults on its debt obligations with China and Japan?

What happens if China and Japan see through the Moody's AAA Debt rating on US Treasuries and force the US to pay back on those Notes? Can they call the notes prior to their expiration? Would this cause a US currency collapse and would the holders of the Notes collect the monies based on our Dollar and not theirs (no adjustment for exchange rates) thus causing them to lose money on their purchase.