The problem is, can they go to war. They'd need to write off a huge portion of the debt before they could afford to launch a full scale offensive. That requires telling one of its major creditors to get lost. Which would potentially cause them to go against you in any way situation.
Also public opinion, would Obama really declare war nearing the end of his term? There is also all the liberal assholes who wouldn't want a war who will try and stop it, and publish material that would go against them, rallying more nations against them.