While it is true that natural capital accounting implies US is really not indebted and if the rest of the world economy remains slow or in recession, dollar being the global reserve currency, US is not in any trouble, real difficulties arise with the rise of China, the only country on a stable and strong growth path at the moment. United States and Europe, under severe social duress because of high structural unemployment, are tentatively recovering with extraordinary government support both on the fiscal and monetary fronts.
Net of social security and Fed holdings, though it is true that US debt is about 68% of GDP, financial crises occur because of triggers. China is a trigger for Wall Street to sell US debt.
US debt is a threat to the United States as early as 2016 should China decide to pull the plug when on a purchasing power parity basis it achieves the status of the world’s largest economy. Wall Street will follow suit. Dollar (and euro) will be dumped for renminbi, the new freely floating reserve currency by 2016.