Jane Galt cites James Hamilton in Good as gold:
Under a pure gold standard, the government would stand ready to trade dollars for gold at a fixed rate. Under such a monetary rule, it seems the dollar is ‘as good as gold.’Except that it really isn’t — the dollar is only as good as the government’s credibility to stick with the standard. If a government can go on a gold standard, it can go off, and historically countries have done exactly that all the time. The fact that speculators know this means that any currency adhering to a gold standard (or, in more modern times, a fixed exchange rate) may be subject to a speculative attack.