For more than half a century, the U.S. dollar has been not just America's currency but the world's. It is used globally by importers, exporters, investors, governments and central banks alike. Nearly three-quarters of all 100 bills circulate outside the United States. The dollar holdings of the Chinese government alone come to more than 1,000 per Chinese resident. This dependence on dollars, by banks, corporations and governments around the world, is a source of strength for the United States. It is, as a critic of U.S. policies once put it, America's ""exorbitant privilege."" However, recen