Fifty years ago this Sunday, President Richard Nixon announced a bold economic plan, including the severing of the U.S. dollar’s ties to gold. Since then, the world’s monetary system has consisted of (mostly) freely floating currencies. The dollar nonetheless remains the primary legal tender used internationally for trade, finance, and as a store of value, which has conferred upon the U.S. enormous advantages. Whether that will continue for the next half-century is far from certain. The Bretton Woods system, in effect back then, reflected America’s economic pre-eminence after World War II. Currency exchange rates were fixed, relative to the dollar,...