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THE 19th century was dominated by the British Empire, the 20th century by the United States. We may now be entering the Asian century, dominated by a rising China and its currency. While the dollar’s status as the major reserve currency will not vanish overnight, we can no longer take it for granted. Sooner than we think, the dollar may be challenged by other currencies, most likely the Chinese renminbi. This would have serious costs for America, as our ability to finance our budget and trade deficits cheaply would disappear.

Traditionally, empires that hold the global reserve currency are also net foreign creditors and net lenders. The British Empire declined — and the pound lost its status as the main global reserve currency — when...