New Delhi, March 19 -- There are two surefire ways for a country to terrify bondholders. One is running up vast debts with no hint of a plan to get borrowing under control. The other is to capture the central bank, so interest rates follow political expediency, not sound economics. Both at once tend to be the preserve of mismanaged emerging economies. And now, if you squint only a bit, also America.

Donald Trump has been on the warpath against the Federal Reserve, demanding cuts to interest rates. On January 11th Jerome Powell, the Fed chair, said he was facing a federal criminal probe relating to the high cost of sprucing up the central bank's headquarters. The president has made zeal for sharp interest-rate cuts a prerequisite for anyo...