In the rise and fall of world powers, the currency market keeps score. A nation’s money gains value when it’s a magnet for global investors; in times of trouble it weakens. The volatility can create havoc. To keep things in check, more than half of all countries have fixed the value of their money to another currency — mostly the U.S. dollar or the euro. They’ve hammered in a peg. The tie-ups can provide stability and foster trade, as Hong Kong’s link to the dollar has since 1983. Until they don’t. To hold most pegs in place, the central bank must......
