In September 2010, Brazil’s former finance minister, Guido Mantega, made headlines when he accused the Federal Reserve, the European Central Bank and the Bank of England of engaging in a currency war.  The complaint was that easy monetary policy was driving down the value of the dollar, the euro and the pound, at the expense of his country and those like it. More recently, similar charges have been levied against Japan: namely, that the Bank of Japan’s extraordinary balance sheet expansion is aimed at driving down the value of the yen, damaging the country’s trading partners and competitors...