NEW YORK – The United States Federal Reserve’s recent decision to launch a third round of “quantitative easing” has revived accusations by Brazil’s finance minister, Guido Mantega, that the US has unleashed a “currency war.” In emerging-market countries that are already struggling with the impact of rapid currency appreciation on their competitiveness, expansionary measures announced in recent weeks by the European Central Bank and the Bank of Japan have heightened the sense of alarm at the Fed’s decision.