The surer prospect that the Federal Reserve will raise interest rates next year has given the dollar a new boost. Its strength against rich-country currencies is fitting: America’s economy is stronger than most. But the dollar’s rapid rise against emerging-market currencies is worrisome. Previous episodes of dollar strength—in the early 1980s and mid-1990s—harmed emerging markets. Bond yields rise and currencies fall as capital is drawn back to America. Governments and companies that borrowed in dollars find debts hard to repay. Inflation rises even as economies stall. So far the currencies of oil exporters, notably Russia, but also Colombia, Mexico and Nigeria, have been the biggest losers. Brazil has been hurt, too. Its offshore oil fields, a source of much recent investment, are now rather less enticing. Emerging markets that are oil exporters, exposed to China’s slowing economy or have large current-account deficits will be most vulnerable in 2015. Brazil is all three.