The European Central Bank’s launch of an aggressive program this week to buy more than €1 trillion in bonds poses important tests for the U.S. economy and the Federal Reserve. Europe’s new program of money printing—and the resulting fall in the euro—means the U.S. economy must deal with a rapidly strengthening dollar that will make American goods more expensive abroad. The stronger dollar could slow both U.S. growth and inflation, giving the Fed some incentive to hold off on its plan to raise short-term interest rates later this year from near zero. U.S. officials have been playing down that scenario, and, more broadly, resisting talk of a global currency war—competitive devaluations by countries eager to keep their currencies as low as possible to protect exports. The U.S. dollar has already soared in the wake of the ECB announcement Thursday […]