Even the weather seemed to conspire against Europe. An unusually dry and hot summer caused the water level in the Rhine River to drop so low that barge traffic became impossible, disrupting transport of chemicals and fuel, causing shortages and driving up prices.

The timing of the overseas slowdown could not be worse from the United States’ perspective. It’s happening just as the American economy is expected to slow down after the economic high of the tax cuts that went into effect last year starts to wear off.

In that environment, the United States and its companies will be more dependent on the health of trading partners like Europe. “The U.S. needs the rest of the world a bit more than last year,” said Brad W. Setser, a senior fellow at the Council on Foreign Relations.

Economic weakness abroad can also send American stock markets into a tailspin.

In 2011, when investors feared that a wave of European governments and companies might default on their debts, American stocks fell nearly 20 percent from their highs. In 2015, the S&P 500 plunged after China’s central bank made a surprise move that reduced the value of the country’s currency against the dollar.

The sudden announcement by the European Central Bank on Thursday was out of character. Typically, the bank changes policy gradually and gives investors plenty of warning.

The bank also displayed an unusually high level of determination and unity. All of the members of the Governing Council supported the decisions on Thursday, Mr. Draghi said. That has not always been the case. During the crisis years the council, which includes central bankers from all 19 countries in the eurozone, was often divided on its response.

Still, the central bank is treading cautiously, waiting to see what happens with the economy. It did not take the more radical step of increasing its purchases of government and corporate bonds. That form of stimulus is associated with extreme stress and might have been taken as a sign of panic.