WASHINGTON — The Federal Reserve cut interest rates on Wednesday for the first time in more than a decade, as it tried to keep America’s record-long economic expansion going by insulating the economy from mounting global risks.

The widely expected quarter-point decrease was the Fed’s first since it slashed rates to near zero in 2008. But unlike those cuts, which were intended to rescue a failing economy, Wednesday’s move was seen as a precautionary effort to protect the United States from slowing growth in China and Europe and uncertainty over President Trump’s trade war.

“The outlook for the U.S. economy remains favorable, and this action is designed to support that outlook,” the Fed chair, Jerome H. Powell, said at a news conference after the decision. The cut, Mr. Powell said, was “intended to ensure against downside risks from weak global growth and trade tensions.”

It was a turning point, but one that disappointed both markets and Mr. Trump, who were hoping for a bigger rate cut. The Fed, which dropped its target rate to a range of 2 percent to 2.25 percent, stopped short of signaling the beginning of an aggressive rate-cutting campaign. By the end of the day, the S&P 500 was down 1.1 percent, the benchmark index’s worst decline since May 31.