All three major U.S. stock indexes snapped their four-session winning streaks, with the Dow ending -0.58%, the S&P finishing at -0.04%, and the Nasdaq closing at -0.40%.

Oppenheimer's chief investment strategist John Stoltzfus:

"Today we're testing some of the good performance that we saw in the markets last week when U.S. stocks, major indices, were up anywhere up on the week, anywhere between five to as high as eight percent in the case of the Nasdaq. So, today there's a little bit of give back, some of that directly related to the fact that Jerome Powell, speaking at an event, said that the Fed would be willing to be more aggressive if it was required."

Powell said on Monday that the Fed must move "expeditiously" to combat inflation, in remarks before the National Association of Business Economics conference, adding that bigger-than-usual interest rate hikes could be deployed, if needed, leading many investors to believe that a 50 basis-point hike in key interest rates at the Fed's next meeting was likely.

Crude prices rose as the EU weighed joining the U.S. in banning Russian oil, which heightened supply concerns and helped push energy shares higher.

The rising geopolitical temperature also helped defense stocks.

Shares of Boeing fell -3.59% after one of its aircraft operated by China Eastern Airlines crashed in southern China with no apparent survivors.

And a Moscow court labeled Meta Platforms an "extremist organization" on Monday, upholding a decision to ban Facebook in Russia. Shares of Meta ended -2.31%.