Even after a fast start to 2018, stock markets finished the first quarter down for the year — the first quarterly decline since 2015. It suggested that a period of calm and steadily rising markets had given way to a turbulent new era with a bearish bent.

The plunge continued Monday, with the Standard & Poor’s 500-stock index sinking 2.2 percent. Investors jettisoned shares of financial, technology and many other businesses, spooked at least in part by a tweet from Mr. Trump aimed at one of the country’s biggest companies: Amazon.

Asian and European markets were down more modestly in midday Tuesday trading.

Monday’s decline left stocks down more than 4 percent so far in 2018. They are now down more than 10 percent from their peak in late January.