Investors have stopped not caring about a trade war.

For months, financial markets shrugged off the escalating language and the imposition of tit-for-tat tariffs between the Trump administration and the United States’ most important trading partners, including Canada, Mexico, the European Union and China.

But in the past several trading sessions, that apathy has been replaced by what looks like mounting alarm about the prospect of a global race to erect trade barriers.

On Monday, the Standard & Poor’s 500-stock index fell sharply, ending the day down 1.4 percent, apparently spooked by signs that the growing trade war could have a negative impact on American companies. That decline, and similar ones in other benchmark indexes like the Dow Jones industrial average, came after the S.& P. slumped 0.9 percent last week.

“I think we’re slowly slipping into a more serious stage in the trade war,” said Ethan Harris, head of global economics research at Bank of America Merrill Lynch. “People are beginning to realize there’s no sign of de-escalation in sight.”