Financial markets imploded again on Monday, as increasingly alarmed investors feared that the global economy could experience a downturn rivaling the cataclysmic recession after the financial crisis a decade ago.

Even President Trump’s boundless enthusiasm for the American economy has wilted in the face of what promises to be a widespread economic slowdown, as he acknowledged on Monday that the country might be heading toward a recession.

The air of inevitable decline was palpable. A day after the Federal Reserve unleashed a major effort to shore up market confidence, the S&P 500 plunged nearly 12 percent on Monday — the latest in a series of big drops since the pandemic knocked it from a record high and the worst one-day decline since 1987.

There are few historical parallels for the shock waves created by the coronavirus pandemic. From still-closed factories in China — where the outbreak first emerged — to Western nations where millions of people are living in a state of semi-house arrest, most of the engines that keep the global economy aloft have simultaneously sputtered to a halt.