NEW YORK — Stocks shook off morning losses on Monday and surged in the afternoon to send the Standard & Poor’s 500 index to its best day in a week. It’s the latest turn for a market suddenly prone to quick shifts not only day to day but also hour to hour, as investors question whether President Trump will really risk a trade war.

The S&P 500 lost as much as 0.6 percent shortly after trading began, only to finish the day 1.1 percent higher after rising 29.69 points to 2,720.94. It’s reminiscent of what happened Friday, when stocks reversed course on speculation that Trump was only making an opening bid when he promised to impose stiff tariffs on imported steel and aluminum, rather than a final offer.

In this March 1, 2018 photo, a customer leaves Dylamatos Market owned by Dianne Shenk in Hazelwood, Pa. About a quarter of Shenk's customers pay with benefits from the federal Supplemental Nutrition Assistance Program, so the government's proposal to replace the debit card-type program with a pre-assembled box of shelf-stable goods delivered to recipients worries her and other grocery operators in poor areas. "These boxes will be full of shelf-stable items, the same things we're being told not to eat," she said. (AP Photo/Gene J. Puskar)

In this Feb. 26, 2018 photo, Dianne Shenk talks to a customer in her Dylamatos Market in Hazelwood, Pa. About a quarter of Shenk's customers pay with benefits from the federal Supplemental Nutrition Assistance Program, so the government's proposal to replace the debit card-type program with a pre-assembled box of shelf-stable goods delivered to recipients concerns her and other grocery operators in poor areas. "These boxes will be full of shelf-stable items, the same things we're being told not to eat," she said. (AP Photo/Gene J. Puskar)

The Dow Jones industrial average jumped 336.70, or 1.4 percent, to 24,874.76, and the Nasdaq composite gained 72.84, or 1 percent, to 7,330.70. Both came back from early-morning losses.

Trump took to Twitter again on Monday to defend the tariffs, which have riled trading partners around the world and already sparked talk of retaliation to heighten the worries about a possible trade war. He highlighted trade deficits with Canada and Mexico, and he said tariffs “will only come off if” a new free-trade agreement between the three countries is signed.

Later in the day, House Speaker Paul Ryan said that he is “extremely worried” about the consequences of a global trade war and urged the White House “to not advance with this plan,” according to a statement issued by his office. “It’s incredibly difficult to try to understand the whims of this current administration and to try to make forecasts,” said Emily Roland, head of capital markets research for John Hancock Investments. If a trade war does occur, it would hurt the global economy and the healthy profit growth that companies have been producing, two of the big drivers for the market.

“But right now, we think the impact should continue to be modest, as long as it’s all talk and no action,” she said.

Boeing offered a good example of how quickly the market shifted. The aerospace giant got the majority of its revenue from outside the United States last year, so it would be hurt if countries put up more barriers to global trade. Boeing was down as much as 2.3 percent in the morning before ending the day up 2.3 percent.

From its low point of the day to its high, the S&P 500 index carried investors through a swing of 1.9 percentage points. It’s the fifth straight day with a gap of more than 1.5 percentage points, as trading has become much more wild since the market’s placid, record-setting run from 2017 into January. During that period, the typical day saw the S&P 500 drift just 0.5 percentage points from its low point to high.

The biggest gain in the S&P 500 came from XL Group, which surged after AXA said that it will acquire the insurance and reinsurance company for $15.3 billion. Investors will get $57.60 per XL Group share, and XL Group stock surged $12.62, or 29.1 percent, to $55.92.

In overseas stock markets, Europe was mostly higher, with Italy an exception after elections there saw no single party emerge with a majority in Parliament. That raises uncertainty about how closely Italy will work with the rest of the European Union.

France’s CAC 40 rose 0.6 percent, Germany’s DAX gained 1.5 percent and the FTSE 100 was up 0.7 percent in London. In Asia, Japan’s Nikkei 225 fell 0.7 percent, South Korea’s Kospi dropped 1.1 percent and the Hang Seng in Hong Kong lost 2.3 percent.

Besides tariffs, investors are also keying in on the upcoming U.S. jobs report that’s looming at the end of the week.

What first jolted the stock market from its peaceful rise to records was last month’s jobs report, which raised the specter of higher inflation. If wages continue to accelerate, investors would likely see it as more evidence that the Federal Reserve will raise interest rates higher and faster than expected.

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