Major stock indexes plunged Thursday after President Trump’s afternoon announcement that he was imposing a 25 percent tariff on imported steel and 10 percent on aluminum.

Investor concern about the news rattled the Dow Jones industrial average, which closed down more than 400 points, or 1.7 percent, at 24,608. The Standard & Poor’s 500-stock index and tech-heavy Nasdaq both finished down about 1.3 percent.

The Dow is down 1,100 points over the last three days, ending with its 420-point drop Thursday. That is a three-day decline of 4.3 percent. The S&P 500 dropped 3.67 percent over the same three days, or about 102 points. The Nasdaq Composite is down 3.25 percent since Tuesday’s open.

Thursday began with positive news. Initial U.S. jobless claims fell by 10,000 to 210,000 in the seven days ended Feb. 24, far better than expectations. The rate of layoffs as measured by jobless claims is the lowest level since 1969. The Institute for Supply Management reported Thursday that its key index, the closely watched PMI, rose to 60.8 percent, the highest reading since May 2004. A reading above 50 percent indicates economic expansion.

But the tariff announcement stamped out the feel-good start to the day, at one point pushing the Dow down 586 points.

Even with the pullback from those depths, the 30-stock, Dow blue-chip index is now in negative territory for 2018. The S&P 500 is holding barely positive for the year. The Nasdaq is still up a healthy 4 percent so far in 2018.

“Traders are, again, focused on talk of tariffs while investors are focused on earnings growth, continuing low inflation and highly accommodative interest rates,” said Daniel Wiener, CEO of Adviser Investments, a Newton, Massachusetts-based firm that manages more than $5 billion in assets. “Volatility is back, but that shouldn’t sway investors from their longer-range objectives.”

The Dow was already reeling from a 380-point decline Wednesday on fears of continued interest rate hikes by the Federal Reserve. New Fed chairman Jerome Powell spoke this week before Congress, where he left open the possibility of four rate hikes this year instead of three.

When February ended Wednesday with another crazy see-saw day, the Dow and S&P were down. The negative month broke 10 consecutive months of gains, which is the most since 1959.

If Trump carries through on his tariffs, some investors expect more down months – or at least continued volatility.

“​If the tariffs are instituted, you won’t have to worry about (the Fed) and extra rate hikes this year,” said Jamie Cox of Harris Financial Group in Richmond, Virginia. “The damage to consumption and manufacturing will slow the economy so dramatically that we may be talking about rate cuts” instead of hikes.

The hardest-hit sectors Thursday were energy, utilities, real estate and technology, which were all down 10 percent, considered to be a correction. Boeing, United Technologies and Intel were among the big drags on the Dow. Trump’s announcement is part of a protectionist approach the president has promised as a way to secure American jobs and industry. But many business people, including manufacturers, don’t welcome protectionism because it can raise their materials costs. Wall Street generally doesn’t like tariffs, either.

“Protectionism doesn’t work,” Washington investment manager Michael Farr said. ” A fair playing field is essential, but anything that dampens trade is bad for the U.S. economy and markets. These broad tariffs with not much insight into current imbalances seem ham-handed and clearly were unwelcome by markets.”

Some others thought the move was long overdue and Wall Street’s reaction was overblown.

Dan Dimicco, former Nucor chief executive, said in an interview with CNBC on Thursday that he viewed the pullback as “a major overreaction.”

“We’ve been in a trade war with China for over 20 years,” Dimicco said. “Now is the first time we are standing up and saying enough is enough and we’re going to fight back. They’ve been cheating.”