President Donald Trump promised to hike tariffs on key imports throughout his presidential campaign — but is just now starting to follow through.

The timing of the announcement of new tariffs on steel and aluminum has taken already-battered markets, which had found some comfort in the tax cuts, by surprise.

"A negative vibe lingered across financial markets after Trump’s vow to impose severe tariffs ... sparked fears of a global trade war," Lukman Otunuga, market analyst at FXTM, said.

There is only one thing about President Donald Trump’s announcement of new tariffs on steel and aluminum imports that should come as a surprise: the timing.

Donald Trump ran for president on an openly anti-trade agenda, and one of his first actions in office was to pull the United States out of the long-negotiated Trans-Pacific Partnership trade deal with 11 other countries.

Since then, Trump’s policies, if not his rhetoric, have been largely within the bounds of global trade norms. For one thing, Trump quickly reneged on a campaign promise to label China a "currency manipulator" and, until this week, had refrained from sharply raising trade barriers.

Yet just as the president’s tax cuts appeared to be getting at least temporary traction and growth projections were moving higher, Trump announced the new measures. While short on details, they include taxes on imports of 25% for steel and 10% for aluminum. Trump said the tariffs would be rolled out next week to help bolster the US metal industries.

The US stock market, already experiencing new volatility amid concerns of a potentially more aggressive Federal Reserve, took a tumble with the Dow Jones industrial average plunging over 500 points during the session.

All about the payback

The market’s losses extended around the world as fears of a trade war spiked, particularly after Trump tweeted that "trade wars are good," doubling down on his earlier threats.

"A negative vibe lingered across financial markets on Friday, after Donald Trump’s vow to impose severe tariffs on imports of steel and aluminium sparked fears of a global trade war," Lukman Otunuga, market analyst at FXTM, wrote in a research note.

"This bombshell development is likely to fuel concerns of retaliatory actions from major US trade partners consequently weighing on risk appetite. Investors are clearly jittery by the threat of a potential global trade war and its possible effect on stock markets."

The president’s announcement comes despite warnings from members of his own party that the tariffs and the likely retaliation would amount to "a massive tax increase on American families."

Chad Bown, former White House economist and currently a trade expert at the Peterson Institute for International Economics (where I used to work,) had a biting response to the president’s trade-war-mongering tweets.

"It’s by opening up these doors of no longer following the rules and blatantly throwing the rules out the window that is ultimately going to give all other countries the opportunity to do the same thing," Bown said at a recent PIIE event.

"And we have to remember that 95% of the world’s consumers are outside of the United States, many American companies and workers get their livelihoods from selling their goods and services outside the United States and those are the ones that will be caught up in that kind of action and behavior and they’re going to be hurt."

The tariffs could hurt NAFTA negotiations and tank the economy

"We have been expecting a solid US GDP growth rate for 2018, but we have also been expecting more downside risks next year," Jason Schenker, economist and president of Prestige Economics, wrote in a research note. "This move presents potential downside risks to the US growth outlook in 2019."

Moreover, swift condemnation from the trading partners likely to be hurt most by the measures, Mexico and Canada, suggests already-tough negotiations to rewrite the North American Free Trade Agreement may be in peril.

"The potential to stay in NAFTA is not a done deal," Schenker added. "The negative economic ramifications of tariffs on steel and aluminum could prove to be a minor preview of the economic catastrophe that an abandonment of NAFTA would likely entail. One thing should be clear: free trade is at risk."

Ed Hyman, Evercore ISI vice chairman, told Bloomberg TV: "I didn’t expect this. And it’s very scary. The problem is not what we do, it’s the retaliation. It’s pretty bad once you go down this road."