President Trump on Friday defended his widely criticized plan to impose steep tariffs on imports of steel and aluminum, tweeting that “trade wars are good, and easy to win.”

“When a country (USA) is losing many billions of dollars on trade with virtually every country it does business with, trade wars are good, and easy to win,” he said.

“Example, when we are down $100 billion with a certain country and they get cute, don’t trade anymore-we win big. It’s easy!” he added.

In announcing the protectionist move, Trump said Thursday the US would slap duties of 25 percent on imported steel and 10 percent on aluminum to protect American producers.

“We’ll be signing it next week. And you’ll have protection for a long time in a while,” Trump said during a meeting with 15 steel and aluminum industry executives.

But White House officials later said some details still needed to be worked out — as the Dow Jones industrial average took a nosedive of as much as 586 points.

Fears of a growing trade war sparked selloffs on Wall Street and in Asia and Europe, hitting the share prices of steelmakers and manufacturers supplying American markets particularly hard.

Trump believes the tariffs will protect US jobs, but many economists say the resulting price hikes for users of steel and aluminum — such as the auto and oil industries — will destroy more jobs than curbs on imports create.

A top economist told CNBC on Friday that Trump’s proposed tariffs — which Democrats and Republicans alike have slammed — are “straight-up stupid.”

“This is fundamentally incompetent, corrupt or misguided,” said Adam Posen, president of the Peterson Institution for International Economics.

Posen, who served at the Federal Reserve Bank of New York in the mid-1990s, now sits on the panel of economic advisers to the US Congressional Budget Office.

US carmaker and plane manufacturer stocks closed sharply lower Thursday, while stocks for metals producers in Europe and Asia plummeted at Friday morning’s open.

“Steel is just a tiny input in the US gross domestic product — which is why it’s so crazy. You mess up your entire trading system for an industry that has a total of 80,000 jobs,” Posen said.

Australia’s trade minister said the measures risked triggering retaliation from other economies and could cost jobs, according to Reuters.

Meanwhile, China predicted harm to trade if other countries followed the US example. In Brussels, the European Commission called the step a blatant intervention that amounted to protectionism.