Just one week after Canada was temporarily spared from across-the-board tariffs, U.S. President Donald Trump said he will put 25 per cent tariffs on all of the country's steel and aluminum imports, a move that will particularly sting Canada.

Canada is the biggest supplier of steel and aluminum to the United States, providing close to one-quarter of its steel imports — $15.9 billion Cdn worth in 2024 — and almost 60 per cent of its aluminum.

Roughly 90 per cent of Canada's steel and aluminum exports go to American companies, according to data from Statista.

The tariffs were already hurting Canadian companies before they became official late Monday, and they're expected to hit a wide range of industries, including energy, construction and automakers.

Rahim Moloo, owner of the manufacturer and distributor Conquest Steel, told CBC News that U.S. distributors are already cancelling orders and that his Toronto company expects a 25 per cent drop in sales.

The auto industry could see "serious layoffs" in a couple of weeks, said Peter Warrian, an economist with the Munk School of Global Affairs and Public Policy at the University of Toronto.

"This is real trouble," he said.

Auto manufacturing is among the industries that are so integrated due to free trade that some parts cross the border multiple times.

Warrian said it wouldn't make sense to put tariffs on a sheet of steel, for example, that comes from Hamilton and crosses the border five times before it lands in a car.

"We should be able to get to some sort of modification or exemption, like we did last time," he said, referring to similar tariffs Trump imposed in 2018.

While details are still unclear, however, Trump said the tariffs will apply "without exceptions or exemptions."

Industry and opposition leaders are pressuring the federal government to respond forcefully.

Industry Minister François-Philippe Champagne said on X, formerly Twitter, that the tariffs are "totally unjustified" and that he was consulting with international partners.

"We will continue to stand up for Canada, support our workers, and defend our industries," he wrote.

Former finance minister Chrystia Freeland, who is running for the Liberal Party leadership, used stronger wording in a video message to Trump: "If you force our hand, we will inflict the biggest trade blow that the United States has ever endured."

Conservative Leader Pierre Poilievre said Monday that, if elected, he would retaliate with matching tariffs, while NDP Leader Jagmeet Singh said he would put 100 per cent tariffs on all products made by electric vehicle manufacturer Tesla, whose CEO, Elon Musk, is one of Trump's top advisers.

In 2018, when Trump imposed a 25 per cent tariff on steel and a 10 per cent tariff on aluminum from Canada, Mexico and other countries, steel exports dropped nearly 40 per cent, and aluminum fell by more than half.

Canada and Mexico struck back with their own tariffs, all of which were lifted after the Canada-United States-Mexico Agreement (CUSMA) was negotiated and signed later that year, replacing the North American Free Trade Agreement.

The University of Toronto's Warrian said Trump is picking points of vulnerability with Canada — with steel, aluminum and oil being our major exports — to "cause pain and get attention."

He said because the move does not make a lot of economic sense for the U.S. on its face, he suspects it is part of a broader plan that involves, among other things, bringing in revenue to offset Trump's planned tax cuts.

Since his November election, Trump has been threatening to impose broad tariffs on Canada and Mexico due to allegedly lax border security.

Trump said on Monday that the tariffs are meant to encourage production in the U.S., and he repeated his suggestion that Canada become the 51st state to avoid further economic punishment.

"All you have to do is make it in the United States. We don't need it from another country," he said.

If the U.S. goes ahead with all of the tariffs it's threatening next month, the Trump administration says those tariffs will indeed compound on top of each other, potentially raising the steel and aluminum tariff to 50 per cent.

The U.S. is threatening both a worldwide tariff of 25 per cent on steel and aluminum starting March 12, and an economy-wide 25 per cent tariff on Canada and Mexico on March 4 that is currently on pause.

A White House official told CBC News on Tuesday: "If the prior tariffs that were paused are reinstated, they would stack on each other, so 25 per cent [plus] 25 per cent."

Readers have asked CBC News whether Canada has accepted Chinese steel or aluminum and resold it to the U.S., allowing China to circumvent American tariffs.

Canada has its own 25 per cent tariff on Chinese steel and aluminum that has been in place since October, so there would be no opportunity to circumvent U.S. tariffs by going through Canada.

In levying the tariffs, the Canadian government criticized Chinese oversupply and overcapacity policy, and accused it of "unfair and harmful marketing distortions." Canada has also used its stand against China to show Trump that Canada is aligned with the U.S. against Beijing.

Julian Karaguesian, an economics professor at McGill University in Montreal, said he's not aware of China ever using Canada to circumvent tariffs on steel or aluminum products. In the wake of Trump's 2018 tariffs on Chinese imports, he said, Chinese exporters shifted production to other countries — such as Vietnam, Thailand, the Philippines and Mexico — or extended their supply chain to those countries, adding value there before shipping to the U.S.