World leaders are vowing to respond with tariffs of their own in response to President Donald Trump's announcement that the U.S. would impose a 25% tariff on steel and aluminum imports starting on March 4.

Speaking from a gathering of world leaders in Paris at the Artificial Intelligence Action Summit, Canadian Prime Minister Justin Trudeau called the United States' upcoming tariffs "unacceptable."

"If it comes to that, our response, of course, will be firm and clear. We will stand up for Canadian workers. We will stand up for Canadian industries," Trudeau said, in a video provided by the CBC.

The threat of tariffs comes as President Trump has continued to insist that Canada should join the United States as a 51st state. Trudeau was heard saying late last week that President Trump is serious about the idea as Canadian opinion polling shows overwhelming opposition to joining the United States.

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European Union President Ursula von der Leyen gave a similar response on behalf of the 27-member bloc.

"I deeply regret the U.S. decision to impose tariffs on European steel and aluminum exports," she said. "Tariffs are taxes - bad for business, worse for consumers. Unjustified tariffs on the EU will not go unanswered—they will trigger firm and proportionate countermeasures. 

"The EU will act to safeguard its economic interests. We will protect our workers, businesses and consumers."

According to senior administration officials, the order reinstates a 25% tariff on steel that was in place in 2018 during President Trump's first term and raises tariffs on aluminum to 25%, with only some exceptions.

Trade groups were immediately critical, warning that tariffs and retaliation against them may increase manufacturing costs for Americans.

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"We have been clear about the damage that tariffs on critical manufacturing inputs will inflict on equipment manufacturers and the 2.3 million men and women who build equipment in America," said Association of Equipment Manufacturers Senior Vice President Kip Eideberg. "The mere threat of tariffs has already contributed to higher input prices, disrupted supply chains, and created uncertainty for equipment manufacturers."

Of course, it is possible President Trump could delay or cancel tariffs. Broad 25% tariffs on imports from Mexico and Canada were set to begin last week, but President Trump delayed their implementation by 30 days.