Donald Trump has announced that he is ending "all trade negotiations" with Canada.
The move is in retaliation for a TV advert in Ontario that opposed US tariffs - and on Truth Social, the president claimed it was factually incorrect.
He wrote: "They only did this to interfere with the decision of the US Supreme Court, and other courts.
"TARIFFS ARE VERY IMPORTANT TO THE NATIONAL SECURITY, AND ECONOMY, OF THE U.S.A.
"Based on their egregious behaviour, ALL TRADE NEGOTIATIONS WITH CANADA ARE HEREBY TERMINATED."
The advert in question had featured former president Ronald Reagan speaking negatively about tariffs - and arguing they cause job losses and trade wars.
But the Ronald Reagan Presidential Foundation has since accused Ontario's government of "using selective audio and video" without permission - and warned it is considering legal options.
Mr Trump's announcement could further inflame trade tensions between the neighbouring countries, which have already been building for months.
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Earlier this month, Canadian Prime Minister Mark Carney had held talks with the president to try and reach a deal.
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Trump turning once again on America's closest ally Canada just proves how flippant his trade decisions are.
The smooth-talking confidence of Prime Minister Mark Carney persuaded Canadians to vote for him in this year's election.
He certainly ran on a pitch to stand up to Trump, but his recent dealings with the US president have largely been diplomatic and cordial.
Carney was last in the Oval Office just over two weeks ago, and the pair laughed off Trump's obsession with Canada becoming the "51st state".
But now it's a single advert from the government of Ontario that has triggered Trump to pause all trade talks between the two, calling its anti-tariff stance "egregious" on his social media platform Truth Social.
The advert uses Ronald Reagan's voice to attack tariff policy - arguing trade barriers "hurt every American worker and consumer… markets shrink, and collapse, businesses and industries shut down and millions of people lose their jobs".
But now, the Ronald Reagan Foundation has said the ad "misrepresents" his words - and they did not give their permission to use it.
Mere hours before Trump's post, Carney was prodding Trump jokingly to bet on the outcome of the baseball World Series.
Given this latest escalation by the President tonight, their next interaction will be far from a laughing matter.
More than 75% of the country's exports go to the US - with C$3.6bn (£1.9bn) of Canadian goods crossing the border daily.
The developments may have caught Mr Carney by surprise.
Just hours earlier, he had challenged Mr Trump to a friendly bet on who would win the baseball world series between the Toronto Blue Jays and Los Angeles Dodgers.