"I have made it very clear to the President that it is not something we relish doing, but it something that we absolutely will do", Trudeau said. "We used fact-based arguments". But a source close to Trudeau told AFP Canada was "prepared for all eventualities".
The U.S. had granted some allies including Canada temporary exemption from import tariffs on steel and aluminum. Blasting Canadian Prime Minister Justin Trudeau for being dishonest and weak during the summit, Trump pulled out from the carefully crafted final agreement, hours after calling it "tremendously successful".
Trudeau had said using national security as a justification for tariffs was "kind of insulting" to Canadian veterans who had stood by their United States allies in conflicts dating back to World War I.
Reacting to the same, Kudlow said, "Trudeau really kind of stabbed us in the back".
Mr Trudeau and the European Union have vowed to take retaliatory action next month over USA tariffs on steel and aluminium.
Trudeau later said he reiterated to Trump that tariffs will harm industries and workers on both sides of the U.S. "That was a mistake". "I don't understand the obsession with trade relations with Canada", he sighed.
"We will stand shoulder to shoulder with the prime minister and the people of Canada", said Doug Ford, the newly-elected anti-tax premier of Ontario and brother to the late crack-smoking mayor of Toronto, Rob Ford. "He'll learn, he can't do that", Mr Trump said.
"POTUS is not going to let a Canadian Prime Minister push him around", said Larry Kudlow, director of the National Economic Council.
It is unfortunate that there is now "so little trust" in this most important relationship, commented University of Montreal professor Frederic Merand.
The United States is Canada's single biggest trade partner, with two-way exchanges of goods and services totalling $673.9 billion in 2017, and Washington enjoys a $8.4 billion surplus.
The US President is also making threats against Canada's dairy producers and automakers. We have put up with Trade Abuse for many decades - and that is long enough. "It's a betrayal. He's double-crossing", he said Sunday.
Canada's approach has been to hope for the best outcome but to always be prepared for the worst, and "to have a plan B, C, D, E and F and maybe to the end of the alphabet", Freeland said.