In a last-minute twist-by-tweet, Donald Trump lashed out, again.
"Canadians, we're polite, we're reasonable but we also will not be pushed around", Trudeau, the host of the two-day summit in La Malbaie, Quebec, told reporters.
He suggested Mr Trudeau was "very dishonest and weak".
Trump left the summit early headed for his meeting with the leader of North Korea next Tuesday.
Despite smiles and jokes for the cameras, the tension among the leaders was clear. Trudeau said Trump's citing of "national security" as a reason for the levies being imposed was "kind of insulting" to Canadian veterans who had stood by their U.S. allies in conflicts dating back to World War I.
He said that other countries were imposing "massive tariffs" on the US.
"No tariffs, no barriers, that's the way it should be", Trump said.
Trump's administration last week ended a two-month exemption to steel and aluminum imports from Mexico, Canada and the European Union.
Trump said that would be a bad idea. "If they retaliate, they're making a mistake", he said.
"We'll win that war 1,000 times out of a 1,000", he said.
Kathleen Wynne says she won't win election
"It's a realization that the Kathleen Wynne brand might be more detrimental than the Liberal party brand", she said. The NDP leader is making her strong push for Liberal ridings with less than a week left in the campaign.
Trudeau said Canada wasn't backing down.
Some countries have retaliated with their own levies on U.S. imports.
Trump said he didn't blame the G7 leaders for "unfair" trade deals but rather their predecessors. "And no subsides, I even said not tariffs". The noise of Mr Trump's helicopter landing was so loud they had to stop talking for a while, in a scene one official compared to the opening from the USA television series M.A.S.H.
Another picture appears to show the 71-year-old Trump wincing slightly as he struggles to get to grips with his 40-year-old counterpart.
Canadian negotiators have said a five-year sunset clause - which would prompt a reworking of the trade deal every five years - is a "poison pill" for trade talks.
Trudeau refused, and the meeting was off.
And as Trump spoke Friday morning, his Defense Secretary James Mattis warned about Russian Federation in remarks to reporters at a North Atlantic Treaty Organisation meeting in Brussels. "European Union treats us very unfairly".
"We're like the piggy bank that everybody's robbing, and that ends", Trump said later. Canada and Mexico are also taking action in retaliation.
"The EU - of course, we operate as a member of the EU, as we now are - the European Union will impose countermeasures to the United States", May said.
Trump repeated his criticism of Canada's supply managed dairy industry, one of his favourite targets in Twitter posts, including this week prior to his arrival in Canada.
"The rules-based worldwide order is being challenged, quite surprisingly, not by the usual suspects but by its main architect and guarantor, the US" - European Council president Donald Tusk vents his frustration at Trump.