"We're like the piggy bank that everybody is robbing", he said at a press conference before making an early exit from the two-day summit in La Malbaie, Quebec, where he met with leaders of Canada, Britain, France, Italy, Germany and Japan.
The letter comes on the day of the Group of 7 (G-7) summit amid heightened conflict between Trump and other G-7 members over the tariffs and following Trump's suggestion that Russian Federation be reinstated into the group.
The attack shattered any hope that Canada could avoid USA tariffs on steel and aluminum or renegotiate the North American Free Trade Agreement by virtue of the charm, patience or measured response it has extended to Trump since he took office.
What else did Trump have to say?
In a statement, the Canadian government did not address Trump's tweets, saying 'we are focused on everything we accomplished here at the #G7 summit, ' spokesman Cameron Ahmad said.
Trump last month directed Commerce Secretary Wilbur Ross to initiate a national-security investigation into imports of cars, trucks and vehicle parts that could possibly lead to tariffs. Mr Trump left the summit early for Singapore, where he will meet North Korean leader Kim Jong Un next week. "It's going to stop or we'll stop trading with them", he said.
The summit was wrapping up just as Chinese President Xi Jinping began hosting the leaders of Russian Federation and Iran at a regional security meeting in a symbol of the power-play between East and West.
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"Every topic we discuss needs to reflect on its impact on women as well as further intersectionalities, whether it be race, gender identity or background or cultures", he said.
In return, Mr Trump made the French president his guest at his first White House state dinner, after which the USA president said of his visitor: "I like him a lot". "And we charge nothing", Trump said.
Justin Trudeau and other leaders have attacked Trump for levying steep trade tariffs on Canada and European nations, despite being allies of the United States.
Trump's change of heart appeared to have been invoked by Trudeau's comments on Trump's steel and aluminum tariffs against Canada and other allies.
"If there were to be a different U.S. government with a different agenda the [relationships] would come back really quickly" because of the United States economy's massive size and the dollar's preeminent role in global finance".
Trump threatened at the meeting that the USA would end all trade with nations that refuse to end "unfair" trade practices.
Free trade is "the way it should be", he said.
"We strive to reduce tariff barriers, non-tariff barriers and subsidies", it said.
The statement also details other things the leaders agreed to during the summit.