QUEBEC CITY — President Trump threw the G7’s efforts refusing to sign a joint statement with America’s allies, threatening to escalate his trade war on the country’s neighbors and deriding Canada’s prime minister as “very dishonest and weak.”
In a remarkable pair of acrimony-laced tweets from aboard Air Force One as he flew away from the Group of 7 summit toward a meeting with North Korea’s leader, Mr. Trump lashed out at Justin Trudeau. He accused the prime minister, who hosted the seven-nation gathering, of making false statements.
Literally moments after Mr. Trudeau’s government proudly released the joint statement, noting it had been agreed to by all seven countries, Mr. Trump blew apart the veneer of cordiality that had prevailed throughout the two days of meetings in a resort town on the banks of the St. Lawrence River.
“Based on Justin’s false statements at his news conference, and the fact that Canada is charging massive Tariffs to our U.S. farmers, workers and companies, I have instructed our U.S. Reps not to endorse the Communique as we look at Tariffs on automobiles flooding the U.S. Market!” Mr. Trump wrote.
A few hours earlier, Mr. Trudeau said the seven nations had reached broad agreements on a range of economic and foreign policy goals. But he acknowledged that deep disagreements remained between Mr. Trump and the leaders of the other nations, especially on trade.
Mr. Trudeau had sought to play down personal clashes with Mr. Trump as he wrapped up the summit, calling the meeting “very successful” and saying he was “inspired by the discussion.” But he also pledged to retaliate against the United States tariffs on steel and aluminum products in defense of Canadian workers.
Mr. Trump, who apparently saw Mr. Trudeau’s news conference on television aboard Air Force One, was clearly enraged.
“PM Justin Trudeau of Canada acted so meek and mild during our @g7 meetings,” Mr. Trump said in a second tweet, “only to give a news conference after I left saying that, ‘US Tariffs were kind of insulting’ and he ‘will not be pushed around.’ Very dishonest & weak. Our Tariffs are in response to his of 270% on dairy!”
Not long after, John Bolton, the president’s national security adviser, tweeted out a dramatic photo of Mr. Trump, arms crossed and scowling, looking defiant as the leaders of the other nations stood in a circle around him.
“Just another #G7 where other countries expect America will always be their bank,” Mr. Bolton wrote as the president’s plane stopped for refueling at Souda Bay on the Greek island of Crete. “The President made it clear today. No more.”
Mr. Trudeau’s office responded to the president’s Twitter barrage with a carefully worded statement.
“We are focused on everything we accomplished here at the summit,” said Cameron Ahmad, a spokesman for Mr. Trudeau. “The prime minister said nothing he hasn’t said before — both in public, and in private conversations with the President.”
The president’s outburst had been foreshadowed for days leading up to the Canada summit, with Mr. Trump and his counterparts trading sharp-edged barbs that included threats of punches and counterpunches on tariffs. President Emmanuel Macron of France accused Mr. Trump of being willing to remain isolated from the world.
That was followed by 48 hours of tense and often confrontational closed-door discussions between Mr. Trump and the leaders of America’s closest allies — France, Britain, Canada, Japan, Italy and Germany — in the hopes of resolving a brewing trade war among friends.
Instead, the gathering apparently served to further inflame Mr. Trump’s belief that the United States is being treated unfairly by countries with which prior presidents had long ago negotiated trade agreements for the flow of goods and services.
The result was a slow-rolling collapse of the fragile alliances that officials at the summit — and even Mr. Tump’s own White House advisers — insisted throughout the day could be maintained in the face of fundamental disagreements.
Reporters on Air Force One had been told that the United States would sign the joint statement. And minutes after the president’s tweets, reporters were sent an email that had clearly been prepared earlier touting Mr. Trump’s participation in the summit, complete with photos.
Earlier in the day, before Mr. Trump left the summit, he brought up the dramatic prospect of completely eliminating tariffs on goods and services, even as he threatened to end all trade with them if they didn’t stop what he said were unfair trade practices.
Mr. Trump, speaking to reporters at the end of the contentious meeting, said that eliminating all trading barriers would be “the ultimate thing.” He railed about what he called “ridiculous and unacceptable” tariffs on American goods and vowed to end them.
“It’s going to stop,” he said, “or we’ll stop trading with them. And that’s a very profitable answer, if we have to do it.” He added, “We’re like the piggy bank that everybody’s robbing — and that ends.”
The other six leaders were defiant in the face of Mr. Trump’s threats.
“I have made it very clear to the president that it is not something we relish doing, but it is something that we absolutely will do,” Mr. Trudeau said. “As Canadians, we are polite, we’re reasonable, but also we will not be pushed around.”
Mr. Macron said the trade debates at the summit were “sometimes quite heated.” Asked who won the tug-of-war with Trump, Mr. Macron said: “There is no winner, there are only losers when you take that strategy.”
Theresa May, the British prime minister, blasted Mr. Trump’s tariffs. She said she had registered “our deep disappointment at the unjustified decision” and that the loss of trade through tariffs would “ultimately make everyone poorer.”