Canada issued its own threat of tariffs, which don't go into effect until July. "I would say that the G-8 is a more meaningful group than the G-7, absolutely", he told reporters as he left the summit in Canada.
And while his bellicosity and volatility has deeply strained relations between the USA and its historic allies, Trump insisted that he had terrific relationships with the G7 leaders, though before arriving at the summit he had unleashed a tweetstorm against some of them.
Before he did, Trump held an impromptu press gathering, where he offered his thoughts on a range of topics, including trade. "Americans stand with you, even if our president doesn't", McCain wrote in a tweet on Saturday in response to a series of messages sent by the president on the social media site. Under Trump, the United States has abandoned its traditional role in the G-7 of pressing for freer global trade. The language, however, was strong, he added.
"If they retaliate, they're making a mistake", he said.
The meeting, once billed as a historic opportunity to rid the Hermit Kingdom of its nuclear arsenal, in recent days has been described more modestly as a chance for the two mercurial leaders to size each other up, and see if the two leaders can do business with one another. "I blame our past leaders".
Such a move could make it almost impossible to renegotiate the terms of the 1994 NAFTA pact between the United States, Canada and Mexico. "They have no choice".
"We have a tremendous trade imbalance", Trump said during Saturday's press conference.
"The Prime Minister said nothing he hasn't said before - both in public, and in private conversations with the President".
He said USA farmers had been harmed by tariffs and other barriers and warned that US trading partners would need to provide him with more favourable terms. "We will win that war 1,000 times".
"And so began a long litany of recriminations, somewhat bitter reports that the United States was treated unfairly, that the trading system was totally unfavorable to the United States, the American economy, American workers, the middle class", the official said.
Trump continued by saying that "we are looking for peace in the world, and not to play games" and emphasized his point, stating "the G8 would be better".
The EU said it would stick to the joint communique despite Mr Trump's decision.
The response from Mr Macron suggests an end to what observers had noted was a budding "bromance" between the French president and Mr Trump.
Former prime minister Stephen Harper said Russian Federation should never be allowed back in the G7 as long as Vladimir Putin is in power.
Trump's tirade came before he was to leave for Singapore for his historic summit with the North Korean leader North Korea's Kim Jong Un.
"Second day of the G7 Summit in Canada", Seibert captioned the photo, according to a translation. "So we are going in with a really positive spirit".
"They understand that. They know it".
Here's a World Bank graph of trade-weighted average tariffs for the G7 countries - with U.S. almost identical to UK/Italy/France/Ger, higher than Japan/Can.