President Trump's break with American allies at the G-7 Summit was the worst foreign policy outing of his presidency, and the worst summit since they started back in 1975, according to Ian Bremmer, president and founder of the Eurasia Group. Bremmer called Mr. Trump's performance "the geopolitical equivalent of the Comey firing."

The tension between the president and America's closest allies unfolded at the end of a weekend that looked promising, as Mr. Trump gave indications that he would sign onto the summit's final declaration.

But en route to his meeting with North Korean dictator Kim Jong Un, Mr. Trump angrily tweeted about Canadian Prime Minister Justin Trudeau, accusing him of making "false statements" at a news conference, and declared that he was rescinding his agreement.

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! — Donald J. Trump (@realDonaldTrump) June 9, 2018

German Chancellor Angela Merkel said the president's reversal "via tweet" from signing onto the G-7 statement was "sobering and a bit depressing."

The decision to break from the agreement drew criticism from lawmakers at home as well. Republican Senator John McCain tweeted: "To our allies ... Americans stand with you, even if our president doesn't."

A widely-shared photo of German Chancellor Angela Merkel and others addressing President Trump at the Group of 7 Summit meeting in Charlevoix, Quebec this past weekend. Steffen Seibert, German gov't spokesman/via Twitter

On "CBS This Morning" Monday, Bremmer, said, "It was Trump unable to control his emotional impulses in doing something that I am sure felt very satisfying at the time. He was forced to go, he didn't want to attend the summit, his advisors all said, 'You have to be there.' He shows up late, he leaves early, he cancels his direct meeting with [French President] Macron. The entire time he's seething, bristling and petulant, but he's on good behavior. Finally he leaves, and he says, 'That's it, I'm going to blow it up.'"

"I'm sure that sending that [tweet] out against Trudeau, America's closest ally, felt really good at the time," Bremmer said. But the result is, "It really damages an important long-term relationship."

"CBS This Morning" co-host Gayle King asked, "What did Trudeau say that was so egregious?"

"Nothing that he hadn't said already," Bremmer replied. "Trudeau was careful not to insult the president personally. He did say that the Canadians were not going to just sit and take it, in terms of the American tariffs, that there would be retaliation. [That's] exactly what he had said on 'Meet the Press' the week before and in other public statements.

"This was not about what Trudeau said. This is about Trump's feelings. And when Trump feels a certain way, he is not to be told otherwise by his handlers. I think he showed very clearly, 'You can't tell me to do whatever you want – I'm my own man, I'm my own president.' And again, at the time it felt great, but this is pretty grave damage to the most important allies around the world."

Will the spat do lasting damage to the Western alliance?

"They're not going to form a G-6, they're not unified, they have no ability to get together on other issues – the Europeans, the Japanese, the Canadians. No. But they don't trust America," Bremmer said. "Trump said that the relationship with the allies was a 10. Maybe out of 100? But the reality is, the relationship with the United States is reasonably uncertain right now. The individual relationship with Trump and those allies is as bad as it could be. It's close to a zero."

Watch more from Bremmer's interview on "CBS This Morning" in the video player above.