“There’s no question it’s a big moment,” said Julianne Smith, once a national security aide to Vice President Joseph R. Biden Jr. and now a senior fellow at the Center for a New American Security. “All the fundamentals are being called into question. We’re at a point where we have a U.S. president who doesn’t value the rules-based international order, and I’m not convinced he even knows what it is.”

Wary leaders of European and Asian allies have spent much of the past year trying to assuage him through flattery, concessions and friendly persuasion, only to find their efforts collapse in recent weeks in a storm of tariffs and a broken nuclear agreement with Iran. Even the leader of the engagement strategy, President Emmanuel Macron of France, seems to have given up, publicly expressing deep frustration with Mr. Trump and warning that the United States will isolate itself.

In the 24 hours before heading to Quebec, Mr. Trump attacked Canada on Twitter six times for “unfair” trade practices and threw in a few jabs at France and the European Union for good measure. While in Canada, he did not hold a separate meeting with Prime Minister Theresa May of Britain and showed up so late that a session with Mr. Macron had to be rescheduled.

At the same time, the American president has in the past few weeks praised Kim Jong-un, the repressive leader of North Korea who has ordered his own relatives killed, as “very honorable.” And this week, Mr. Trump even hailed Iran’s theocratic rulers as a “much, much different group of leaders” who have pulled back from malign activities in the Middle East since he ripped up a nuclear agreement with them, a conclusion few others share.

With no country has Mr. Trump’s attitude been more opposite from the rest of American foreign policymakers than Russia. Speaking with reporters on Friday before leaving the White House for the Group of 7 meeting in Canada, the president proposed readmitting Russia. “They should let Russia come back in because we should have Russia at the negotiating table,” he said.