But he came alive whenever trade was mentioned, mocking and insulting other leaders, particularly Mr. Trudeau, Mr. Macron and Chancellor Angela Merkel of Germany, according to the witnesses. Ms. Merkel was clearly not happy but largely kept quiet, evidently not wanting to provoke more conflict. Mr. Trump’s conversation was described by European officials as stream of consciousness, filled with superlatives but not following a linear argument.
Negotiators worked late Friday night to craft a statement that all seven leaders could sign. Mr. Trump’s delegation objected to the term “rules-based international order” until negotiators compromised by expressing support for “a” rules-based order rather than “the” rules-based order.
In a section on trade, the Americans insisted on using the word “reciprocal,” one of Mr. Trump’s favorite terms. They compromised by expressing support for “creating reciprocal benefits.” They did not refer directly to the Iran nuclear agreement, which Mr. Trump just pulled out of, but the Europeans included a reference to the United Nations Security Council resolution that endorsed the deal.
The negotiators included agree-to-disagree language on climate change and on an international commitment to reduce plastics in the world. And then the statement was released on Saturday night, only to have Mr. Trump disavow it. Mrs. May learned only as her helicopter arrived at the airport for her to board her plane to return to London.
“It started out as a good summit because we were actually talking to each other, instead of past each other,” said Peter Beyer, the German government’s coordinator on trans-Atlantic relations. But he added, “It looks like the U.S. is no longer a reliable partner in international agreements, and that’s bad.”
Josef Braml of the German Council on Foreign Relations said Mr. Trump considered diplomacy a waste of time. “He is about to destroy what’s left of the liberal world order because he thinks rules and institutions help America’s rivals, China and Europe,” he said.
Laurence Nardon, director of the North American program at the French Institute of International Relations in Paris, said Mr. Trump’s behavior was a negotiation trick. “It’s like when a person leaves the room and slams the door, but is hoping the other person will run after them and follow them into the corridor,” she said.