Mr. Trump is the black sheep of this family, the estranged sibling who decided to pick fights with his relatives just before arriving to dinner. The dispute, Larry Kudlow, the president’s top economic adviser, acknowledges, is “much like a family quarrel,” but with the potential for vast diplomatic and economic consequences for the world…

“Patently absurd” is what Liam Fox, the British trade minister, called them. Chancellor Angela Merkel of Germany said they were “illegal,” while Mr. Trudeau said they were “insulting and totally unacceptable” — and that was in the carefully worded public statement. In a phone call with Mr. Trump, he was said to be even more blunt.

Before the summit meeting, finance ministers from the other six countries that form the Group of 7, or G-7, condemned Mr. Trump’s trade decisions in an extraordinary rebuke of a member nation’s president. And some of the leaders themselves have threatened to boycott the usual end-of-meeting communiqué. A senior Canadian official said a statement by only Mr. Trudeau, the gathering’s host, is possible.

Asked about the upcoming discussions in Canada, Ms. Merkel, the famously taciturn leader of Germany, said they would be “difficult.”