LONDON — It was just hours after Boris Johnson, then Britain’s foreign secretary, returned to London from a whirlwind trip last year to try to persuade the White House to abide by the Iran nuclear accord.

Kim Darroch, then the British ambassador to the United States, fired off a withering assessment of President Trump’s wish to quit the deal. Mr. Trump, he wrote in leaked diplomatic cables that were published on Saturday, was “set upon an act of diplomatic vandalism, seemingly for ideological and personality reasons — it was Obama’s deal.”

The vice president, the national security adviser and the secretary of state had all failed to “articulate why the president was determined to withdraw, beyond his campaign promises,” Mr. Darroch wrote. And the American government had no plan for what would follow.

“Even when you pressed,” Mr. Darroch wrote to Mr. Johnson, “none had anything much to say about the day after, or a Plan B, beyond reimposition of U.S. sanctions.”