For Mr. Trump’s advisers, the biggest risk at the United Nations General Assembly this year is the reverse of what it was last year: not that he will be dangerously undiplomatic, but that he will be overly enthusiastic about engagement with wily adversaries.

Far from restraining Mr. Trump’s belligerent tendencies, his senior aides are engaged in a quiet effort to avoid a direct encounter with Iran’s leader that he would be unprepared to handle or concessions that they fear could undermine their effort to keep pressure on North Korea.

Either of those possibilities would rattle Mr. Trump’s aides, who are uniformly hawkish about Iran and North Korea, and favor squeezing those countries over talking to them. A meeting with President Moon Jae-in of South Korea on Monday also looms large, since Mr. Moon is likely to press him to make concessions to keep the talks with Mr. Kim going.