WASHINGTON — Three weeks from now, in New York, President Trump will find himself in the setting he most relishes: seated at the head of a polished table, calling on those seated around him, rewarding those he likes and cutting off those who displease him.

It is not a revival of “The Apprentice,” or even a meeting of his cabinet. Mr. Trump will be presiding at the United Nations Security Council, a rotating role that falls to the United States this month. His star turn is prompting anxiety among people, inside and outside the administration, who worry that the president will bring reality-TV antics to the world stage.

Exercising the prerogative of the chairman, Mr. Trump plans to focus on Iran and its malign activity around the Middle East. European diplomats said they fear that this will only underscore the disunity of the West, given the unpopularity of Mr. Trump’s decision to pull the United States out of the Iran nuclear deal.

Already, the president’s choice of subject has drawn objections from Russia, which said the focus of the meeting should be entirely on the nuclear deal and Mr. Trump’s exit from it, and Iran, which accused Mr. Trump of abusing his leadership of the council to vilify a single country.