The reversal followed private appeals from the leaders of Saudi Arabia and the United Arab Emirates, who told Trump that serious talks were underway and assured him that a deal will be made. Mediators are now pressing Iran to adopt a more flexible position on the nuclear demands put forward by Washington.

President Trump said he was “an hour away” from striking Iran on Monday when he called off the military strike at the request of Middle East leaders — and warned another “big hit” could...