The Middle East has also changed for the better since Democrats were last in charge. Israel and the Sunni Arab states are normalizing relations after decades of conflict, and one reason is their shared worry about Iran. The Arab-Israeli conflict over Palestine is no longer the dominant issue.

President Trump gave both sides assurance that the U.S. is on their side, in contrast to Barack Obama’s loud disdain and his strategic embrace of Iran. If Mr. Biden rushes head-long back into the nuclear deal, he runs the risk of blowing up that regional progress.

Why would Mr. Biden want to throw that away? We understand the emotional attachment that Mr. Sullivan and other Biden advisers have for the 2015 nuclear deal. But time and Iranian behavior have exposed that deal as even worse than it looked at the time. Mr. Biden’s foreign-policy team should be thankful for the stronger hand Mr. Trump is leaving them.