It wasn’t until American negotiators were close to a nuclear deal with Iran that U.S. Secretary of State John Kerry turned to his Iranian counterpart and asked about the wild card. “Do you have the mandate of the Supreme Leader?” Mr. Kerry asked, referring to Tehran’s paramount political figure, Ayatollah Ali Khamenei. Iranian Foreign Minister Javad Zarif said he was confident he did. The exchange, described by U.S. aides accompanying Mr. Kerry, was both a breakthrough in the final wrenching days of negotiations and a warning of the challenges that lie ahead if the deal is to be successfully implemented. Throughout two years of negotiations, Mr. Khamenei was the ghost in the machine. U.S. officials could never be fully certain he was on board—indeed, they never spoke to him directly, one Kerry aide said—and he repeatedly blindsided participants on both sides. Through the final hours of talks, he continued […]