As the eighth round of talks to restore the 2015 nuclear deal continues in Vienna, negotiators are struggling to contend with one of Iran’s most difficult demands: a guarantee from Washington that the United States will not withdraw from the deal and reimpose sanctions, as former President Trump did in 2018. Delegations from Iran, China, France, Germany, Russia, and the United Kingdom have gathered in Vienna for eight rounds of talks since April 2021, under the chairmanship of the European Union. The U.S. team is also in Vienna with an aim to negotiate U.S. re-entry to the nuclear deal, known...