VIENNA—Efforts to bring the United States back into the 2015 deal on Iran’s nuclear program stepped up a gear Tuesday as Iran and the five world powers still in the accord were meeting in Vienna while the United States is due to start indirect talks with Tehran. Friday’s announcement that Washington and Tehran would begin indirect talks through intermediaries was one of the first signs of tangible progress in efforts to return both nations to the terms of the accord, which restricted Iran’s nuclear program in return for relief from U.S. and international sanctions. Then-President Donald Trump pulled the U.S. unilaterally out of the accord in 2018, opting for what he called a maximum pressure campaign of stepped-up U.S. sanctions. Since then, Iran has been steadily violating the restrictions in the deal, like the amount of enriched uranium that it can stockpile and the purity to which it can enrich …