Trust is one big problem. The Iranian regime was established by a revolution more than four decades ago that replaced the American-backed Shah of Iran with a complicated government overseen by clerics and the strong hand of the Supreme Leader, Ayatollah Ali Khamenei. The ayatollah only reluctantly agreed to the 2015 deal with the “Great Satan” of America. After Mr. Trump pulled out, Mr. Khamenei’s mistrust only deepened.

Mr. Trump also imposed many economic sanctions on Iran beyond those originally lifted by the deal, trying “maximum pressure” to force Iran to negotiate much more stringent terms. Iranian officials now say as many as 1,600 American sanctions must be lifted, about half of them imposed by Mr. Trump. Some are aimed at terrorism and human rights violations, not nuclear issues. Lifting some of them would create opposition in Congress.

Many in Washington, let alone in Israel and Europe, also disbelieve Iran’s assertions that it has never pursued a nuclear weapon and would never do so.

Further complicating restoration of the accord are its “sunset” clauses, or time limits, that would allow Iran to resume certain nuclear enrichment activities. The Biden administration wants further negotiations with Iran to extend those time limits as well as put limits on Iran’s missile program and other activities.

Iran says it simply wants the United States to return to the deal it left, including the lifting of sanctions, before it will return, too. It has so far rejected any further talks.

Even under the Islamic regime, Iran has politics, too. There are presidential elections in June, with candidates approved by the clerics. The current president, Hassan Rouhani, who cannot run for another term, and the foreign minister, Mohammad Javad Zarif, are considered relatively moderate and negotiated the 2015 nuclear deal. But powerful forces in Iran opposed the deal, including the Islamic Revolutionary Guards Corps. The moderates hope that quick progress on lifting economic sanctions will help them in the presidential elections; the hard-liners are expected to oppose any quick deal in Vienna that might benefit the moderates.

Iran has lived with tough Trump sanctions for three years now and survived popular discontent and even protests, and hard-liners will argue that another six months are not likely to matter.

The meeting of senior diplomats is formally a session of the Joint Commission of the deal, called by the European Union as chairman. Since the United States left the accord, its representatives will not be in the room, but somewhere nearby. Diplomats from Britain, France, Germany, Russia, China and Iran will meet, with a European Union chair, and start to discuss how to revitalize the accord.

Iran refuses to meet face-to-face with American diplomats. So the Europeans suggest that they will either meet the Americans with proposals, or that the Iranians will leave the room before the Americans enter. This process of indirect talks could take time.

But European diplomats say that after a few days, the job will be left in Vienna to working groups on the complicated political and technical issues. If a rough agreement can be reached on a synchronized return to compliance, the expectation is that officials of Iran and the United States will meet to finalize the details.

The talks may take a long time, and some in Washington hope at least for an agreement in principle in the next few months that would bind any new Iranian government after the June elections.

But some European diplomats fear that too much time has already elapsed, and that the deal is effectively dead, and will essentially serve as a reference point for what may be a fundamentally new negotiation.

So the timeline is unclear, as is the prospect for success.

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