BRUSSELS — In Vienna last week, the signers of the 2015 Iran nuclear deal came together with what would appear to be a simple task. They wanted to restore compliance with an agreement that puts strict controls on Iran’s nuclear enrichment, to ensure that it cannot build a nuclear weapon, in return for the lifting of punishing economic sanctions.
The first sessions went well, with working groups established to try to synchronize the return of both countries — a group on sanctions to bring the United States back into compliance and a group on nuclear issues, to bring Iran and its nuclear program back within the limits established in the accord.
But with Iran pressing ahead with enrichment in the meantime, and a mysterious explosion Sunday night at its Natanz nuclear-enrichment plant that American and Israeli officials say is the work of Israel, the atmosphere for the talks will be much more strained.
Israel has always opposed the accord, regarding it as weak, and is no fan of President Biden’s efforts to reinvigorate it. While Iran on Monday threatened retaliation against Israel, European officials still believe that both Tehran and Washington want the talks to succeed regardless of Israeli opposition.
Defense Secretary Lloyd J. Austin III is in Israel now, raising the question of whether the attack on Natanz was done with the knowledge or even the permission of the United States. He will join Secretary of State Antony J. Blinken in Brussels later this week. While Russian military maneuvers near Ukraine and Afghanistan are major topics, Iran will figure as well.
President Donald J. Trump, with Israeli support, pulled the United States out of the nuclear accord in May 2018, calling it “the worst deal ever negotiated,’’ and restored and then enhanced harsh economic sanctions against Iran, trying to force it to renegotiate.
Iran responded about a year later by enriching uranium significantly beyond the limits in the agreement, building more advanced centrifuges, and acting more aggressively in support of allies in the Middle East, like Hezbollah, Hamas, Shia militias in Iraq and the Syrian government of Bashar al-Assad.
So returning to a deal made six years ago will likely be harder than many people realize. And the attack on the Natanz plant is likely to further complicate the talks.
What are the talks about?
The Vienna talks are intended to create a road map for a synchronized return of both Iran and the United States to compliance with the 2015 deal. It has been at risk of collapse since Mr. Trump withdrew American participation.
The accord was the outcome of years of negotiations with Iran. Under the chairmanship of the European Union, Britain, France and Germany made the first overtures to Iran, joined by the other permanent members of the United Nations Security Council: Russia, China and the United States.
But it was not until the United States started secret talks with Iran under President Barack Obama and agreed that Iran could enrich uranium, though under safeguards, that a breakthrough occurred.
Even then, the deal was widely criticized as too weak by many in Congress and by Israel, which saw Iran’s possible reach for a nuclear weapon — an aspiration always denied by Iran — as an existential threat.
After Mr. Trump restored American sanctions, the Europeans tried to keep the deal alive, but proved unable to provide Iran the economic benefits it was due under the deal’s terms. The American sanctions, based on the global power of the dollar and the American banking system, kept European and other companies from doing business with Iran, and Mr. Trump intensified the pressure by adding many more sanctions.
Iran responded in various ways, including attacks on shipping and on American allies in Iraq, but more importantly by restarting uranium enrichment at a higher level and with centrifuges banned under the deal.
The estimated time it would take Iran to make enough enriched uranium to produce a nuclear weapon has now shrunk from a year, which was what the deal wanted to preserve, to just a few months. Iran is also making uranium metal necessary for a warhead, also banned under the deal, and is aggressively supporting allies in the Middle East, including many the West regards as terrorist groups.
In a further pressure tactic, Iran has interpreted the inspection requirements of the deal narrowly, and has separately declined to answer questions from the International Atomic Energy Agency about radioactive particles that inspectors found at sites that have never been declared by Tehran as part of the nuclear program.
In late February, Iran agreed to keep recording information on its inspection equipment for three months, but without granting I.A.E.A. access. If economic sanctions are not lifted in that time, Iran says, the information will be deleted. That would leave the world in the dark about key parts of the nuclear program.
Iran insists it can return to compliance with the deal quickly, but wants the United States to do so first. The Biden administration says it wants Iran to go first.
What are the obstacles?
One big problem is trust.
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, who is now Ayatollah Ali Khamenei. The ayatollah agreed only reluctantly 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 the country to renegotiate the agreement and accept 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, as well as in Israel and Europe, do not believe Iran’s assertions that it has never pursued a nuclear weapon and that it never will.
Further complicating a 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 negotiations with Iran to extend those time limits, as well as put limits on Iran’s missile program and other activities.
Iran says it wants the United States to return to the deal it left, including the lifting of sanctions, before it will come back to it, 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 agreement, among them 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.
The Iranian government has lived with the tough Trump sanctions for three years now, surviving popular discontent — including protests — and hard-liners will argue that another six months are not likely to matter.
How are the talks structured?
The meeting of senior diplomats is formally a session of the Joint Commission of the deal, called by the European Union as chairman.
Because the United States left the accord, its representatives will not be in the room, but instead are in a hotel nearby. Diplomats from Britain, France, Germany, Russia, China and Iran are meeting, with a European Union chair, 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.
The more formal meeting authorized the two working groups, and judged their progress satisfactory last Friday. The working groups continue; senior diplomats are to meet again on Wednesday. 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.
What is the prospect for success?
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. They think it will essentially serve as a reference point for what may be a fundamentally new negotiation.
So the timeline is unclear. And so is the prospect for success.