BRUSSELS — In Vienna last week, the signers of the 2015 Iran nuclear deal came together with what might appear to be a simple task. They wanted to reestablish compliance with an agreement that places strict controls on Iran’s nuclear enrichment, to make sure that it can’t build a nuclear weapon, in return for the lifting of punishing economic sanctions.

The two Iran and the United States insist that they want to return to the deal, known as the Joint Comprehensive Plan of Action. But nothing about the meetings is straightforward.

The initial sessions went well, with working groups established to try to synchronize the return of both nations — 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 which 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 think that both Tehran and Washington want the talks to succeed no matter Israeli resistance.

Defense Secretary Lloyd J. Austin III is in Israel today, raising the question of whether the attack on Natanz was done with the understanding or even the permission of America. He’ll join Secretary of State Antony J. Blinken in Brussels later this week. While Russian military maneuvers near Ukraine and Afghanistan are important issues, Iran will figure as well.

President Donald J. Trump, with Israeli aid, pulled the United States from 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 reacted about a year later by enriching uranium significantly beyond the limits in the agreement, building more advanced centrifuges, and behaving 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 probably be harder than a lot of people realize. And the assault on the Natanz plant is very likely to further complicate the talks.

The Vienna talks are intended to create a road map to get a synchronized return of both Iran and the United States to compliance with the 2015 deal. It’s been at risk of collapse since Mr. Trump withdrew American participation.

The accord was the outcome of years of discussions 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 America.

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 happened.

Then, the deal was widely criticized as too weak by many in Congress and by Israel, which saw Iran’s possible reach to get a nuclear weapon — an aspiration always denied by Iran — as an existential threat.

After Mr. Trump revived American sanctions, the Europeans tried to keep the deal alive, but proved unable to provide Iran the economic benefits it was due under the bargain ’s terms. The American sanctions, depending on the worldwide power of the dollar and the American banking system, kept European and other companies from doing business with Iran, and Mr. Trump resisted the pressure by adding a lot more sanctions.

Iran responded in various ways, including attacks on transport and on American allies in Iraq, but more importantly by restarting uranium enrichment at a higher level and with centrifuges banned under the offer.

The estimated time it would take Iran to make enough enriched uranium to make a nuclear weapon has now shrunk from a calendar year, which was what the deal needed to preserve, to just a month or two. Iran is also making uranium metal necessary for a warhead, also banned under the deal, and is vigorously encouraging allies in the Middle East, including many the West regards as terrorist groups.

In a further pressure strategy, Iran has translated the review needs of the bargain narrowly and has independently declined to answer questions from the International Atomic Energy Agency about radioactive particles that inspectors found at sites which have never been announced by Tehran within 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 moment, Iran says, the data will be deleted. That would leave the world in the dark about key areas of the nuclear program.

Iran insists it can return to compliance with the deal immediately, but needs the United States to do so first. The Biden government says it wants Iran to go first.

1 big problem is trust.

The Iranian regime was established by means of a revolution over four decades ago that substituted the American-backed shah of Iran with a complicated government overseen by clerics and the powerful hand of the supreme leader, who’s now Ayatollah Ali Khamenei. The ayatollah agreed only reluctantly to the 2015 deal with the “Great Satan” of America.

After Mr. Trump pulled , Mr. Khamenei’s mistrust only deepened.

Mr. Trump also enforced many economic sanctions on Iran beyond those originally lifted by the agreement, trying “maximum pressure” to induce the country to renegotiate the agreement and take a lot more stringent terms.

Iranian officials now say as many as 1,600 American sanctions must be lifted, about half of them enforced by Mr. Trump. Some are aimed at terrorism and human rights violations, not nuclear problems. Lifting some of these would create resistance in Congress.

Many in Washington, as well as in Israel and Europe, do not think Iran’s assertions that it’s never pursued a nuclear weapon and that it never will.

Further complicating a restoration of the accord are its “sunset” exemptions, or time limits, that would enable Iran to resume certain nuclear enrichment activities. The Biden government wants talks with Iran to extend those time limits, as well as putting limits on Irans missile program and other actions.

Iran says it wants the United States to return to the deal it abandoned, including the lifting of sanctions, before it is going to return to it, also. 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 present president, Hassan Rouhani, who can’t run for another term, and the foreign minister, Mohammad Javad Zarif, are considering relatively moderate and negotiated the 2015 atomic deal. But powerful forces in Iran opposed the agreement, among them the Islamic Revolutionary Guards Corps.

The moderates hope that rapid 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 — such as protests — and hard-liners will argue that another six months are unlikely to matter.

The meeting of senior diplomats is officially a session of the Joint Commission of the deal, called by the European Union as chairman.

Since the United States abandoned the accord, its agents won’t be in the area, but instead are in a hotel nearby. Diplomats from Britain, France, Germany, Russia, China and Iran are fulfilling, with a European Union seat, to discuss how to revitalize the accord.

Iran refuses to meet face to face with American diplomats. So the Europeans imply that they will either meet the Americans with suggestions, or that the Iranians will leave the room before the Americans enter.

This practice of indirect talks could take time.

The more formal meeting authorized both 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.

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

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

So the timeline is unclear. And so is the prospect for success.