By Massimo Calabresi
March 19, 2015

The ceremonial entrance to the Palais Coburg hotel in the heart of Vienna sits atop an elaborate staircase rising from the end of the former palace’s carriage drive, creating what the hotel’s promotional literature calls “the anticipation of a grand event.” But on a balmy day last June, the five American nuclear experts arriving at Palais Coburg needed no added drama. Ahead of them lay a tense confrontation with their Iranian counterparts in a day of high-stakes talks over Tehran’s nuclear program.

In one of the building’s ornate 19th century staterooms, the Americans faced off against the Iranians across a large conference table. Over the next hour, the U.S. team presented excerpts from a series of highly classified Iranian documents that U.S. intelligence had obtained from Tehran’s top-secret nuclear-weapons program, according to several sources familiar with the talks. The documents, the U.S. officials said, proved what the International Atomic Energy Agency (IAEA) had said publicly: that Iran had researched components of a missile-deliverable nuclear warhead, including specialized firing mechanisms, warhead designs and modified re-entry capsules for use atop Iran’s existing medium-range Shahab-3 missiles. That and other technology, plus a supply of weapons-grade uranium, would have given Iran the bomb.

The Iranian negotiators dismissed out of hand the evidence the U.S. team laid in front of them, calling the documents fabrications and denying that the country had ever sought a nuclear weapon or the means to build one. The Americans expected that–but the purpose of the presentation was not to win an argument. Rather, the message from Washington to Tehran was simple, according to the sources familiar with the briefing: We will never trust you. Ever. Any agreement by the U.S. and its international allies to ease economic sanctions on Iran would have to come in exchange for broad and lasting nuclear transparency by Iran–not simply promises or pledges.

The Vienna meeting was just one of dozens that have taken place over the past 16 months as the U.S. and Iran have struggled to reach just such a deal, and this month Secretary of State John Kerry and his counterpart, Iranian Foreign Minister Mohammad Javad Zarif, are attempting to agree to the outlines of one before a self-imposed March 24 deadline. But last spring’s exchange in Vienna captures the central challenge of the talks: Is it even possible to construct a complex nuclear agreement between two countries whose mutual distrust runs so deep?

Building bridges across distrust is the point of diplomacy–just ask the negotiators who hammered out nuclear agreements between the U.S. and the Soviet Union during the Cold War. But with the U.S. and Iran it’s hard to know where to begin laying bricks. The two nations are sworn enemies that have had no formal diplomatic relations since the 1979 revolution brought the anti-American ayatullahs to power. Iran developed its nuclear program secretly and in violation of international law: as a signatory to the Nuclear Non-Proliferation Treaty, it forswore the right to develop nukes. It was caught thanks only to Western spies and inspectors from the IAEA. For more than a third of a century, the U.S. and Iran have dealt with each other through threats, covert attacks and sabotage.

So what has kept both sides at the bargaining table these past 16 months? Fear. The U.S. worries that a nuclear Iran could threaten its Middle East allies. Tehran has long sponsored overt and covert war throughout the region, including against Israel; with nuclear weapons Iran would become even more powerful and dangerous. The ayatullahs, for their part, fear the destabilizing effect of continued international sanctions, which have crippled their economy. Most of all, both sides know that the failure of diplomacy could bring war, as Israel and the U.S. have sworn to use force to prevent Iran from getting the bomb.

As the talks have unfolded, the distrust between the U.S. and Iran has spread from the negotiating table to capitals around the globe. Differences over the merits of a deal have badly damaged relations between Israel and America. Congress accuses President Barack Obama of endangering U.S. security in the talks, while Obama says that it’s Congress threatening the U.S. by meddling. Iranian hard-liners in Tehran don’t trust their negotiators, while the negotiators aren’t sure their theocratic rulers will sign off on any deal they make.

With those global tensions building, participants in the talks say they are worried about the fallout should they fail to reach a deal by the March 24 deadline. As Kerry and Zarif try to hammer out a long-term political agreement at their hotel in Lausanne, Switzerland, it’s unclear whether there’s enough time–and trust–to keep talking at all.

The “Orchid” Master

The International Community’s distrust of Iran’s nuclear ambitions largely traces back to one man: Mohsen Fakhrizadeh, who has been called Iran’s Robert Oppenheimer–the American scientist who led the creation of the world’s first atomic bomb. The 54-year-old physicist is a ranking member of Iran’s hard-line Revolutionary Guards and has guided the Islamic Republic’s nuclear-weapons efforts. Starting in the late 1990s, he consolidated the research physics and defense arms of the Iranian nuclear program in what came to be called the Orchid Office, named after the street on which it was located in Tehran, according to a lengthy and detailed November 2011 report by the IAEA.

Once the Orchid Office was established, Fakhrizadeh set about pursuing three main branches of nuclear-weapons research. The first, known as the “green salt project,” aimed to develop the material for a nuclear warhead. Iran is blessed with a natural supply of uranium, and in what he labeled Project 5, Fakhrizadeh organized its mining, milling and conversion into gas. Over time, in secret and on the open market, Iran procured the materials needed to construct thousands of high-speed centrifuges that could refine the gas into weapons-grade uranium, and it built two secret facilities to house them, one at Natanz, the other at Fordow.

Fakhrizadeh’s second task, the IAEA has reported, was to design and develop the mechanical system for initiating a nuclear explosion with the uranium–the bomb part of a nuclear bomb. First, the uranium needs to be carefully cast into the right shape. Then, to initiate the atomic chain reaction, fast-acting detonators need to go off within one microsecond of each other, driving the implosion of the uranium. And for the uranium to reach supercritical density and initiate a fission explosion, charges around the outside of a spherical casing need to yield a uniform blast wave that evenly drives the uranium inward. The IAEA found that Fakhrizadeh and his Orchid Office had overseen the development of each of these elements.

Most worrying to Iran’s neighbors, the triggers and explosives from his first two projects, when put together, would form a warhead that exactly matched the dimensions Fakhrizadeh was using in his final area of research, Project 111. That was designed to show how a new spherical payload could be incorporated atop Iran’s existing Shahab-3 missile, which could be modified to reach targets up to 1,200 miles (1,930 km) away, bringing Israel within its range.

Iran says the U.S. and IAEA documents detailing Fakhrizadeh’s work are fabrications and argues that every part of his research was aimed at peaceful, civilian projects, or at least nonnuclear military ones. The U.S. and the IAEA admit that some of his projects could conceivably be used that way, but most others couldn’t. Taken together, there is no project for which the research collectively applies other than building a missile-deliverable nuclear warhead, according to the IAEA, the U.S. and many of its allies. And by 2003, Fakhrizadeh was well on his way to developing one.

Yet after the shock and awe of the U.S. invasion of Iraq in 2003, Iran suddenly stopped its pursuit of nuclear weapons, according to a 2007 U.S. intelligence assessment. Fakhrizadeh complained that his funding had been cut off. And right after the invasion, the Iranian regime sent a secret letter to the U.S. via the Swiss ambassador to Tehran, offering to engage in talks. The Administration of then President George W. Bush rebuffed the overture. By the time Obama made an offer to engage with Iran based on “mutual respect” in March 2009, the Supreme Leader Ayatullah Ali Khamenei was no longer in the mood.

Against those fleeting and fumbled efforts to build trust, the two sides continued to act on their deep-seated suspicion of each other. Iran may have stopped its research into a nuclear weapon, but it accelerated its production of low- and medium-enriched uranium, increasing the stockpile of potential material with which it could rapidly assemble a bomb. In a September 2009 announcement with France and the U.K., Obama revealed the existence of the secret Fordow enrichment plant. Soon after, centrifuges at Natanz began malfunctioning. As many as 1,000 of the 9,000 centrifuges at Natanz were taken offline, thanks to a computer virus called Stuxnet that was traced back to a cyberattack launched by the U.S. and Israel.

More threatening, certainly for Fakhrizadeh, were magnetic “sticky bombs” that unidentified assailants attached to the cars of his colleagues in the Iranian nuclear program, blowing them up. One bomb killed nuclear scientist Majid Shahriari as he was being driven to work in late 2010. Another nearly killed a colleague named Fereydoon Abbasi the same day. At least 11 people have been killed in unexplained attacks against Iran’s nuclear program. No one has taken credit for them; the Iranians blame Israel.

For Iran’s leaders, though, the greatest danger came not from cyberweapons or sticky bombs but from the new U.N. sanctions imposed on Iran in June 2010 with the support of the usually obstructionist Russia and China. The European Union, Iran’s most important trading partner, tightened its own sanctions in 2010 by joining the U.S. oil embargo. The U.S. Congress added to its already tough sanctions in July 2013 by closing off much of Iran’s foreign financial access. By 2013 inflation was running at nearly 40% in Tehran, joblessness was rising, and basic foods had more than doubled in price within a year.

Exhausted by their confrontations and fearing worse, the two sides started secret talks in March 2013 in Oman. Eight months later in Geneva, after the surprise election of moderate Iranian President Hassan Rouhani, the U.S. and Iran unveiled a temporary deal. In exchange for no new international sanctions and limited access to some of the country’s frozen overseas assets, the Iranians agreed to freeze their nuclear program, grant the IAEA daily access to Natanz and Fordow and get rid of all their medium-enriched uranium. Iran also agreed to halt work at a plutonium reactor at the city of Arak, temporarily closing off another nuclear pathway.

In a prime-time address, Obama called the deal “a new path toward a world that is more secure.” But there were many skeptics. Israeli Prime Minister Benjamin Netanyahu called it “the deal of the century” for Tehran because it suggested that Iran might get to keep its civilian nuclear capability. Eric Cantor, the Republican majority leader of the U.S. House of Representatives at the time, called it a “mistake,” saying it “will not stop Iran’s march toward nuclear capability.” But the agreement has been surprisingly durable as the U.S.-led sanctions coalition has held together and the IAEA says Iran has complied with its obligations.

Distrust and Verify

The deal was supposed to last only a few months while a long-term pact was hammered out. Instead, over the past 16 months, the talks have progressed in a kind of movable feast along the shores of Switzerland’s Lake Geneva, migrating from Geneva to Montreux to Lausanne. While the technical teams made progress on verification measures, the Iranian diplomats resisted key political compromises, refusing to address IAEA concerns about Fakhrizadeh’s program and resisting truly intrusive inspections. On Dec. 4 last year, 10 days after talks had been extended for a third time, the top U.S. negotiator, Ambassador Wendy Sherman, walked into a secure room on Capitol Hill in Washington for a closed-door briefing with Senators to explain why.

The wiry, white-haired Sherman is an experienced dealmaker, the No. 3 diplomat at the State Department and a veteran of nuclear negotiations with North Korea under the Clinton Administration. But observers on both sides of the aisle say she’s bad at managing skeptical members of Congress. When asked by Senators in the Dec. 4 briefing about a possible minor Iranian violation of the November 2013 deal, she said that the Iranian negotiators couldn’t have known about it. Answers like that, says one Senator who was present, “make you very concerned about our U.S. negotiators and what they deem important.” Senators worry that Iran is playing for time, extracting concessions from Sherman while giving little in return. And they worry the U.S. is being outmaneuvered by Foreign Minister Zarif.

Zarif speaks excellent English, thanks to years spent living in the U.S. and a Ph.D. in international law and policy he earned at the University of Denver. Compared with his predecessors, who could be curt and conspiratorial, Zarif is described by those who’ve watched him in action as affable, smart and tough. But he can be frustratingly theatrical and bombastic. And as a Western-educated liberal Iranian, it’s not clear that he truly speaks for the Islamic Republic’s leadership. Why, the Senators asked Sherman, should we trust him?

The U.S. negotiators insist they don’t. They say they’re focused not on showmanship but on the hard math of a deal. The U.S. says Iran must restrict all of its nuclear activities to a level that keeps it at least a year away from being able to build a nuclear weapon–time enough for the U.S. to detect it and prepare a military response. IAEA monitors must have access to every part of the program, from the uranium mines and mills to the centrifuges and the fate of any enriched uranium. They must have answers to all their questions about Fakhrizadeh’s program. And all of that has to be in writing.

In exchange, the U.S., U.N. and E.U. would temporarily remove sanctions, step by step over time. If Iran violated the deal, the sanctions would “snap back” into place without new action by the international community. The deal would last anywhere from 10 to 20 years, after which Iran would be allowed to have a civilian nuclear program under IAEA safeguards.

But Fakhrizadeh’s Orchid Office effort shows Iran is unlikely to scramble for a nuclear weapon in broad daylight. “The real problem is the covert path,” says a senior Administration official. So the U.S. is also insisting on the IAEA’s right to check out suspicious sites and any indications of covert activities anywhere in the country. The Administration argues that the best way to detect the covert path is with such an intrusive inspection regime and says that overall, the deal on the table is the best way to prevent Iran from getting a bomb–if Tehran will agree to it. “It would take multiple, highly unlikely sets of catastrophes at the same time” for the U.S. to miss an Iranian attempt to cheat and get a nuclear weapon, says the official.

Not everyone agrees. With Fakhrizadeh’s “spherical payload” theoretically a missile launch away from Tel Aviv, Netanyahu is not inclined to rely on even intrusive inspections. The Israeli Prime Minister thinks Iran can’t be trusted with any nuclear infrastructure and that all its centrifuges should be demolished. The current deal, Netanyahu told the U.S. Congress in a March 3 speech, “would all but guarantee that Iran gets those weapons–lots of them.” The only solution, Netanyahu believes, is to end the talks and impose new sanctions on Iran. In private, some Israeli officials argue that intermittent military strikes against Iran’s nuclear facilities may be the only way to ensure it won’t get a weapon. Israeli attacks could draw the U.S. into a new Middle Eastern war on unfavorable terms.

The speech solidified Netanyahu’s support in Congress, especially among Republicans who argue that real constraints on the nuclear program can be achieved only by the threat of more sanctions. After Kerry failed to reach a deal with Zarif last November, and after Sherman’s unconvincing performance at her Dec. 4 closed-door meeting, Senators from both parties began rallying behind a bill authored by Republican Senator Mark Kirk and Democratic Senator Robert Menendez that would impose new sanctions on Iran if it didn’t agree to a deal by June 30.

That has set up a dangerous moment in the talks. Senate majority leader Mitch McConnell says he may bring the Kirk-Menendez sanctions bill up for a vote as soon as March 24. The U.S. intelligence community has told Senators that Iran may claim that the bill’s threat of new sanctions breaks the November 2013 agreement, collapsing negotiations. Iran could then restart its nuclear program, dial back the IAEA’s daily access to Iran’s nuclear facilities and begin production of medium-enriched uranium immediately. If it did, it could produce enough weapons-grade uranium for a bomb in as little as two to three months, according to former Administration expert Robert Einhorn. Iran could blame America for breaching the deal and get some countries to break with the sanctions regime. But many of the Senators don’t believe the CIA’s warnings.

The fight over new sanctions has damaged the relationship between the U.S. and Israel. Senate Foreign Relations Committee chairman Bob Corker agrees with the U.S. intelligence assessment and is working to prevent a collapse in the talks. Israel’s intelligence service, the Mossad, also believes new sanctions pose a danger to diplomacy. In January, Corker requested a briefing by Mossad for six of his Senate colleagues who were traveling to Israel. When he arrived in Jerusalem to join them for the briefing, Corker was told that Netanyahu had pulled it from the agenda.

Corker threatened to cancel a meeting with Netanyahu and leave immediately; only the personal intervention of the Israeli ambassador to the U.S., Ron Dermer, persuaded Netanyahu to put the meeting back on the agenda. When the briefing went ahead, the Senators were told by Tamir Pardo, the head of Mossad, that a new sanctions bill would be like “throwing a grenade” into the U.S.-Iran diplomatic process. Netanyahu’s office declined to say why he had tried to prevent the Senators from receiving the Mossad briefing.

If Iran’s leaders had somehow missed the fact that Washington itself was divided over the merits of diplomacy, Senator Tom Cotton of Arkansas and 46 fellow Republican Senators on March 9 sent them an open letter threatening to block parts of any deal that Obama signed. One Iranian analyst worries the missive could derail talks. “It only shows to the Iranian public that the United States is untrustworthy,” says Mohammad Marandi, a Tehran academic who supports the regime.

Keep Talking

Even if the talks in Lausanne were over something comparatively simple, Kerry and Zarif would face steep odds overcoming the global network of distrust working against any agreement. In private, U.S. officials involved in the talks are skeptical, saying a long-term deal is a long shot: even if they manage to get Iran to agree in principle to intrusive inspections and “snap-back” sanctions, they’ll know they have a deal only when the details are written out before the June 30 deadline. And the ultimate success of a long-term deal depends on a more moderate Iranian regime eventually coming to power–something Washington has hoped for, largely in vain, since 1979.

With all that, it’s easy to lose sight of how far the two sides have come. Fakhrizadeh’s pursuit of a nuclear weapon remains, as far as the IAEA knows, on hold. Iran’s uranium-enrichment program has been capped and rolled back, thanks to the 2013 agreement. The U.S.-led sanctions coalition remains in place. Several key players in the talks tell TIME that barring a dramatic set of concessions by the Iranians, the best outcome would be a continuation of talks. Corker has introduced a substitute bill to replace Kirk-Menendez that would punish Iran with new sanctions only if it walks away from the bargaining table. For now, that may be as far as the U.S. and Iran are willing to distrust each other.

–WITH REPORTING BY NAINA BAJEKAL/LONDON AND KAY ARMIN SERJOIE/TEHRAN

Contact us at editors@time.com.

This appears in the March 30, 2015 issue of TIME.

SPONSORED FINANCIAL CONTENT

Read More From TIME

EDIT POST