Iranians take to the streets of Tehran on July 14 to celebrate their country’s historic nuclear accord.
Majid Saeedi
By Massimo Calabresi
July 16, 2015

As Secretary of State John Kerry and his Iranian counterpart, Foreign Minister Mohammad Javad Zarif, sat in Room 103 of the Palais Coburg Hotel in Vienna just before midnight on July 13, 20 months of negotiations hung on a set of issues that had nothing to do with nuclear weapons. Iran had agreed to most of the provisions of the 100-page deal hours earlier, committing Tehran to deep and unprecedented constraints on its atomic program. But Zarif had demanded that U.N. limits on Iran’s sale and purchase of conventional weapons be lifted. The dispute had become so heated that the diplomats had asked their staffs to wait outside the room.

Kerry couldn’t agree to give Iran immediate access to conventional arms. Just 15 months earlier, on March 5, 2014, the Israeli navy had intercepted an Iranian-supplied tanker, the Klos C, carrying scores of rockets and mortars and nearly 400,000 rounds of ammunition hidden in crates of cement bound for the anti-Israel forces of Hamas in the Gaza Strip, according to Kerry’s own State Department. Iran is also arming Syrian dictator Bashar Assad in a war that has killed more than 200,000 people and fueled the rise of the terrorist group ISIS. In Yemen, Kerry says, Iran is flying supplies several times a week to rebel forces that toppled the U.S.-backed government and are openly at war with Washington’s longtime regional ally Saudi Arabia.

With Iran’s nuclear concessions hanging in the balance, and Russia’s top diplomats for the first time siding with Iran and angrily pushing for a deal, Zarif met Kerry halfway. Under the agreement, Iran would be allowed to purchase and sell conventional weapons again in five years if the rest of the pact were implemented. With that and the other final issues resolved, the two men stood up, shook hands and went to tell their leaders in Tehran and Washington they had a deal.

The last-minute haggling in Room 103 was more than just the dramatic climax of the long-running nuclear talks. It framed the gamble at the heart of the historic agreement President Barack Obama announced to the world hours later from the White House. The deal represents a strategic trade-off between Iran and the five nuclear powers that emerged to shape the global order after World War II as well as the nonnuclear Germany. Iran agrees to strict controls of its nuclear program, including 10-to-15-year limits on production of nuclear fuel and tough, permanent international inspections. In exchange, the Islamic Republic gets increased regional leverage, establishing itself as an expanding power in the Middle East, flush with cash and arms, and with international recognition as a potential nuclear power.

In his White House statement, Obama emphasized what he saw as the strategic importance of the deal. The great danger of our time, he said, “is that nuclear weapons will spread to more and more countries, particularly in the Middle East, the most volatile region in our world.” Today, he continued, “because America negotiated from a position of strength and principle, we have stopped the spread of nuclear weapons in this region.” In the wake of the deal, senior Administration officials say the President hopes to not just contain Iran but also set a new standard for nuclear arms control, reversing decades of spreading nuclear know-how.

That optimism makes others wary. The deal’s critics worry that it could pave the way for a spiraling nuclear confrontation in the Middle East and beyond. Iran is already the world’s leading state sponsor of terrorism and is involved in every single serious conflict in the Middle East, usually on the side of America’s enemies. Whether it finds a way around the deal’s constraints or simply waits 10 years for them to begin to expire, Tehran will only get stronger with time. Saudi Arabia has said it will not stand by while Iran gains nuclear capabilities, raising the prospect of an atomic standoff between the region’s two ancient enemies. “Instead of stopping the spread of nuclear weapons in the Middle East,” said Republican House Speaker John Boehner, “this deal is likely to fuel a nuclear arms race around the world.”

Boehner’s opinion matters, as Congress can block the deal if a veto-proof majority opposes it in the next 60 days. For Obama, that debate represents a notable triumph after years of patient diplomacy. As a newly elected President, he promised to pursue a peaceful solution with Iran and grounded his entire national-security strategy on revived international nuclear agreements. He has delivered on his promise of diplomacy. The question now is: Will it work?

Late in the administration of George W. Bush, French intelligence analysts poring over satellite photos of Iran spotted unusual construction work in a mountain at the town of Fordow outside the holy Iranian city of Qum. Soon multiple foreign intelligence services were covertly trying to verify the West’s suspicion: Iran was trying to build a massive uranium-enrichment facility in violation of its international treaty commitments.

As a candidate for President, Obama had promised to try nuclear diplomacy with Iran, and when Bush’s intelligence officials briefed his team on Fordow in late 2008, he wasn’t dissuaded. But after nine months of public and private outreach failed to entice Iran into negotiations over its nuclear program, Obama publicly revealed the existence of the Fordow facility in the middle of an international summit in September 2009. At the time, it seemed like a death knell for negotiations. By the end of the year, Israel was threatening to attack Iran and the U.S. was scrambling to find a way to avoid getting drawn into war.

As it happened, the Fordow revelation helped push the major powers to realize the world had reached a turning point. After the fall of the Soviet Union in 1991, a brief spurt of arms control brought the number of countries known to have nuclear weapons down from 14 to eight. But in the decades since, North Korea had defied diplomatic and economic pressures and gone nuclear. Iraq and Syria had been stopped only by military intervention. If Iran were allowed to build a weapons program, its regional enemies including Saudi Arabia, Turkey and Egypt might all go nuclear too. The nuclear club that had maintained a relatively stable balance of power for decades was in danger of collapse.

On June 9, 2010, the big powers pushed back: Germany joined the five permanent members of the U.N. Security Council–Russia, China, Britain, France and the U.S.–to impose tough new sanctions on Iran. Even more surprising, they enforced the sanctions with unity and resolve over the following five years, even though it cost them in trade and diplomatic ties. The reason was basic power politics: all five had an interest in trying to reimpose the exclusivity of the nuclear club. “The Russians disagree with us on Ukraine and Syria and a whole host of issues,” says the senior Administration official, but “this was the one issue on which the great powers were aligned: that it would be bad if Iran got nuclear weapons.”

The 2010 U.N. resolutions imposed sanctions on Iran’s banking and financial sectors and blocked its ability to get paid for oil sales. That cut Iran off from its main source of income, but Tehran was defiant. Rather than backing down on its nuclear program, it sped it up. By 2013, it had enough enriched uranium for around eight nuclear bombs, if it chose to refine its stockpiles, and the technical capacity to generate fuel for the first bomb within two months. As recently as 2003, the country had been testing how to make and then fit a nuclear warhead atop a missile. Once again the talk turned to the possibility of a war against Iran to attempt to take out its nuclear facilities.

But the sanctions did their work. With Iran’s inflation rate topping 40% in 2013, Tehran secretly agreed to negotiate, and the U.S. dispatched diplomats to Oman to initiate covert talks about a nuclear deal. Three months later, the talks got a boost with the election of moderate Iranian President Hassan Rouhani, a longtime believer in diplomacy, who appointed the Western-educated Zarif as his Foreign Minister. In November 2013 the nuclear powers, Germany and Iran announced a deal that would freeze Iran’s nuclear program where it was in exchange for the major powers’ agreeing not to impose any new sanctions on the country.

Twenty months later, after Kerry’s last, late session with Zarif, the details of the deal were unveiled. Iran agreed to reduce the amount of uranium it keeps on hand for the next 15 years to no more than half of what it would take to make a bomb. It agreed to remove two-thirds of its 2,700 uranium-refining centrifuges from Fordow and to stop refining there. It promised to operate only about 5,000 at an aboveground site. Those and other constraints mean that for a decade, Iran would remain a year away from having on hand the material needed for a nuclear weapon.

Most important, Iran agreed to tough, permanent international monitoring, including new protocols that allow for visits by International Atomic Energy Agency (IAEA) inspectors to suspicious sites. Critics fear that Iran will cheat and secretly go nuclear, as it has tried to in the past. They worry that Iran can hide parts of a covert program and stall inspectors by using the deal’s lengthy, bureaucratic dispute-resolution mechanism. The Administration says between the IAEA inspections and the intense scrutiny by national intelligence services of every part of Iran’s program, from uranium mining, milling and refinement to international procurement, it will be nearly impossible for it secretly to get a nuclear weapon.

For Obama and his team, the deal was a vindication. “The agreement proves that you can use diplomacy to bring countries back into compliance with the nuclear nonproliferation regime,” says the senior Administration official. Some on the outside agreed. “This moment could represent a seminal achievement in the history of nuclear nonproliferation negotiations,” wrote Ilan Goldenberg and Avner Golov of the Administration-friendly Center for a New American Security in National Interest magazine. “The United States should take the most positive elements of the agreement with Iran and turn them into global best practices.”

In Tehran, the news of the deal was met with unalloyed joy. Some youths danced in public squares, while others drove through the streets honking horns and hanging out the windows of their cars. Many were simply relieved at the prospect of economic relief for the country; others saw larger benefits. “Iran is now at its peak of power in centuries,” said veteran Iranian diplomat Sadegh Kharrazi on state television. “This is why the world superpowers have been negotiating with us for so long. That’s why we were able to reach a deal which guarantees our interests.”

Indeed, for all the nuclear concessions Iran promises under the deal, it arguably emerges stronger than it was when Obama first unveiled the existence of Fordow in 2009. When the agreement is implemented and verified by the IAEA, which could come as soon as December, Iran will get access to more than $100 billion in frozen overseas assets. And once the U.S. and U.N. lift the ban on bank transfers, Iran can expect as much as an additional $20 billion in oil revenue per year, according to some estimates.

That money means power, and not the soft kind. Last November, Iran’s Supreme Leader Ayatullah Ali Khamenei tweeted nine points about eliminating Israel. The tanker that Israel seized in March 2014 in the Red Sea was the fourth Gaza-bound arms shipment intercepted by Israel since 2002, Israeli officials say. Iran has given hundreds of millions of dollars to Hamas over the past decades, according to testimony by U.S. and other officials. Iran has been directly violating U.N. Security Council resolutions since 2006 through its support for Lebanese Hizballah; one Iranian general declared last November that the organization is effectively one with the Iranian military.

Against weaker foes, Iran has even more influence. In neighboring Syria, Tehran has spent more than $1 billion to prop up the Assad regime, according to documents leaked to the Israeli paper Ha’aretz by the hacking collective Anonymous. In April, Kerry told the PBS NewsHour, “There are a number of flights every single week that have been flying in” from Iran to provide military assistance to Houthi rebels in Yemen. Concludes Iranian diplomat Sadegh Kharrazi: “Iran’s sphere of influence stretches from the Mediterranean to the Indian peninsula, from Kazakhstan to Yemen.”

Which is precisely what worries America’s regional allies. Israeli Prime Minister Benjamin Netanyahu called the deal “a stunning historic mistake.” The Saudis privately have expressed repeated and increasingly desperate concerns to Obama and his aides about Iran’s growing influence. After the deal, one Saudi official told Reuters that he feared the deal would allow Iran “to wreak havoc in the region.” The danger, said Abdulaziz Sager, head of the Gulf Research Center, a Jidda-based think tank, is that Iran will use sectarian divides to push “Saudi Arabia to go into war by proxy.”

That is already happening. The multiple conflicts that have erupted in the wake of 2011’s so-called Arab Spring nearly all break down along sectarian lines, with Iran backing Shi’ite Muslims and Saudi Arabia and others backing Sunnis. For years the U.S. has been closely allied with many of the Middle East’s ruling Sunni families, even as elements of those regimes directly or indirectly backed al-Qaeda and ISIS with money, weapons and training. Some see a natural alliance between Iran and the U.S., or at least see a more powerful Iran as balancing out dominant Sunni power.

Obama has downplayed in public the prospect of improved U.S.-Iran relations, but his top aides say the deal opens the space for closer ties if Iran were to change its behavior. They believe the country’s pro-Western youth and even some in its leadership would like to pursue the kind of openness that could change the balance of power in a region frozen in antagonism since 1979. But few are holding their breath. In Iraq, Iran has been fighting on the same side as the U.S. against the ISIS militants who are attempting to overthrow the government. The relationship is uneasy, and both sides insist there is no coordination. In Syria, Yemen and Lebanon, sectarian fighting appears only to benefit terrorist groups of all stripes, whether al-Qaeda, ISIS or the Shi’ite groups Iran supports. And those in favor of confrontation in Iran remain powerful: just last April, Iranian naval vessels repeatedly threatened U.S. vessels in the Gulf.

All of which means anything that advances the Iranian-Saudi competition may end up looking more like an arms race than a balance of power. In May, Saudi officials meeting with Obama said they would match whatever nuclear capability Iran gained through negotiations. With their historically close ties to nuclear Pakistan, the Saudis could start down that road with one phone call. “We can’t sit back and be nowhere as Iran is allowed to retain much of its capability and amass its research,” a Saudi official told the New York Times.

If Obama’s marathon diplomatic outreach to Iran has achieved its goal, the effort to convince everyone else of the deal’s merits is just getting started. His message to Iran’s Gulf enemies like Saudi Arabia, Bahrain and Kuwait is that there is no good alternative to diplomacy with the Islamic Republic. “The President said you can’t put your head in the sand and pretend that Iran doesn’t play a role in every single important conflict in the region,” says one Administration official involved in the diplomacy with Iran’s neighbors. “There can’t be a solution without them.”

More immediately, the deal faces a huge test back home. The Republican chairman of the Senate Foreign Relations Committee, Bob Corker of Tennessee, says he will hold multiple hearings on the deal, after he and others read the classified annexes he says are crucial to understanding it. Corker helped the Administration keep the talks on track last winter but says he is skeptical the deal “actually meets the goal of preventing Iran from obtaining a nuclear weapon.” Then, during the long August recess, both sides will take the fight out into the country. Most of the 16 GOP candidates running for President will oppose the deal, which will put additional pressure on Republicans in Congress. “Opponents of the deal are going to marshal tens of millions of dollars to oppose this thing,” says one senior Administration official.

If Obama can sustain a veto in the face of a congressional effort to block the deal, the question is what happens then. Iran’s hard-liners appear to have accepted the agreement, after Khamenei endorsed it in a tweet. In private, Obama’s diplomatic team admits the deal may just push the problem of Iran’s nuclear status down the road 10 years. But in the meantime, they hope, progress can be made toward bringing Iran further into the international mainstream. Perhaps Iran’s pro-Western youth and moderate political leaders can change the country in that time. Even if that doesn’t happen, Administration officials say, at least the big, insurmountable disagreement between Iran and the rest of the world will be off the table, clearing the way for diplomacy on other fronts, like Syria, Yemen and Iraq.

Most of all, they say there is no other viable choice, since walking away from a deal would make an Iranian dash for a bomb more likely. If getting breathing room for diplomacy and increased monitoring of Iran’s nuclear program means giving it a little more power regionally, for the Administration, it’s a risk worth taking. “That may sound like a big gamble, except when compared to the alternative, which is not stopping Iran from crossing the nuclear threshold and over-turning the current order as a nuclear weapons state,” says the senior Administration official.

And as a final fail-safe, Obama says that if the IAEA finds Iran has broken the terms over the next decade, the big-power sanctions can “snap back” into place. That process is untested, and somewhat vague in the deal. More important, after 10 years, Iran can again approach the verge of nuclear-power status without asking anyone’s permission. At that point, Iran may be a very different kind of regional power–for better or worse.

–WITH REPORTING BY KAY ARMIN SERJOIE/TEHRAN AND ZEKE MILLER AND MAYA RHODAN/WASHINGTON

Contact us at editors@time.com.

This appears in the July 27, 2015 issue of TIME.

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