An inside look at how the Iran talks unfolded

The shouting match — a little over a week before Tuesday’s historic deal was sealed — began after Zarif attempted to back away from some of the commitments that had previously been agreed to. Recriminations flew and both sides began yelling.

The confrontation laid bare long-simmering tensions between two countries working to overcome decades of distrust and reach a historic deal to curb Iran’s nuclear ambitions. It underscored frustrations spawned by mixed signals, shifting demands, and uncertainty on both sides over whether the opposite negotiating team really wanted a deal.

At one point, Kerry directly questioned whether Zarif had been empowered by Iranian Supreme Leader Ayatollah Ali Khameini to come to final terms, officials said Tuesday.

“The secretary asked foreign minister Zarif directly, ‘Do you have the authority to negotiate this deal and bring it to closure?’ ” said a senior administration official. “Zarif was explicit that he did.”

With the demands of their respective leaders looming in the background, the relationship between Kerry and Zarif formed the centerpiece of the exhausting, sometimes testy, and ultimately successful negotiations in Vienna that led to the history-making deal announced Tuesday.

As Kerry pursued the pact, he spent more time with Zarif than with any other diplomat, a State Department official said. But as the possibility of a deal grew, so did frustrations with Zarif, who has a flair for the dramatic that seemed fitting in the neoclassical Palais Coburg, a richly appointed hotel that was the setting for the 18-day diplomatic finale.

Zarif, in a public example of his theatrical sensibility, would take to his hotel balcony nearly each day to shout updates down to the reporters craning for snippets of news. When a European negotiator threatened to walk out on the talks if progress wasn’t made, Zarif looked at her and declared, “Never threaten an Iranian.”

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Kerry quietly began building a bridge to Iran in December 2011, when he was chairman of the Senate Foreign Relations Committee, more than a year before President Obama named him secretary of state. In a trip coordinated with the White House, Kerry boarded a commercial flight and traveled to Oman for a secret meeting with Sultan Qaboos bin Said al Said.

The wooing of the sultan began when Kerry presented him with an antique book about Frederick Law Olmsted, the famed 19th-century Boston landscape designer.

That overture led to a first meeting between Kerry and Zarif, in 2013, after Kerry had been installed at the Department of State. Obama, defying numerous critics, was convinced engagement with Iran was an important goal for US policy in the Middle East.

Conditions for negotiation improved after June 2013. Obama exchanged letters with Iran’s newly elected president, a relative moderate, Hassan Rouhani. In September 2013, Obama spoke with Rouhani by phone in the first direct contact between leaders from the two nations since 1979.

But handshakes and phone calls were one thing. Hammering out the nitty-gritty of a contentions, highly technical accord would fall to Kerry and his team at the State Department, as well as, beginning this year, a newcomer to global diplomacy, a former MIT professor, Energy Secretary Ernest Moniz.

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In January, Wendy Sherman, a high-ranking State Department official, received a message from her Iranian counterpart, Seyed Abbas. Iran was adding Ali Salehi, the chief of their atomic weapons program, to its team of nuclear negotiators and wondered if the Americans had an equivalent.

Officials at the White House and State Department tapped Moniz, whose department oversees the US nuclear weapons program. Not only did he come from a science background, as a physicist, but his calm disposition might smooth over conflicts that had developed over technical issues like limits on Iranian centrifuges, eliminating nuclear infrastructure, and transferring radioactive material.

From their first meeting, it was clear Moniz and Salehi would get along.

“If they were on an online dating site, they probably would have been matched together,” said one aide.

Salehi earned his doctorate from MIT and two of his children were born while he studied there. They exchanged gifts. Salehi brought fruits and pistachios. Moniz brought Salehi, a new grandfather, baby gifts from MIT, including a onesie with the symbols of copper and tellurium (spelling out CuTe), and a stuffed beaver, the school mascot. The Moniz-Salehi side talks helped establish a steady sense of progress on technical matters.

They met for nearly 60 hours, often in a basement room the Americans began calling Little Fordo, a reference to a nuclear development site buried within an Iranian mountain.

In contrast to some of the heated outbursts Kerry and Zarif would have, Moniz and Salehi rarely raised voices. Moniz would occasionally wander over to the Iranian dining room for dessert, where they were importing food from a local Persian restaurant. On the recommendation of Moniz — who considered the Iranian food a better option — Kerry also entered the dining room for lunch on July 4, to the surprise of some of the Iranians.

“Like President Reagan said,” Zarif said during one lull in the negotiations, remarking on the separate dining arrangements, “Tear down this wall!”

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Kerry and Zarif exchanged e-mail addresses and private phone numbers in 2013 so they could communicate privately, particularly after one or the other would make public comments about nuclear talks aimed at their respective domestic audiences.

“Both exchanged private thoughts periodically — particularly at tough stages — to try and understand why they were running into difficulty and to find a way forward, outside the spotlight or the glare of the cameras,” said a former administration official.

They referred to each other by first names, Javad and John. When Kerry broke his leg last month, one of the first messages from well-wishers he received was Zarif’s.

Kerry showed up at meetings with thick binders, scribbling out in long hand on a legal pad. Zarif usually had little more than a smartphone.

As Zarif held court from his balcony with reporters, he made faces, gestured with his hands, and provided incremental updates. Kerry, by contrast, was understated. He occasionally hobbled over to a tree-filled terrace and conferred with aides.

Over the past two weeks, Kerry and Zarif met with each other almost every day, spending about an hour together sketching out the parameters in broad brush strokes, and then leaving it to aides to follow through with details.

“He has really immersed himself in this. He’s put a lot of political capital in,” said Daryl G. Kimball, executive director of the Arms Control Association, a Washington think tank. “The time with Zarif is well spent because good diplomacy is achieved when personal rapport is achieved. That’s been critical.”

But over time, Kerry and Zarif let loose their frustrations. Twice over nearly two years of talks Kerry refused offers that Zarif said were Iran’s last and best, according to a former senior administration official.

“It was unclear whether Iran would come back to the table,” the official said. “But Kerry refused to beat his chest publicly about that, even as his aides encouraged him to do so as proof that he was tough.”

“That’s a great way to end up with nothing in the long term in return for some short-term talking points,” Kerry told his aides. “We’re not playing the short game here. The only yardstick that counts is where we are when we decide we’re through.”

The deal has been all the more complex because it involves six world powers on one side of the table — the United States is joined by Britain, China, France, Germany, and Russia — and just Iran on the other. It was often hard to keep a unified front — one aide compared Kerry to a circus performer who was spinning several plates at the same time — and the Iranians at times used those divisions to their advantage.

At one point, a former senior administration official said, Kerry told Zarif that the US position on an issue was shared by others, and they would be willing to walk away.

Zarif smiled and said, “The French companies and their ministers of finance are in Tehran planning for the day after the deal. They can’t wait to sell to Iran.”

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On Saturday, Kerry emerged into the Palais Coburg gardens, a lush and green spot where he often retreated during the negotiations to clear his head or hold a small side meeting in the fresh air. It was days after the shouting match with Zarif. A more serene mood prevailed.

Sitting in shirtsleeves in a wicker chair, he began writing notes for his “closing argument’’ with Zarif when a bride in an ornate dress passed by. Members of the wedding party returned with her, and the silver-haired politician tossed on his coat and posed for photos.

He sat back down with one of the US Senate legal pads he prefers and continued jotting notes. Using his training in legal arguments learned at Boston College, and the debating skills honed during the 2004 presidential campaign and his years in the Senate, Kerry decided to appeal to Zarif’s sense of history. He made his final pitch that night.

“He told him he had the chance to be known as the person who took the steps to ultimately end the nuclear sanctions against Iran,” a senior State Department official said.

There were still two more dramatic days left. But aides said the Saturday evening meeting was a turning point, and it began to show that both sides were invested in the deal and ready to try and close it. By Tuesday morning, Kerry and Zarif wore exhausted smiles.

Kerry called Zarif “a tough, capable negotiator, and patriot, a man who fought every inch of the way for the things he believed.’’

“Even when there were times of a heated discussion,’’ he added, “I think he would agree with me at the end of every meeting we left with a smile and with a conviction that we were going to come back and continue.’’

Matt Viser can be reached at mviser@globe.com. Follow him on Twitter @mviser.