Iranians celebrated on the streets of Tehran following the announcement that a framework agreement has been reached on Iran's nuclear program between the country and the major world powers. (The Washington Post)

It started with a simple handshake. On Sept. 26, 2013, the top diplomats of the United States and Iran — bitter foes for nearly 35 years — exchanged awkward greetings on the sidelines of a U.N. conference in New York.

Though it lasted only minutes, the encounter between Secretary of State John F. Kerry and Foreign Minister Mohammad Javad Zarif was a breakthrough of sorts: the highest-level exchange between a U.S. and an Iranian official in decades. It would be followed by phone calls, secret talks and hours of direct negotiations, all culminating in Thursday’s historic framework agreement on limiting Iran’s nuclear program.

The ultimate success of the nuclear deal may not be known for months or even years. But for Iranians, the negotiations have already produced something unparalleled since the birth of the Islamic Republic: direct engagement with a country that Iranian leaders have long denounced as “the Great Satan.”

Statements by U.S. and Iranian officials made it clear that the two countries are nowhere close to normalizing relations. But to longtime Iran observers, the talks themselves were a milestone achievement, a pivotal point in Iran’s post-revolutionary history and the beginning of a new chapter in the country’s relations with the outside world.

Highlights of the Iran deal

“We have entered uncharted territory,” said Karim Sadjadpour, an Iran expert at the Carnegie Endowment for International Peace. “A decade ago, you could go to prison for advocating dialogue with the United States. Now that taboo has been smashed.”

Tehran and Washington remain sharply at odds on a host of issues, from Iranian support for militants to the detention of U.S. citizens in Iranian prisons. President Obama, in announcing the agreement, acknowledged that it “will not end the deep divisions and mistrust between our two countries.”

Yet the deal represents a remarkable shift for a country that, just three years ago, was firing at U.S. surveillance drones over international waters in the Persian Gulf and allegedly plotting to assassinate a Saudi diplomat by bombing a restaurant in Washington. While Iran’s security establishment and many of its leading clerics remain vehemently anti-American, ordinary Iranians will now see rapprochement with the West as not only possible but desirable, said Trita Parsi, president of the National Iranian American Council.

“For many people, this closes the chapter on 1979,” said Parsi, referring to the year when Iranians overthrew the country’s pro-American ruler, Shah Mohammad Reza Pahlavi, allowing the fiercely anti-American Ayatollah Ruhollah Khomeini to come to power. “It’s not that the revolution is undone, but the enmity with the United States is finally neutralized.”

For most of the past three decades, that enmity not only defined relations between the United States and Iran, but also served as a rallying cry, particularly at times when the regime faced external or domestic crises. The defining event in the Islamic Republic’s history was the storming of the U.S. Embassy in Tehran on Nov. 4, 1979, and the seizing of American hostages, 52 of whom were held for more than 14 months in a crisis fueled by rage against the United States for its support of the shah’s corrupt and brutally repressive regime.

Every year since 1979, tens of thousands of Iranians have participated in rallies commemorating the anniversary of the embassy takeover, burning U.S. flags and chanting, “Death to America.” One of the largest rallies in recent years occurred in 2013, the same year Iranians overwhelmingly elected moderate cleric Hassan Rouhani as president.

The hostage ordeal marked the end of diplomatic relations between the two countries, but it was only the first of a long series of crises, from allegedly Iranian-sponsored terrorist attacks on U.S. Marines and diplomats in Lebanon to the downing of an Iran Air passenger jet by the guided missile cruiser USS Vincennes in 1988.

Since 2003, the tensions have centered on Iran’s nuclear ambitions, following allegations of secret nuclear-related research programs by Iran’s military and the discovery of partially constructed, heavily fortified facilities intended to produce enriched uranium. Iran has maintained that it has a right to develop a peaceful nuclear energy program, but Western suspicions about Iran’s nuclear intentions elevated concerns about a possible military conflict.

As nuclear talks between Iran and six world powers stumbled, the United States and its allies sought to slow the nuclear program through other means imposed by the Obama administration, from covert cyberattacks to sweeping economic sanctions that slashed Iran’s oil profits and crippled its banking sector. At least five Iranian nuclear scientists were killed by unknown assassins in the space of two years.

The first real signs of a thaw emerged after the surprise election of Rouhani, a moderate who campaigned for president on a promise to improve relations with the West and win relief from economic sanctions. The historic first handshake between Zarif and Kerry took place a month after Rouhani’s inauguration. Two months after that, on Nov. 24, 2013, Kerry, Zarif and diplomats from five other world powers signed an agreement freezing Iran’s nuclear program and setting the stage for the comprehensive framework announced Thursday.

Iran experts noted that the agreement sidestepped major obstacles to genuinely improved U.S. relations with the Iran. The negotiators did not, for example, discuss the growing crisis over Iran’s support for Houthi rebels who this year overthrew Yemen’s Sunni-led government in Sanaa.

“This understanding is unlikely to lead to immediate changes in Iran’s domestic and foreign policies,” said Ali Nader, an Iran analyst for the Rand Corp. “But for now, this is an important breakthrough.”

Whether Iranians will see tangible benefits from the accord — and if they do, how quickly — is unclear. Negotiators deferred a final decision on the timing for easing Western sanctions against Iran, meaning that improvements in Iran’s chronic jobless rate and high inflation could be many months away.

Thus, while many Iranians reacted jubilantly Thursday to news of the agreement, the euphoria could quickly turn to despair if the nuclear deal fails to translate into economic improvements for ordinary Iranians, said Mehdi Khalaji, a senior fellow with the Washington Institute for Near East Policy.

“People need to see change,” Khalaji said. “If Rouhani is not able to do much in the short term, it will lead to popular disappointment and strengthen the hand of hard-liners.”

But other observers believe that the diplomatic opening achieved over the past 18 months will be difficult to reverse. After years of sanctions, many ordinary Iranians are more than eager to see their country shed its isolation and experience freedoms that residents of other countries take for granted, said Haleh Esfandiari, director of the Middle East Program at the Woodrow Wilson International Center for Scholars, a Washington think tank.

“The population is dying for a deal,” Esfandiari said. “They are saying, ‘We are sick and tired of being treated as a pariah state. We are sick and tired of being marginalized.’ ”

Iran’s most powerful leaders, including Ayatollah Ali Khamenei, the supreme leader, will likely continue to resist change, and even Rouhani may feel he lacks the capital to undertake major reforms of the country’s foreign policy. Yet, sanctions relief is likely to provide “some space to Iranians who want an end to Iran’s isolation,” said Nader, the Rand expert.

“The conservatives won’t be able to blame the nuclear crisis with the West for the intense atmosphere of repression they have maintained for the past decade,” Nader said.

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