VIENNA, Jan 16 — The United States and European nations lifted oil and financial sanctions on Iran and released roughly $100 billion of its assets after international inspectors concluded that the country had followed through on promises to dismantle large sections of its nuclear program.
This came at the end of a day of high drama that played out in a diplomatic dance across Europe and the Middle East, just hours after Tehran and Washington swapped long-held prisoners.
Five Americans, including a Washington Post reporter, Jason Rezaian, were released by Iran hours before the nuclear accord was implemented. The detention of one of the released Americans, Matthew Trevithick, who had been engaged in language studies in Tehran when he was arrested, according to his family, had never been publicly announced.
Early on Sunday, a senior United States official confirmed that “our detained U.S. citizens have been released and that those who wished to depart Iran have left.” The Washington Post also released a statement confirming that Mr. Rezaian and his wife, Yeganeh Salehi, had left Iran.
“Iran has undertaken significant steps that many people — and I do mean many — doubted would ever come to pass,” Secretary of State John Kerry said Saturday evening at the headquarters of the International Atomic Energy Agency, which earlier issued a report detailing how Iran had shipped 98 percent of its fuel to Russia, dismantled more than 12,000 centrifuges so they could not enrich uranium, and poured cement into the core of a reactor designed to produce plutonium.
But Mr. Kerry was clearly energized by the release of the Americans, an issue he took up on the edges of almost every nuclear negotiation, and pursued in separate, secret talks that many involved in the nuclear issue were only vaguely aware were happening.
The release of the “unjustly detained” Americans, as Mr. Kerry put it, came at some cost: Seven Iranians, either convicted or charged with breaking American embargoes, were released in the prisoner swap, and 14 others were removed from international wanted lists. Many of the presidential candidates, including Senator Marco Rubio of Florida and Donald J. Trump, denounced the swap as a sign of weakness, and they have long promised to review or withdraw from the nuclear agreement.
They particularly object to the release of about $100 billion in frozen assets — mostly from past oil sales — that Iran will now control, and the end of American and European restrictions on trade that had been imposed as part of the American-led effort to stop the program. It was not only sanctions that forced Iran to the table: the United States and Israel also developed one of the world’s most sophisticated cyberweapons to destroy the centrifuges that Iran has now been dismantling.
With the start of the so-called implementation day, the day that the accord goes fully into operation, the structures are finally in place for Tehran to re-engage with the world after decades of isolation.
But even in a week that started with the release of 10 sailors who drifted into Iranian waters — the Defense Department still has not provided an explanation of how that happened — and ended with a prisoner swap that seemed drawn from the pages of the Cold War, it was far from clear whether Tehran would choose to re-engage — at least very quickly.
In Tehran and Washington, political battles are still being fought over the merits and dangers of moving toward normal interchanges between two countries that have been avowed adversaries for more than three decades. But Mr. Kerry suggested that the nuclear deal had broken the cycle of hostility, enabling the secret negotiations that led up to the hostage swap. It was far from a sure thing: Just weeks ago, Iran was demanding the release of nearly 20 Iranians convicted or indicted in the United States; an administration official said that number had been whittled down to seven, but even that still rankled some.
“Critics will continue to attack the deal for giving away too much to Tehran,” said R. Nicholas Burns, who started the sanctions against Iran that were lifted Saturday as the No. 3 official in the State Department during the George W. Bush administration. “But the fact that Iran’s nuclear ambitions will be effectively frozen for the next 10 to 15 years is a real advantage for us,” he said, adding that “it was achieved by tough-minded diplomacy and not war.”
Still Mr. Burns, who now teaches diplomacy at Harvard and has advised Hillary Clinton, a Democratic candidate for president, argued that recent encounters with Iran — including its ballistic missile tests and its propping up of President Bashar al Assad of Syria, “demonstrate how complicated our relationship with Iran will continue to be.” He urged President Obama to issue new sanctions against Iran this weekend for the ballistic missile tests — a violation of United Nations Security Council resolutions — to demonstrate that he will keep up the pressure.
A copy of the proposed sanction leaked three weeks ago, and the Obama administration pulled it back — perhaps to avoid torpedoing the prisoner swap and the completion of the nuclear deal. Negotiations to win the release of Mr. Rezaian, who had covered the nuclear talks before he was imprisoned on vague charges, were an open secret: Mr. Kerry often alluded to the fact that he was working on the issue behind the scenes.
Mr. Rezaian was held in Tehran’s notorious Evin Prison for practically all of his incarceration, and spent the first several months in solitary confinement. He suffered vision problems and relatives said he lost about 40 pounds. It wasn’t until about a year ago that the Iranian authorities publicly explained the nature of the charges against him.
His mother, Mary Rezaian, who lives in Turkey and went to Tehran last June to be closer to her son, said then that he had only just become aware of the global support for him, including an appeal for his release from Muhammad Ali, the former heavyweight boxing champion. Mr. Rezaian’s mother said she was permitted to visit him only occasionally and said it had become “ever harder” for him to cope.
Then, several weeks ago the Iranians leaked news that they were interested in a swap of their own citizens, which American officials said was an outrageous demand, because they had been indicted or convicted in a truly independent court system.
But behind the scenes, one senior American official said, “it was clear this would be the only way.” There was discussion inside the administration of similar swaps during the Cold War, a practice moviegoers have been reminded of recently in “Bridge of Spies,” about the negotiations to win the release of Francis Gary Powers.
Mr. Kerry insisted that the two sets of negotiations were completely separate, but he acknowledged they were related: The intense diplomatic contact with Iran — Mr. Kerry has spent more time with his American-educated Iranian counterpart, Mohammad Javad Zarif, than any other foreign minister — made the prisoner talks possible.
The result was two parallel races underway — one involving implementing the nuclear deal, the other to get the prisoner swap done while the moment was ripe. The Iranians beat, by months, the C.I.A. and Energy Department estimates of how long it would take them to dismantle the far-flung elements of its nuclear empire, a long checklist that occupies scores of pages in the nuclear accord.
“They were highly motivated to get it done,” said one American official who was closely involved, because President Hassan Rouhani wanted money flowing into Iran, and more oil flowing out, before a critical election next month.
But there were last minute hitches. For example, the United States and Iran were struggling late Saturday to define details of what kind of “advanced centrifuges” Iran will be able to develop nearly a decade from now — the kind of definitional difference that can undermine an accord.
The result was that Mr. Kerry and Mr. Zarif veered from the monumental significance of what they were accomplishing — an end to a decade of open hostility — to the minutiae of uranium enrichment.
But Mr. Kerry emerged to tell reporters he had reached the goals he has talked about for two years. “Each of the pathways that Iran had to a nuclear weapon have been verifiably closed down,” he declared. Noting that Tehran has frozen much of its activity during the negotiations, he responded to critics of the deal — including, without naming them, the Republican presidential candidates — who say that Iran will immediately cheat.
“We have now two years of compliance under our belt,” he said. “Obviously, past performance does not guarantee future results.” But, he argued, “we know without doubt that there is not a challenge in the entire region that wouldn’t become much more complicated if Iran had the ability” to produce nuclear weapons.”
But Iran has something it desperately needs: Billions in cash, at a time oil shipments have been cut by more than half because of the sanctions, and below $30-a-barrel prices mean huge cuts in national revenue.
Just how much cash is a matter of dispute. A senior American official said Saturday that Iran will be able to access about $50 billion of a reported $100 billion in holdings abroad, though others have used higher estimates. The official said Iran will likely need to keep much of those assets abroad to facilitate international trade.
The Obama administration on Saturday also removed 400 Iranians and others from its sanctions list and took other steps to lift selected restrictions on interactions with Iran. Another 200 people, however, will remain on the sanctions list under for other reasons, including terrorist activities, human rights abuses, involvement in civil wars in Syria or Yemen or ties to the country’s ballistic missile program.
Under the new rules put in place, the United States will no longer sanction foreign individuals or firms for buying oil and gas from Iran. The American trade embargo remains in place, but the government will permit certain limited business activities with Iran, such as selling or purchasing Iranian food and carpets and American commercial aircraft and parts.
It is an opening to Iran that represents a huge roll of the dice, one that will be debated long after Mr. Obama he has built his presidential library. It is unclear what will happen after the passing of Iran’s Supreme Leader, Ayatollah Ali Khamenei, who has protected and often fueled the hardliners — but permitted these talks to go ahead.
The president and Mr. Kerry, with a year and four days left in office, are hoping to foster new discussions that will bear fruit in other areas, including ending the war in Syria and moving, slowly, to the eventual restoration of diplomatic relations.
Rick Gladstone and William J. Broad contributed reporting from New York, Peter Baker from Washington and Thomas Erdbrink from Tehran.