The United States and Iran took a historic step towards ending more than three decades of estrangement when President Barack Obama and Iranian counterpart Hassan Rouhani spoke by phone and agreed to work on resolving global suspicions that Tehran is trying to build a nuclear weapon.
The 15-minute call capped a week of seismic shifts in the relationship that revolved around Mr Rouhani's participation in the annual United Nations meeting of world leaders.
The night before the leaders spoke, US and European diplomats hailed a "very significant shift" in Iran's attitude and tone in the first talks on the nuclear stand-off since April.
The diplomatic warming began shortly after Mr Rouhani's election in June. But it is rooted in both presidents' stated campaign desires - Mr Obama in 2008 and Mr Rouhani this year - to break through 34-year-old barriers and move towards diplomacy.
Iran is also seeking quick relief from blistering economic sanctions imposed on Tehran for refusing to scale back its nuclear activities. Iran insists its nuclear programme is for peaceful purposes only, but years of stonewalling inspections and secrecy about its activities have fuelled fears it is seeking to build warheads.
Mr Rouhani and Mr Obama spoke while the Iranian president was in his car and heading to the airport to fly back to Tehran, with Mr Obama at his desk in the Oval Office. Mr Rouhani's aides initially reached out to arrange the conversation, and the White House placed the call.
The last direct conversation between the leaders of the two countries was in 1979 before the Iranian Revolution toppled the pro-US shah and brought Islamic militants to power.
Mr Obama said the long break "underscores the deep mistrust between our countries, but it also indicates the prospect of moving beyond that difficult history".
"While there will surely be important obstacles to moving forward, and success is by no means guaranteed, I believe we can reach a comprehensive solution," he said at the White House.
Iran's nuclear programme has been a major concern not only to the United States but to other Middle Eastern nations - especially Israel - and to the world at large.
Earlier, at a news conference in New York, Mr Rouhani linked the US and Iran as "great nations", a remarkable reversal from the anti-American rhetoric of his predecessors, and expressed hope that at the very least the two governments could stop the escalation of tensions.
"I want it to be the case that this trip will be a first step, and a beginning for better and constructive relations with countries of the world as well as a first step for a better relationship between the two great nations of Iran and the United States of America," Mr Rouhani said at the end of his four-day debut on the world stage at the annual UN General Assembly.
Iran scholar Gary Sick at Columbia University described the events as "breathtaking" and said the weeks of slow warming led to yesterday's dramatic step.
"This is part of a pattern that has led to a real breakthrough," he said. "And basically what's happening is that the ice that has covered the US-Iran relationship for over the last 30 years is starting to break. And when ice starts to break up, it goes faster than you think."
But the groundwork for the detente was set years ago.
During his 2008 presidential campaign, Mr Obama repeatedly said he would be willing to negotiate with Iran to ease tensions and move towards a nuclear settlement. That fell by the wayside, however, when Mahmoud Ahmadinejad was re-elected president in 2009 in a disputed vote that spurred the worst domestic unrest since the 1979 Iranian Revolution and, in turn, a violent crackdown on the political opposition.
The nuclear talks between Iran and world powers have stagnated since then, prompting a series of blistering economic sanctions on the Islamic Republic that have slashed oil exports, made it difficult to conduct international bank transfers, drastically driven up inflation and devalued the currency.
Mr Rouhani took office on August 4 after overwhelmingly defeating several conservative candidates in the first round of elections on a promise to seek relief from the sanctions - and has said he has "full authority" from supreme leader Ayatollah Ali Khamenei to do so.
Khamenei may be mindful that the pressure of sanctions could fuel a wave of unrest like the one in 2009 and experts believe this is one reason he appears to have given his blessing to Mr Rouhani to pursue negotiations.
Mr Rouhani is no stranger to the nuclear talks. In 2003 he was on the Iranian team that negotiated a settlement with European nations under which it agreed to an additional safeguards protocol, suspend enrichment and allow additional inspections by the International Atomic Energy Agency.
The deal fell through when Iran reneged on its pledge to allow inspectors unfettered access to the nuclear sites, and then it withdrew from the protocol.
In a sign of modernisation in Iran, the news broke on Twitter a couple of minutes before Mr Obama spoke, in an account that people close to Mr Rouhani say is written by a former campaign aide who remains in close contact with the president's inner circle.
A Rouhani adviser said the president did not tweet himself or direct what was written. The White House said that the tweets were an accurate description of the call.
The two men talked through interpreters, but the tweet from @HassanRouhani said they ended by signing off in each other's language.
"In a phone conversation b/w #Iranian & #US Presidents just now: @HassanRouhani: 'Have a Nice Day!' @BarackObama: 'Thank you. Khodahafez,'" the tweet said, quoting Mr Obama as using the Farsi word for goodbye.
Both sides said the presidents directed their top diplomats, Zarif and Kerry, to continuing pursing an agreement, with Iranian and UN officials agreeing to meet again on October 28.