United States President Donald Trump was widely expected to decline to renew a 120-day waiver of sanctions that America's Congress imposed on Iran in 2012 - meaning, in effect, that the US is walking out of a multilateral nuclear deal with Iran.
Under the Joint Comprehensive Plan of Action (JCPOA), which took effect in 2015, international sanctions were lifted in exchange for Teheran suspending its nuclear programme.
The deadline for Mr Trump's signature to extend the waiver on sanctions is May 12, but on Monday, he tweeted that he would announce his decision yesterday afternoon.
Besides the US, China, France, Germany, Russia, the United Kingdom and the European Union all signed the deal with Iran. In recent weeks, the US' European allies France and Germany have tried to persuade Mr Trump to stay in the deal.
Analysts warn that a US pullout from the deal may lead to negative consequences. For one thing, it will give Iran an opportunity to exploit divisions between the US and its European allies, analysts say.
A US withdrawal from the deal would also ironically strengthen hardliners in Iran who did not like it in the first place, believing that Iran took a bad deal which restricted its sovereign right to have nuclear weapons - a position that most of the population would support, Iran experts say.
"The moderates and reformists, championed by President Hassan Rouhani and his chief negotiator, Foreign Minister Javad Zarif, continue to hail the deal as a win-win solution to the nuclear crisis," Dr Ariane Tabatabai, a senior associate with the Proliferation Prevention Programme at the Centre for Strategic and International Studies, wrote last month in The Bulletin of Atomic Scientists.
For them, the agreement removed the international sanctions that had stifled the economy, and paved the way for Iran's re-entry into the world community while ridding it of the threat of war.
"But Iranian hardliners see the nuclear deal differently," wrote Dr Tabatabai, who is also a visiting assistant professor at Georgetown University's Edmund A. Walsh School of Foreign Service. "From their point of view, the country made a number of concessions that weakened its hard-won nuclear infrastructure, for very little in return."
Separately, a diplomat in Washington, DC, told The Straits Times that the gap between moderates like Mr Rouhani and so-called hardline conservatives, like Supreme Leader Ali Khamenei, on the issue of the nuclear programme was not very wide.
"At the end of the day, Rouhani or any of these people who are relatively moderate compared to the Supreme Leader Ali Khamenei, cannot survive in that system unless they adapt to a harder line on the nuclear programme," he said.
"And if the clerical establishment and Rouhani say we believe we must have a nuclear programme, they will have most of the people with them automatically, without coercion; it is a symbol of national security and sovereign rights," he added.
However, there is a possibility that Mr Trump may leave room to negotiate a deal that addresses his concerns.
Mr Trump, who calls the Iran deal one of the "worst deals ever", has objected to Iran's ballistic missile programme; its influence in Yemen and Syria and Lebanon; and the deal's "sunset" provisions which, in 10 to 15 years, would allow Iran to gradually restart its nuclear programme.
In a speech last October, he said: "In just a few years, as key restrictions disappear, Iran can sprint towards a rapid nuclear weapons breakout."
He added: "In other words, we got weak inspections in exchange for no more than a purely short-term and temporary delay in Iran's path to nuclear weapons."
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Curbs on Teheran's nuclear programme
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• Formally titled the Joint Comprehensive Plan of Action, the Iran deal was signed in 2015. It placed strict limits on Iran's nuclear programme to keep it from developing nuclear weapons.
• Six nations signed the deal with Iran: the United States, Britain, France, Germany, China and Russia. It was approved by the United Nations Security Council.
• Iran got relief from some economic sanctions in return for allowing inspections and the destruction of nuclear equipment. The International Atomic Energy Agency, which enforces the deal, and US officials say Iran complies with its terms.
• The US Congress has passed numerous sanctions against Iran but given the president the power to waive their enforcement. They come up for waiver or renewal every few months, and the next deadline is Saturday.
• President Donald Trump has criticised the deal and threatened to stop issuing the waivers, meaning they would be reimposed on Iran. He says the deal does not address Iran's ballistic missile programme or its role in wars in Syria and Yemen, and does not permanently prevent Teheran from developing nuclear weapons.
• If Mr Trump reimposes sanctions, it would place the US in breach of the deal. If they were reimposed, they would penalise countries or firms buying oil from Iran by making it hard for them to do business with US banks and markets.
Mr Trump may not immediately reimpose sanctions targeting Iran's Central Bank. And a dispute resolution mechanism gives parties to the JCPOA 35 days to consider claims of violations.
But Mr Rouhani has said Iran wants the JCPOA or nothing at all.
Last year, Israel's Institute for National Security Studies ran an exercise to simulate outcomes from the US walking out of the JCPOA.
Its conclusion: The US "might well find itself, in a relatively short time, faced with a number of unsavoury choices, from settling for achievable but far less ambitious goals on Iran's nuclear programme, to prolonged tensions with key allies, to a dramatic escalation with Iran at a time when other priorities, like North Korea, make that undesirable".
