In a hard-won deal struck in 2015, Iran agreed to freeze its nuclear programme in return for the lifting of punishing international sanctions.
It was a breakthrough that ended a 12-year standoff between Iran and the West, concerned Tehran was developing a nuclear bomb.
On Tuesday US President Donald Trump is due to announce his verdict on the US sanctions relief underpinning the accord, which he insists was "very badly negotiated."
Here is some background:
21 months of talks
Negotiations start in June 2013 between Iran and the five permanent members of the UN Security Council — Britain, China, France, Russia and the United States — plus Germany.
By November they reach an interim deal that is finalised in April 2015 and signed on July 14 that year.
The UN Security Council adopts the deal on July 20, 2015 and it comes into force on January 16, 2016.
The accord brings to a minimum of one year, for at least 10 years, the "breakout time" that Iran needs to produce enough fissile material to make an atom bomb.
Among other points, Tehran agrees to slash the number of centrifuges, which can enrich uranium for nuclear fuel as well as for nuclear weapons, from more than 19,000 to 5,060, maintaining this level for 10 years.
The deal limits all enrichment to only one facility.
It also stipulates that Iran's pre-deal stockpile of 12 tonnes of low-enriched uranium — enough for several nuclear weapons if further enriched — must be reduced to 300 kilogrammes (660 pounds) for 15 years.
Only enrichment to low purities is allowed, also for 15 years.
The International Atomic Energy Agency (IAEA) is charged with regular inspections of facilities such as uranium mines and centrifuge workshops for up to 25 years.
UN inspectors have certified Iran's compliance with the deal nine times, most recently in November.
The accord paved the way for a partial lifting of international sanctions on Iran, opening the door for foreign investors with French energy giant Total and carmakers PSA and Renault quick to strike deals.
UN embargoes on the sale of conventional arms and on ballistic missiles to Iran have however been maintained up to 2020 and 2023 respectively.
May 12 ultimatum
In October 2017, Trump refuses to certify that Iran is respecting its commitments on the agreement but he does not re-impose sanctions or abandon the deal itself.
Trump again waives sanctions in January but he demands that European partners "fix the terrible flaws" in the accord before May 12, the next decision deadline for Trump on the deal.
Ahead of the deadline the US leader says he will announce his decision Tuesday, putting an abrupt end to efforts over the past weeks by the UK, Germany and France to save the deal.
"If the United States leaves the nuclear agreement, you will soon see that they will regret it like never before in history," Iranian President Hassan Rouhani says on Sunday.