Here is a snapshot:

- US withdrawal -

On May 8, 2018, President Donald Trump withdraws the US from the deal, saying, "We cannot prevent an Iranian nuclear bomb under the decaying and rotten structure of the current agreement".

Iran has always denied its nuclear programme has any military dimension.

Tehran urges the remaining parties -- Britain, China, France, Germany and Russia -- to salvage the deal.

- US sanctions -

In August and November 2018, Washington reimposes sanctions on Iran and companies with ties to it, notably hitting Iran's vital oil sector and central bank.

Major international firms halt their activities and projects in Iran.

In May 2019, Washington ends its sanctions exemptions for countries buying Iranian crude.

- Iran starts walk-back -

Iran in May 2019 announces its first step back from the deal, suspending commitments to limit its heavy water and enriched uranium stockpiles.

Trump sanctions Iran's steel and mining sectors.

In July, Tehran says it has exceeded the accord's restrictions on its enriched uranium reserves and uranium enrichment level.

- 'Highest sanctions' -

In September, tensions soar after aerial attacks on two major Saudi oil facilities, blamed on Tehran, which denies involvement.

Trump announces new sanctions on Iran's central bank.

- Iran takes more steps -

Later that month, the International Atomic Energy Agency (IAEA) nuclear watchdog says Iran has started using advanced centrifuges to enrich uranium.

On November 4, Tehran says its enrichment increased tenfold and that it has developed two new advanced centrifuges.

Later that month, it resumes enrichment at its underground Fordo plant in its fourth walk-back and says its heavy water reserves have passed the accord's limit.

In December, Britain, France and Germany accuse Iran of developing nuclear-capable ballistic missiles. Tehran dismisses the charge.

- Brink of war -

Tensions between Washington and Tehran spiral after a January 2020 US drone strike kills top Iranian military commander Qasem Soleimani in Baghdad, sparking a tit-for-tat confrontation in which Iran fires missiles at two US bases in Iraq.

A Ukrainian plane is then brought down "unintentionally" by an Iranian missile on January 8, killing all 176 on board.

Iran announces its fifth step back from the deal on January 5, forgoing a limit on its number of centrifuges.

- Dispute mechanism launched -

In mid-January, the deal's European signatories trigger its dispute mechanism, accusing Iran of repeated violations, while saying they remain committed to the deal.

Iranian President Hassan Rouhani responds that Iran's daily uranium enrichment is higher than it was before the deal was concluded.

Later that month, Tehran threatens to walk out of the Non-Proliferation Treaty (NPT) on nuclear weapons if the dispute goes before the UN Security Council, which could reimpose international sanctions.

On February 14, Iran demands significant economic advantages from Europe in return for cancelling all or part of its rollback measures.

- Enriched uranium stocks -

On March 3, the IAEA says Iran's stockpile of enriched uranium is more than five times the deal's limit and that it has breached the 3.67 percent threshold on uranium enrichment.

It says that Iran refused access to two sites that it wished to visit in January as part of its nuclear verification mission.