Here is a snapshot:

- US withdrawal -

On May 8, 2018, President Donald Trump withdraws the US from the deal negotiated between Iran and the five permanent members of the UN Security Council (Britain, China, France, Russia and the United States) plus Germany.

"We cannot prevent an Iranian nuclear bomb under the decaying and rotten structure of the current agreement," Trump says.

- 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, seeking to pressure the European signatories to help it get round the sanctions.

Trump then 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.

- More steps -

In September, 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 Fordow plant in its fourth walk-back and says its heavy water reserves have passed the accord's limit.

- More centrifuges -

Tensions between Washington and Tehran spiral after a January 2020 US drone strike kills top Iranian commander Qassem Soleimani in Baghdad, sparking a tit-for-tat confrontation.

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

- Dispute mechanism -

In mid-January, the European signatories trigger a dispute mechanism set out in the deal, accusing Iran of repeated violations, while saying they remain committed to the accord.

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

On March 31, European signatories say they have delivered medical goods to Iran in the first transaction under the Instex mechanism set up to bypass the US sanctions.

- Over the limit -

On May 27, Washington ends sanctions waivers for nations that remain in the accord, which allow companies, primarily from Russia, that are still present in Iran to carry out the agreement.

On June 5, the IAEA says Iran has accumulated enriched uranium at nearly eight times the limit of the accord and has for months blocked inspections at sites where historic nuclear activity may have occurred.

Iran's highest level of enrichment in the stockpile is 4.5 percent, over the deal's limit of 3.67 percent, according to its report.

On June 19, IAEA governors pass a resolution critical of Iran, the first of its kind since 2012, urging Tehran to provide its inspectors with access to two historic sites.