The nuclear challenge at the heart of Trump's Iran negotiations

Low-enriched uranium - typically 3-5% purity - is enough to produce reactor fuel required for a nuclear power station, but weapons-grade uranium needs to be at least 90% enriched.

The International Atomic Energy Agency (IAEA), the UN nuclear watchdog, reported that Iran was complying with the agreement until the US withdrew from it in 2018.

"The deal was remarkably successful," argues Kelsey Davenport from the Arms Control Association (ACA), a national nonpartisan membership organisation.

"Any move to nuclear weapons, any deviation from the JCPOA's terms would have been detected," Davenport told BBC Verify, noting that both the IAEA and US intelligence repeatedly assessed Iran was complying.

In an April 2018 report, the US Department of State said Iran was "transparently, verifiably, and fully implementing the JCPOA".

However, when President Trump announced the US withdrawal from the agreement in May 2018, he called it a "horrible, one-side deal that should never, ever have been made".

He said it failed to address Iran's ballistic missile programme, that the inspection requirements lacked mechanisms "to prevent, detect, and punish cheating" and that Israeli intelligence showed Tehran's "history of pursuing nuclear weapons".

Jacob Olidort, chief research officer at the America First Policy Institute, says Trump was right.

"All of these issues were completely pushed to the sidelines, completely deprioritised and not included in the arrangement", he told BBC Verify.

Baroness Ashton, who negotiated the deal on behalf of the UN Security Council, rejects this.

"There was always a criticism that we should have covered all kind of things. But the critical question was, 'Could we prevent any fear that Iran was going to build a nuclear weapon?' And we did that."

"There was plenty of opportunity afterwards to talk about other issues, ballistic missiles, drones etc. And indeed the Trump administration in its first term could have done that," she adds.

"If President Trump felt that the deal was inadequate, then the answer was to build on it, not to rip it up."

Olidort says the time-limited nature of the deal meant Iran could have eventually pursued a nuclear weapon.

"It was always made explicit in the deal that the terms of the deal would expire… The sunset clauses in effect nullify their effectiveness," he argues.

Davenport says that because some limits on the uranium enrichment level and stockpile size were only set for 15 years, "by January 2031, Iran could theoretically expand its enrichment programme".

But many other features were permanent, including IAEA safeguards, she said, adding: "There was still a whole host of other provisions that would have provided assurance that any move in that direction [towards a nuclear weapon] would have been quickly detected".

Money has been a recurring theme of Trump's criticism, taking aim at former President Barack Obama multiple times.

"Obama signed that stupid deal where he paid them billions, and billions of dollars. He thought he could bribe them," he told NBC on 7 June.

Under the JCPOA, Iran gained access to billions of dollars in previously frozen assets and benefited from the lifting of international sanctions.

Baroness Ashton, however, says sanction relief was a necessary requirement to secure the agreement.

"If you sanction someone because they're doing some behaviour and they change the behaviour, then by definition the sanction cannot stay."

Olidort argues lifting sanctions helped Iran fund its conventional weapon programmes in addition to its nuclear one.

In the years after the US withdrew from the agreement, Iran began to accelerate its uranium enrichment programme.

In June 2022, the IAEA assessed that Iran held 43.1kg (95lbs) of 60% enriched uranium.

The US and Israel attacked Iran's facilities in June 2025, which American officials said significantly set back the prospect of Tehran building a nuclear weapon.

At the time of the attacks, the IAEA estimated Iran had obtained 440.9kg (972lbs) of ⁠uranium enriched up to 60% purity.

The head of the IAEA, Rafael Grossi, told the Associated Press in April that the majority of Iran's highly enriched uranium (roughly 200kg; 440lbs) was likely located in underground tunnels at its Isfahan nuclear complex, about 273 miles (440km) south of Tehran.

Grossi said inspectors had been unable to verify the site since the conflict began.

"We haven't been able to inspect or to reject that the material is there and that the seals - the IAEA seals - remain there," he said.

Grossi added the the IAEA also wanted to inspect Iran's nuclear facilities at Natanz and Fordo.

In a Truth Social post last month, Trump declared his envisaged deal would be "far better" than the JCPOA.

While the exact terms are unclear, Davenport argues Iran will expect to benefit economically from any agreement and that Trump may attempt to resist any comparison with the JCPOA given his past criticism of sanctions relief.

"Iran is not going to agree to a deal that does not include sanctions relief and assets or access to its frozen assets. Tehran has made very clear that those are key issues", she says.

Trump will likely want to show he secured concessions that Obama could not, she adds. That could include a temporary suspension of enrichment and the disposal of Iran's existing stockpile.

"He's going to point to two factors to claim victory, that he cleaned up a mess that he'll say accelerated under Biden and he got what Obama couldn't, which was a multi-year suspension".

However, Davenport also points out that Iran's nuclear programme is very different from the one negotiators faced in 2015 - due to the apparent destruction of most of its enrichment capacities - making it hard to draw direct comparison with the JCPOA.

Olidort believes the US is negotiating from a position a strength and does not see a deal being weaker than the JCPOA.

Iran "is in a much more weakened state from... its capabilities perspectives, but also the state of different proxies in the region".

While the details of any agreement remain unclear, Baroness Ashton argues that military pressure alone is unlikely to secure a lasting settlement.

"All I can say is in my experience, the way that negotiations work is that people have to feel that they've got enough to make it worthwhile participating in that negotiation".