Enrichment to levels suitable for domestic nuclear power (the goal, and follow on decoupling from Russia as the supplier and extractor of fuel for existing Iran nuclear power station) is a magnitude and more less time and effort than enrichment to levels suitable for weapons.
Isotope separation by centrifuge as a physical process follows the laws of diminishing returns, getting rid of all the uranium variations save the rare target weight takes more and more time as percent purity increases.
"Just a step away" was more a hard bridge to cross back when third party inspectors were at the enrichment centres and leaving locked and logged "long soak" spectrometer instruments behind. It's hard to enrich to greater levels without leaving a ratio fingerprint behind in the gamma spectrum.
"The revelation that Iran had built major nuclear facilities in secret, without required disclosure to the IAEA, ignited an international crisis and raised questions about the program's true aim."
That's a tad Descartes before the hordes .. the response that situation in 2002 was the Joint Comprehensive Plan of Action of 2015 which had plenty of carrots, sticks, and ability to peer into dark places .. but not real support from Isreal or the US who scuppered the plan under Trump.
You are aware the deal was entirely dependent on Russia, and the follow-on Biden wanted to sign (but couldn't since Iran wouldn't fully cooperate with IAEA) involved Russia even more heavily? There's no other place that both sides accept can store the enriched Uranium or supply fuel rods to Iran.
>Isotope separation by centrifuge as a physical process follows the laws of diminishing returns
It's the other way around. Going from 3 to 20 percent is much harder than 20 to 60 which is harder than 60 to 90. Going to 99.9999% would be tough, but is unnecessary even for nukes.
>"Just a step away" was more a hard bridge to cross back when third party inspectors were at the enrichment centres
They were allowed to enrich to that level under the deal starting in 2031, inspections would have tested if the enriched material was diverted.
Even if they could be effective at such short notice, it would have taken the US being distracted by some other crisis and being unable to act in the short period between detection and weaponization to lead to a nuke.
Isotope separation by centrifuge as a physical process follows the laws of diminishing returns, getting rid of all the uranium variations save the rare target weight takes more and more time as percent purity increases.
"Just a step away" was more a hard bridge to cross back when third party inspectors were at the enrichment centres and leaving locked and logged "long soak" spectrometer instruments behind. It's hard to enrich to greater levels without leaving a ratio fingerprint behind in the gamma spectrum.