It was all about centrifuges, industrial spinning tubes designed to enrich uranium and crucial to making a nuclear bomb.
Five years ago, Iran was set to spin thousands of them, but the deal brokered in 2015 stopped almost all of them and Iran’s nuclear programme was frozen.
But Donald Trump has never liked it, once even labelling it as “insane”. And today he has the chance to pull his country out of the Iran nuclear agreement for good.
Foreign Secretary Boris Johnson is amongst many who have urged him to stay along with the other signatories. The leaders of Israel and Saudi Arabia have egged him on to pull out.