As the two rivals came close to fighting an all-out war, here's a look back at how Tehran's nuclear ambition first began—with help from Washington.
Iran's desire to invest in a nuclear program dates back to the 1950s, when the country's autocratic ruler Reza Shah Pahlavi, or the Shah of Iran, signed a treaty with the US in which Washington would help Tehran to build its nuclear program.
As the Iranian Revolution toppled the Shah's regime in 1979, taking Iran away from America's ambit and turning it from a friend to foe, its nuclear ambitions became the cause of major concern for Washington.
Fast forward to 2020, amid heightened tensions between the two rivals after the US killed Iran's towering military commander Qasem Soleimani, Tehran gave strong indications that it will not abide by the nuclear deal, officially known as the Joint Comprehensive Plan of Action (JCPOA). Iran insists that it is no longer bound by the limits imposed on the numbers or types of centrifuges it can operate or the level of enrichment of uranium it can pursue.
As the West has once again sharpened its focus on Iran's nuclear program, fearing that Soleimani's killing has given Tehran a reason to roll back on the promises it made under the JCPOA, here's a quick look at how the country attained its first nuclear arsenal before it was transformed into an anti-American Islamic Republic.
As part of US President Dwight Eisenhower’s Atoms for Peace program, Iran, a US ally under the Shah's rule, received nuclear assistance from Washington from 1957 to 1979 for 'peaceful' purposes.
The US also helped Iran to develop its first nuclear reactor, the Tehran Research Reactor (TRR), and supplied enriched uranium to fuel the plant. Iran was also provided with "hot cells" by the US, which are critical for the production of plutonium, an essential ingredient for developing nuclear weapons.
While Tehran operates several research reactors, the TRR is still the largest one with a five megawatt-thermal (MWth), pool-type light water research reactor.
Under Eisenhower’s program, Israel, India and Pakistan received the same technology, such as small reactors and their own dollops of fuel. However, following the oil boom in the 1970s, Iran’s nuclear program was converted into a fully-fledged civilian nuclear program.
At the time, Tehran had money to gather nuclear knowledge by sending scholars and students to attend the Massachusetts Institute of Technology and study nuclear engineering. They returned to Iran as trained nuclear scientists and technicians and became indispensable professionals for the country's nuclear program.
In the 1970s, the US officials started thinking that their procurement might be a mistake and feared that Tehran may turn toward building nuclear weapons.
US officials started negotiating to limit Tehran’s nuclear capability. The Shah however said: “Unless it was clear Iran was not being treated as a second-class country, I would look for alternative vendors and I would not work with US companies to acquire nuclear technology for Iran."
In the following term, Iran bought nuclear plants from France and West Germany. The nuclear reactor provided by the US continued to function at Tehran University. But soon after, the university became the bastion of Iranian revolution in 1979.
With the Shah gone and the new conservative regime taking control of the country, Tehran’s Atomic Energy Organisation paid $5.5 million to Argentina’s Applied Research Institute in order to convert fuel for the reactor from 93 percent enriched uranium - which is weapons-grade - to slightly less than 20 percent enriched.
Between 1988 and 1992, Iran conducted covert plutonium reprocessing experiments using fuel pellets irradiated in the TRR.
By the early 1990s, Iran's quest for acquiring nuclear weapons came close to completion as the TRR began conducting laboratory-scale plutonium-reprocessing experiments, allegedly violating the nuclear safeguards failures according to the IAEA. By 2003, Iran acknowledged that it had used the TRR to produce small amounts of polonium-210 - a well-known radioactive material used in a neutron initiator that starts the chain reaction in a nuclear weapon.
However, Iran maintained that the polonium was produced as part of a study to support thermoelectric generators and meet its energy needs and not for building weapons.
After years of trading allegations, the US and Iran finally found a way to sort out differences over Iran's nuclear ambitions, with Tehran agreeing to put a cap on uranium enrichment and reversing all the gains it had made to transition an energy-oriented program into a military one.
But when Donald Trump came to power, the new US president called JCPOA a "bad deal", re-introducing harsh sanctions against Tehran. The escalation reached a dramatic turn with the killing of Soleimani last week and it remains to be seen whether Tehran will return to the 2015 nuclear agreement or completely walk away from it.