Iran signed the Nuclear Non-Proliferation Treaty in 1968 and ratified it two years later. By the mid-1970s, Shah Mohammad Reza Pahlavi had approved plans for up to twenty-three nuclear power stations. Western companies rushed in. The German firm Kraftwerk Union, a joint venture of Siemens and AEG, signed a contract to build two pressurized water reactors at Bushehr for an estimated four to six billion dollars. France granted Iran a ten percent stake in the Eurodif uranium enrichment consortium. In 1976, President Gerald Ford signed a directive offering Iran the opportunity to buy and operate a US-built plutonium reprocessing facility. A 1974 CIA assessment observed bluntly that if the Shah remained in power into the mid-1980s, and if other countries like India had pursued weapons, Iran would follow suit. The revolution of 1979 ended these plans before most of them materialized.

The Islamic Revolution severed nearly every international nuclear partnership Iran had built. Kraftwerk Union halted work at Bushehr in January 1979, with one reactor half complete and the other eighty-five percent finished. The United States cut off highly enriched fuel for the Tehran research reactor. Eurodif stopped deliveries. Iran's new leaders initially showed little interest in nuclear technology, but by 1981 they concluded the program should continue. Iraq's use of chemical weapons during the Iran-Iraq War proved decisive in shifting attitudes. Former president Akbar Hashemi Rafsanjani later acknowledged that during the war, Iran considered pursuing weapons of mass destruction as a deterrent against the possibility that Saddam Hussein might develop nuclear weapons. Through the late 1980s and 1990s, Iran rebuilt quietly, obtaining centrifuge designs from the Abdul Qadeer Khan network in Pakistan and establishing a joint research organization with Russia called Persepolis.

In 2002, Iranian dissidents revealed what intelligence agencies had missed: undeclared uranium enrichment facilities at Natanz and a heavy water reactor under construction at Arak. The IAEA investigation that followed found Iran had systematically failed to report nuclear activities for years. Under pressure, Iran briefly suspended enrichment in 2003 and signed the IAEA Additional Protocol. The pause collapsed after Mahmoud Ahmadinejad's election in 2005. Iran resumed enrichment, the UN Security Council imposed sanctions beginning in 2006, and Tehran steadily expanded its capabilities. In 2009, the existence of the Fordow underground enrichment plant, built into a mountain near the holy city of Qom, was publicly revealed. The Stuxnet cyberattack in 2010 damaged Iranian centrifuges but did not halt the program. By the early 2010s, Iran's estimated breakout time to produce enough material for a weapon had shrunk to a few months.

The Joint Comprehensive Plan of Action, reached in 2015 between Iran and the P5+1 nations, represented a diplomatic achievement years in the making. Iran agreed to cap uranium enrichment at 3.67 percent, reduce its enriched uranium stockpile to 300 kilograms, and redesign the Arak reactor to limit plutonium production. In exchange, broad international sanctions were lifted. The IAEA verified Iranian compliance. The arrangement held for three years. In May 2018, the United States unilaterally withdrew, citing Iran's missile program and regional activities. Reimposed American sanctions hammered the Iranian economy. Beginning in 2019, Iran responded by methodically exceeding JCPOA limits, enriching uranium to levels far beyond what the agreement allowed and installing advanced centrifuges at Fordow and Natanz.

As of 2025, Iran's nuclear program is significantly more advanced than it was a decade earlier. The IAEA found Iran non-compliant with its nuclear obligations for the first time in twenty years. Iran retaliated by announcing a new enrichment site and installing additional advanced centrifuges. In June 2025, Israeli and then American military strikes targeted the Fordow facility near Qom. The physical infrastructure can be rebuilt or relocated. The knowledge accumulated over seven decades of nuclear work cannot be unbombed. The program that began with an American handshake in 1957 has become one of the defining challenges of international security, its facilities scattered across a country roughly twice the size of Texas, buried in mountains and woven into the fabric of Iranian national identity.