Ever since the United States, alongside Israel, went to war against Iran in late February, U.S. President Donald Trump has struggled to define the objectives of the conflict. His emphasis has shifted from demands for regime change to degrading Iran’s military to securing safe passage through the Strait of Hormuz. Most frequently, he has seemed to emphasize a need to roll back Tehran’s nuclear program. It is an odd frame for a war that has barely touched Iranian nuclear capabilities, but a fitting one for a leader who withdrew from President Barack Obama’s 2015 nuclear deal with Iran, the Joint Comprehensive Plan of Action, in 2018 and claimed a year ago to have “obliterated” Iran’s program. “The DEAL that we are making with Iran will be FAR BETTER than the JCPOA,” Trump boasted on Truth Social in late April.

But achieving such an agreement will be more difficult than the president seems to realize, in large part because Iran’s nuclear capabilities have advanced since Trump withdrew the United States from the 2015 treaty during his first term. Public reports suggest that ongoing U.S.-Iranian negotiations over Tehran’s program are focused on two elements: the length of a moratorium on its uranium enrichment activities and the fate of its stockpile of highly enriched uranium. Both are necessary components of any successful future nuclear deal, but they are also insufficient. Over the past seven years, Iran has markedly improved its ability to manufacture and install more powerful enrichment centrifuges, shrinking the time needed to produce the material for a nuclear weapon. And there are now more gaps in international inspectors’ knowledge about the extent of the program.

For bragging rights, Trump doesn’t only need a deal that differs from the JCPOA; he needs a dramatically different one. A deal in 2026 must go well beyond addressing enrichment and stockpiles; it must also create new, detailed procedures that allow inspectors to understand Iran’s current capabilities and to prevent the country from making covert progress toward a weapon. Without addressing these concerns, it doesn’t matter how much Washington bombs Iran, or how long an enrichment moratorium lasts, or what happens to the country’s highly enriched uranium. Tehran could emerge from the war closer to a nuclear weapon than it was before.

Last June, Iran’s enrichment program was severely degraded by U.S. and Israeli strikes in what is now known as the 12-day war. Nuclear scientists were killed, and enrichment capacity at the country’s underground Fordow and Natanz facilities was badly damaged or destroyed. Yet Iran’s nuclear program still poses a greater challenge now than it did in 2015, when the last nuclear treaty was finalized. The pact capped not only the number of centrifuges Iran could operate but also, and more important, the kinds it could operate or manufacture. And it prevented Iran from carrying out certain research that would advance its centrifuge program. After the United States withdrew from the deal, these provisions were no longer implemented.

In the years that followed, Iran acquired technical knowledge in centrifuge production and operation that cannot be bombed away. By June 2025, its best centrifuges were roughly six times as efficient as the ones that existed in 2015. Iran also improved its installation speed: the rate at which a country can produce material for nuclear weapons depends not only on how many and what type of centrifuges it has but also on how quickly it can install additional ones into “cascades,” or networks of connected devices that accelerate the concentration process. In 2015,........

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