Despite frantic, overnight negotiations, peace talks between Iran and the United States have broken down. The two sides had no shortage of dispute to settle, and so it was always going to be hard for them to forge a permanent settlement to their war. But one issue, above all, appears to be responsible for the failure: Iran’s nuclear energy program. “The meeting went well, most points were agreed to,” U.S. President Donald Trump wrote on social media. “But the only point that really mattered, NUCLEAR, was not.”

It is not surprising that the nuclear issue is Trump’s main focus, or that it is why talks collapsed. Managing Iran's nuclear ambitions has been a defining challenge of global diplomacy for decades. But during both of his terms in office, the United States has tried to force Iran to fully give up its nuclear program through economic strangulation and military action. And each time, it has failed. “We will ensure that Iran does not obtain a nuclear weapon,” Trump said on February 28—the day Washington began bombing Tehran. But six weeks later, the fundamental challenge remains. The war may have dealt immense damage to Iran, but it has not erased the country’s underlying nuclear knowledge or its long-term capacity to rebuild the program.

This danger is now more acute politically, even if Iran’s near-term technical capacity has been badly disrupted. The lesson many in Tehran may draw from the war is not that restraint brings security, but that vulnerability invites attack. That does not mean a rapid or covert sprint to a nuclear weapon is likely–any serious move to reconstitute such a capability would take time and would be highly detectable. It does mean, however, that the argument for retaining the option of a future deterrent is likely to have grown stronger.

These results confirm what should have been clear from the start: diplomacy is the only viable way to ensure that Iran’s nuclear energy program is peaceful. It has, after all, worked before. For more than a decade, U.S. diplomats joined their counterparts from China, France, Germany, Russia, the United Kingdom, and the European Union to pursue a negotiated agreement with Iran on its program. The result was the 2015 Joint Comprehensive Plan of Action (JCPOA)—in which Iran set verifiable limits on that program in exchange for the lifting of sanctions. One of us, Mogherini, led the negotiations and implementation of that deal; the other, Shah, has spent years working on the policy architecture surrounding it. This diverse group of states negotiated with Tehran not because they trusted it or because they were naive about the complicated nature of the regime or because they believed diplomacy alone could resolve every concern they had. They did so because they understood that the alternative to diplomacy is the chaos and destruction playing out now.

The JCPOA, of course, did not last. Less than two years after taking office, in 2018, Trump unilaterally abrogated the deal even though the International Atomic Energy Agency certified that Iran was in compliance and even though every other party wanted it to last. But this failure is not a case against trying again. In fact, it means Washington must go further this time around by creating systems that make a deal more durable and thus much harder for any party to back out of it. Doing this may not please those who see Tehran as fundamentally untrustworthy and hope to bludgeon it into surrender. But this war has proved that Washington can’t force Tehran into submission. To prevent the country from pursuing nuclear weapons, the United States has to reach an agreement with the Islamic Republic. And given that Iran might now be more incentivized than ever to get a deterrent, Washington must make sure that the next deal works.

The U.S.-Israeli war against Iran was, from the start, illegal and reckless. American and Israeli officials claimed that bombing the Islamic Republic was essential to ensuring it does not obtain a nuclear weapon. But there was no evidence that Tehran posed an imminent nuclear weapons threat or that diplomacy was ineffective. In fact, talks were actively underway, and multiple parties involved in........

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