A weakened Iran still has a major deterrent: the nuclear option

For months, Iranian officials have said that Tehran has accumulated most of the knowledge needed to build a weapon and that it might reconsider Supreme Leader Ayatollah Ali Khamenei’s two-decade-old pledge not to procure weapons of mass destruction.

In late September, a former head of Iran’s atomic agency, Fereydoun Abbasi, suggested that Tehran could start producing 90% enriched, weapons-grade uranium. U.S. officials have said it would take Iran less than two weeks to convert its current 60% nuclear-fuel stockpile into weapons-grade material.

The 2015 nuclear deal curbed Iran’s program in return for sanctions relief. In the years since the U.S. pulled out, Tehran has significantly advanced its program, leaving it on the cusp of being able to develop a nuclear weapon.

“The weakening of its capabilities versus Israel will force Tehran to develop new sources of deterrence, increasing pressure on expanding the nuclear program," said Gregory Brew, senior analyst on Iran and energy at the Eurasia Group consulting firm. “What we’re likely to see is more pressure to advance the program and warnings that it may not stay ‘peaceful.’"

How close is Iran to a nuclear weapon?

While it claims its nuclear program is for purely peaceful purposes, Iran is the only nonnuclear weapons power that produces highly enriched uranium. It currently has enough near-weapons-grade fuel for almost four nuclear weapons, according to the most recent data from the International Atomic Energy Agency. Iran also has been conducting experiments with uranium metal, a key component of a nuclear weapon, and has cut back much of the international oversight granted by the nuclear deal.

U.S. intelligence officials and the IAEA no longer offer what were once standard assurances that Tehran isn’t working on a weapons program. U.S. officials said this summer that Tehran had begun activities to gain more of the knowledge needed to build a bomb. Iran’s weaponization work would be harder to swiftly detect. Some experts believe Iran could produce a crude nuclear device in a matter of months.

What could Israel and the U.S. do in response?

Israel has in the past taken action against Iran’s nuclear program through sabotage without permanently setting back Tehran’s enrichment efforts. Still, a dash for a nuclear weapon—which the U.S. has vowed to prevent—would carry huge risks for Iran.

Israel has demonstrated deep intelligence penetration into Iran and its allies through its recent attacks, including one that killed Hamas political leader Ismail Haniyeh in a military-run guesthouse in Tehran. Israel could choose to strike Iran’s nuclear sites if Tehran accelerates its nuclear work.

Israeli Prime Minister Benjamin Netanyahu said last week that Iran had “made a big mistake" with its missile attack and would pay for it. In a separate speech last week directed at the Iranian people, he said, “There is nowhere in the Middle East Israel cannot reach."

Hezbollah, the world’s most heavily armed nonstate militia, has long been seen as Iran’s main insurance policy. The Lebanese group, which is designated by the U.S. as a terrorist organization, is positioned on Israel’s border. But the recent Israeli military campaign has weakened the group, killing its longtime chief, Hassan Nasrallah, and targeting its weaponry. Last week, Israel launched the first ground operation into southern Lebanon in about two decades to attack Hezbollah positions in villages near the border.

Iran’s other major threat is its estimated stockpile of at least 3,000 ballistic missiles. While a number of missiles made it through Israel’s air defenses in a 180-missile barrage last week, they did little damage. Tehran has threatened to unleash a bigger attack.

After Iran’s first-ever direct attack on Israel—an April barrage of more than 300 missiles and drones—Israel responded with a small warning shot, knocking out air-defense batteries located just 13 miles from Iran’s Natanz nuclear-enrichment facilities.

The head of Iran’s atomic agency, Mohammad Eslami, said last week that the country’s armed forces had created the necessary deterrence against what he said were Israeli threats against his country’s nuclear facilities.

Israel so far has chosen not to act alone in striking Iran’s nuclear facilities. While the Biden administration has said the U.S. would ensure that Iran faces severe consequences for last week’s attack, there is no appetite in Washington for a broader war, particularly ahead of November’s presidential election.

Israeli airstrikes would damage Iran’s nuclear enrichment sites, but fully disabling its nuclear program would pose significant challenges, said people familiar with Iran’s nuclear program. It is unclear how deeply Israeli strikes could penetrate new facilities Tehran has built deep underground at Natanz or the Fordow site, which is dug into a mountainside.

There are challenges beyond the fortifications, the people said. Iran holds more than five tons of enriched uranium—including 165 kilograms of highly enriched uranium—outside of its enrichment facilities and next to mobile vans that can move it quickly. Airstrikes would also be of limited value against the work Tehran would need to do to build a bomb—its potential weaponization efforts. In the past, Israel is believed to have assassinated several of Iran’s top nuclear scientists as a way to prevent that work.

The risk of setting off race to a bomb

An attack that merely pushes back Iran’s nuclear program by months or a year would be a major gamble. Iranian officials have previously warned that an attack could prompt them to follow in North Korea’s footsteps and quit the treaty that ensures international oversight of nuclear programs.

“The big question then will be whether Iran withdraws from the Non-Proliferation Treaty and kicks out the inspectors," said Eric Brewer, deputy vice president at the Nuclear Threat Initiative’s Nuclear Materials Security Program.

An Iranian race for a nuclear bomb could force the U.S. to act to stop Tehran from attaining a nuclear weapon.

Iran may decide for now to continue its incremental approach of inching closer to nuclear-weapons capabilities, said Michael Horowitz, the Israel-based head of intelligence for the consulting firm Le Beck.

“Iran will focus on its own security first, before taking any drastic and risky steps, including building a bomb," he said.

A nuclear push also comes with political liability for Iran. Recently elected President Masoud Pezeshkian intends to use diplomatic outreach to win relief from Western sanctions on Iran’s ailing economy. Any hope of achieving that likely hinges on Iran’s taking steps to end its production of highly enriched uranium and granting improved access for the United Nations’ nuclear agency, the IAEA.

“I don’t think Iran would have made the decision to retaliate without some level of introspection when it comes to its nuclear program," said Nicole Grajewski, fellow at the Nuclear Policy Program at the Carnegie Endowment for International Peace.

“This doesn’t mean like all out weaponization but perhaps more work on ensuring their ability to quickly weaponize if need be," she added.

Write to Laurence Norman at laurence.norman@wsj.com and Sune Engel Rasmussen at sune.rasmussen@wsj.com