Last month, Iran’s nuclear program entered dangerous new territory: Tehran now possesses enough highly enriched uranium for a nuclear bomb. That material, enriched to 60 percent, would need to be further enriched to roughly 90 percent—so-called weapons-grade uranium—before it could be used in a nuclear weapon. But that process, known as “breakout,” will now take just weeks due to Iran’s advances since 2019, when Tehran began casting off the constraints of the 2015 Iran nuclear deal following the U.S. withdrawal from the agreement. Although this action alone would not give Iran a bomb, it is the most important step in building one.

The consequences of this milestone are profound. Until now, the international community has had months, if not years, to prevent any Iranian dash to bomb-grade material—plenty of time to resolve the crisis diplomatically. Should that fail, the United States has always kept military options as a last resort. Indeed, this fact has helped deter Iran from trying to build a bomb. But as U.S. envoy Robert Malley noted last month, Iran’s capabilities have reached the point where Tehran “could potentially produce enough fuel for a bomb before we could know it, let alone stop it.” Given that Democrats and Republicans have long maintained that they will not allow Iran to produce nuclear weapons, the fact that the United States might not be able to prevent an Iranian dash should be deeply worrying.

The easiest solution to this problem, and the one the United States appears to still be banking on, would be a return to the Iran nuclear deal. This would buy time by rolling back many of these nuclear gains, putting Iran’s breakout timeline at roughly six months. But talks to revive the accord have stalled over Iran’s demand that the U.S. State Department remove the Islamic Revolutionary Guard Corps from its designated terrorist list—apparently a bridge too far for the Biden administration. The problem with waiting for a bargain to materialize, however, is that the longer the stalemate drags on, the less likely a deal becomes as its benefits diminish for both Tehran and Washington.

Unfortunately, the international community might be faced with an Iran at the threshold of a nuclear weapon for the foreseeable future. Washington will have to think creatively about how to manage this state of affairs if it wants to avoid an Iranian bomb and the negative consequences that would follow.  

Breaking Down Breakout

It is useful to think about the challenges posed by breakout as being governed by three clocks. The first clock measures the time it would take Iran to produce enough material for a bomb. The second, the time it would take international inspectors or Western capitals to detect those activities. And the third, the amount of time required for the international community to respond. Historically, the time on the first clock has been sufficiently longer than the time on the second and third clocks. But today that is no longer the case.  

According to U.S. officials, Iran would need “a matter of weeks” to produce enough material for a bomb, while some outside experts have estimated that it could be done in about ten days (the first clock). This timeline will probably continue to shrink as Iran’s program advances. Inspectors visit Iran’s enrichment sites about once a week (the second clock). Thus, Iran could time a breakout so that inspectors arrive and find out too late or with just days left before Iran produces enough material for a bomb. Iran could also fabricate an excuse to deny inspectors their normal access and complete production in their absence.

Inspectors would report the situation back to International Atomic Energy Agency (IAEA) leaders, but that information would then need to reach Washington. It is also possible, although by no means certain, that the United States or one of its allies would detect preparations for a breakout through their own intelligence collection. Even so, the United States would want to analyze the information and convene senior officials to discuss and debate options—a process that would take more time.

With Iran at or near the end of its breakout, Washington would have to quickly respond (the third clock). Unfortunately, there would be no time for diplomacy, and the United States would need to intervene militarily. Whether a military option is available in that time frame would depend on a range of other factors. The United States would probably want to use the Massive Ordnance Penetrator, reportedly the weapon most capable of reaching the deeply buried Iranian nuclear facilities where this breakout would be taking place, which is carried by B-2 bombers based in Missouri. Flight time to Iran could be over 30 hours, possibly too long to prevent a breakout in this scenario. That flight would also require refueling aircraft and presumably multiple B-2s: would those planes be available at a moment’s notice?   

Iran may be at the threshold of a nuclear weapon for the foreseeable future.

Things get even more complicated—and potentially more time-consuming—from there. The United States might want to strike multiple nuclear sites, target Iran’s radar and air defense systems to minimize the risk of U.S. aircraft being shot down, or have missile defense and other capabilities in place in the region to defend against Iranian retaliation. Some of these options would be impossible to execute in such narrow time frames, which could dissuade the United States from acting at all.

Rather than trying to break out at known sites, Iran could also try to divert its nuclear material to a covert facility between inspections for further enrichment to 90 percent. To do so, Iran would need to have a clandestine enrichment facility, and there are no indications that it does (although a lack of inspector access to cameras monitoring Iran’s centrifuge production since February 2021 makes that harder to confirm). But unlike in the past, when Iran would be starting from slower, first-generation centrifuges and low-enriched uranium, Iran now has growing stockpiles of 60 and 20 percent enriched material and has mastered more advanced centrifuges. This means that Iran could build a smaller enrichment plant that would be harder to detect, enabling it to enrich the material to 90 percent much faster than before. The United States would need to know the location of the covert facility and the missing nuclear material were Washington to use military power to stop Iran’s march toward a nuclear weapon.

Of course, having the fissile material for a bomb is not a bomb itself. Iran would need longer—perhaps a year or two—to build a nuclear device and mount it on a missile. But fissile material production remains the most heavily monitored, and therefore the most detectable, part of building a bomb. Weaponization activities can take place at a variety of scattered facilities, which are not subject to any robust monitoring and carry fewer telltale signatures. The United States may struggle to detect the remaining weaponization processes after Iran produces the requisite fissile material.

Even if Iran never produces a bomb or the necessary fissile material, a nuclear-capable Tehran would still generate serious policy challenges. Iranian foreign policy would grow bolder and more aggressive if Tehran believes it can hang the nuclear breakout sword of Damocles over the head of the international community. Iran could also consolidate its nuclear hedge in ways that do not require a full-fledged nuclear weapons program, including by developing intercontinental ballistic missiles. Finally, faced with an Iran on the cusp of a bomb and doubts about Washington’s ability to stop it, countries in the region could embark on their own nuclear hedging efforts or bomb programs, posing a further challenge to the global nonproliferation regime. Those same allies and partners might try to leverage the threat of going nuclear to press the United States for stronger security assurances and defense assistance—a strategy that has been used by U.S. allies in Asia. Washington would find itself caught between two unpalatable options: deeper military commitments in the Middle East at a time when it would prefer to allocate attention elsewhere, or remaining at a distance and risking further nuclear and missile proliferation.

Winding Back the Clock

With the fate of the Iran nuclear deal hanging in the balance, Tehran has little incentive to halt its nuclear advances, which it believes put pressure on the West. That becomes doubly true if talks to revive the deal collapse. While it waits on diplomacy, Washington should therefore focus on what it can control: clocks two and three—speeding up detection and its response.

To increase the odds that the international community would detect an Iranian breakout, the United States, its allies, and, if possible, China and Russia should push Iran to allow daily IAEA visits to Iran’s two enrichment sites and nuclear material storage locations. In addition, Iran should resume using online enrichment monitors—an automated technology that continuously monitors enrichment levels when IAEA officials are not present. These measures were in place under the Iran nuclear deal, but Iran has since discarded them. In addition, the United States should increase its own intelligence collection efforts and coordinate with allies to help provide as much warning time as possible. When just days might separate Iran and enough material for a bomb, every second counts. These measures would provide valuable time and help deter a breakout.

There are reasons to believe that Iran might adopt such conditions. First, there is a powerful, apolitical argument that these added precautions are necessary for the IAEA to do its monitoring job since Iran is the only country producing highly enriched uranium that does not possess nuclear weapons. Second, these measures could help provide vital assurance to the international community that Iran was not sprinting for a bomb and therefore reduce the chances of a military strike, something Iran would presumably see as in its interests. With such a narrow margin of error, any delay in inspector access to Iran’s sites—even if genuinely rooted in a misunderstanding or accident and not a breakout attempt—could lead to a miscalculation that Iran should be keen to avoid. Finally, none of these steps would require Tehran to halt any of its nuclear advances, such as 60 percent enrichment, increasing its stockpile of material, or adding advanced centrifuges, which it believes provide important negotiating leverage.

A nuclear-capable Tehran would generate serious policy challenges.

The United States and its allies would also need to speed up their ability to respond. The National Security Council should establish a modified committee at the principal level that would convene immediately upon receiving information indicating an Iranian breakout. As in any crisis, indicators may be ambiguous and data points may be conflicting. To simulate that reality, this group should practice convening, assessing likely types of information they might receive, and game out options beforehand.

The most impactful step the United States could take, however, would be to shorten military response time. This step might also be the hardest. One option would be increasing readiness and ensuring that all capabilities required for a strike, such as refueling aircraft, would be available on short notice. Another would be positioning aircraft, missile defense systems, and other support assets in the region. U.S. B-2 bombers, for instance, periodically deploy outside the United States but have no sustained overseas presence. Washington would need to examine the requirements, and risks, of more frequent deployments or permanent stationing abroad. Still, these steps would give the United States more flexibility should a crisis arise and would signal to partners in the region as well as to Iran that the United States is prepared to act if needed.

The United States must properly communicate and sequence these diplomatic and military moves. Washington needs to monitor intelligence about Iran’s threat perceptions closely and think carefully about which steps to disclose and which to keep secret so that it does not inadvertently trigger a scenario it hopes to avoid: an Iranian dash to a nuclear weapon. Although Iran may well reject the idea, the United States should also push to establish a direct line of communication with Tehran to help with crisis management. Finally, the United States should be open to holding off on some military steps in the unlikely event that Iran opts for nuclear restraint and transparency. The goal, after all, is not to bomb Iran but to prevent an Iranian bomb.

This approach would be compatible with U.S. efforts to reach a diplomatic settlement. But it would also require the United States to confront the uncomfortable reality that it cannot pin its hopes solely on the eventual revival of the Iran nuclear deal to solve the current dilemma. These options are not ideal, but in the absence of a restored nuclear agreement, they may be all Washington has left to prevent a nuclear-armed Iran.

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  • ERIC BREWER is a Senior Director at the Nuclear Threat Initiative and has served on the National Security Council and National Intelligence Council.

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