Iran’s Nuclear Threshold Challenge


Iran is overtly inching closer to the possession of nuclear weapons, but thus far has refrained from crossing the line. Crucially, Iranian officials are already acting and speaking as if Iran has a threshold nuclear capability, claiming that they possess all the needed technical elements for nuclear weapons. Moreover, they are threatening to acquire nuclear weapons if attacked and expressing satisfaction at the deterrent effect they have already achieved.

Tehran’s behavior raises acute (but not unprecedented) problems for the United States and the world: how to deal with a state that has succeeded in acquiring the capability to quickly produce nuclear weapons and how to prevent it from going all the way. An effective strategy for dealing with Iran should also address the specific problem of its leveraging a threshold nuclear capability to enable other provocative behavior. This includes its direct missile and drone attacks (most recently on Israel) and capture of commercial shipping vessels, as well as its use of proxies to carry out other violent or destabilizing acts in the region. Yet this challenge extends beyond Iran, to other states that might be inspired to emulate its behavior or respond to the threat it poses. To navigate these twin problems — dealing with Iran and preventing the nuclear threshold from becoming a desirable status for others — policymakers ultimately will need to reconfigure nuclear energy and nonproliferation policy.



The Nuclear Threshold

Nuclear weapons are the sum of a disparate set of policy, technical, and organizational activities: producing fissile material (highly enriched uranium or plutonium); designing a weapons system and engineering a warhead; building delivery systems (ballistic and cruise missiles or gravity bombs for aircraft); creating a military infrastructure to command and control them; and deciding on strategies and plans for how to utilize nuclear weapons for political and military effect. Bringing these elements together indigenously takes time and serious effort. Iran has been working towards this moment for the better part of three decades.

The nuclear weapons development process is lengthy not simply because of the cost and complexity of these activities, but also because of a global system of treaties aimed at incentivizing restraint and dissuading and stymieing states that seek the bomb. This system is backed by unilateral and collective enforcement efforts from major powers. Since the 1950s, many states have contemplated or even pursued nuclear weapons. Most gave up, failed, or stopped somewhere short of the threshold, which is what makes Iran’s status today unique for now but perhaps not for long.

Japan, among other potential threshold states, for example, has ample weapons usable fissile material and is developing missiles that could be capable of nuclear delivery. Although it has long adhered to the three non-nuclear principles it adopted in 1967 and does not appear to be currently considering nuclear weapons acquisition, let alone actively working on its necessary constituent elements, Tokyo’s latent capabilities allow it to quickly put all the missing pieces together if it decided to do so.

Japan constitutes a clear example of nuclear latency, in which a state unintentionally acquires many of the capabilities needed to develop nuclear weapons (especially fissile material). Other states may intentionally obtain such capabilities to create the potential for a nuclear weapons option down the road, a phenomenon referred to as nuclear hedging. Iran has clearly gone a step further, necessitating specific analytic focus on the threshold between hedging and nuclear weapon possession.

Three distinct elements distinguish a state that has achieved a threshold status. First, the conscious pursuit of this combined technical, military, and organizational capability to rapidly (probably within three to six months) obtain a rudimentary nuclear explosive capability after a decision to proceed. Second, implementation of a strategy for achieving and utilizing this status. And third, the application of this status for gain vis-à-vis adversaries, allies, and/or domestic audiences. Nevertheless, a threshold state remains sufficiently short of weapons possession and even from the capacity to assemble disparate components into a nuclear weapon within days (a “screwdriver’s turn away”). This allows it to try to claim that it is staying within the rules, allay some of its security concerns, and leverage the potential of possession while attempting to evade efforts to roll back the capability.

Iran as a Threshold State

Iran began a full-fledged clandestine nuclear weapons program in the 1990s. The program was exposed and truncated in 2003, but not before it had made considerable strides. In this early phase the program lacked the necessary fissile material to power the bomb, a critical deficiency Iran has successfully labored to make up for since. There are various estimates today for how long it might take Iran now to acquire its first nuclear weapon, the gist of which is months, not years.

Consecutive reports from the International Atomic Energy Agency detail the numerous achievements made by Iranian scientists in developing a substantial uranium enrichment enterprise and accumulating a stockpile of many bombs’ worth of highly enriched uranium. Rafael Grossi, the head of the International Atomic Energy Agency, has criticized Iran for “accumulating a vast amount of highly enriched uranium. And this is something that, of course, draws our attention because no other country without nuclear weapons is enriching at these high levels.” Indeed, in one respect, Iran has been using inspectors to validate and advertise its nuclear gains. There is also a body of evidence of research, development, and testing of dual-use delivery vehicles and various weapons design components and materials, including extensive studies documented in the Iranian archive covertly taken by Israel.

Although Iran has been relatively close to the capability to assemble nuclear weapons for several years, it only consolidated its threshold states after 2018. President Donald Trump’s decision that year to unilaterally pull the United States out of the Joint Comprehensive Plan of Action freed Iran from the negotiated restraints on its nuclear program. Since then, with increasing assertiveness and consistency, Iranian officials have begun to overtly acknowledge and attempt to leverage its threshold status. Here, the statements of various regime figures are telling:

Ali Akbar Salehi, the former head of Iran’s nuclear agency, stated in February 2024:

We possess all the nuclear science components and technology. We’ve crossed all the lines, overcome all obstacles. It’s like having all the parts to build a car: we have the chassis, the engine, the transmission, everything. Each component serves its purpose, and everything is in our hands.

In January 2024, the current head of the same agency, Mohammad Eslami, asserted:

This is not about having the capability [to produce nuclear weapons]. Rather, it is about us not wanting to do this. In terms of our national security, we do not want to do it. It is not about the lack of capability. This is a very important point. Our national security in this field requires us to continue to seek our objectives and to gain influence. I think we have achieved such deterrence.

In April 2024, Mahmoud Reza Aghamiri, the president of Shahid Beheshti University, argued:

The issue is not about producing an atomic bomb. When you have high capabilities, it means power. As you said, going in this direction is problematic and forbidden in the Leader’s view at the moment, but since he is a religious jurist, this could change tomorrow or later.

Also in April 2024, Iranian Majles member Javad Karimi Ghodousi posted on X (formerly Twitter) about a nuclear weapons capability, “If permission is issued, it would take one week to conduct the first test.”

And Kamal Kharrazi, a senior advisor to Supreme Leader Ayatollah Ali Khamenei, stated in May 2024, “We have no decision to build a nuclear bomb but should Iran’s existence be threatened, there will be no choice but to change our military doctrine.” Similarly, Brig. Gen. Ahmad Haghtalab, commander of the Revolutionary Guard Corps Nuclear Protection and Security Corps, declared that “a revision in the nuclear doctrine and policies of the Islamic Republic and departure from previously stated considerations is possible and conceivable.”

These statements serve a clear communication purpose for both foreign and domestic audiences: to validate the achievement of the nuclear weapons threshold and to justify the economic sacrifices involved; to warn that the threshold to nuclear weapons possession could be quickly crossed at any time based on reversal of the leader’s oft-discussed original 2004 fatwa; and, yet, to convey restraint, for now. Interestingly, Eslami’s statement suggests a motivation for maintaining the status quo, namely that Iran is accruing the benefits of nuclear deterrence without having to take on the risks of actually possessing nuclear weapons. He also asserted, “This deterrence has been achieved with the help of God, without having to violate any rules and regulations.” Worryingly, this statement underscores Iran’s belief, largely endorsed by Russia, that — notwithstanding repeated protestations from Grossi — reaching the nuclear weapons threshold has been done legitimately and without violating any international norm.

Detection, Deterrence, and Reassurance

Iran’s achievement of a threshold status reveals clear gaps in the global nonproliferation system that policymakers should close if they are to prevent others from following in Tehran’s footsteps. The international nuclear nonproliferation system is built mainly on deterrence through detection. The International Atomic Energy Agency is charged with the latter mission. A state that fails to completely and correctly declare to the agency all its fissile materials and related activities risks being found out by inspectors (as was the case with North Korea in the early 1990s). Exposure of such cheating can trigger members of the international community to respond unilaterally and collectively with varying degrees of punishment. Iran’s economy and people have no doubt suffered the toll of the sanctions levied over the years due to revelations of its nuclear cheating.

This system can work well at detecting and dissuading proliferation at an early stage, but its efficacy declines the closer a state openly comes to acquiring weapons-grade fissile materials. The reason is simple: There is no formal prohibition on amassing highly enriched uranium (or weapons-grade plutonium), even though this practice defies reasonable commercial rationale, and the mandate of the International Atomic Energy Agency does not extend to the other activities necessary to acquire a bomb, namely nuclear weapons design activity, delivery capability, or even nuclear militarization. For a state like Iran that already possesses abundant fissile material, the timeliest indicators of actions to acquire nuclear weapons are likely to occur in areas not currently monitored by the agency.

Therefore, the nuclear threshold challenge means that other states need to be able to impress on the leaders of a proliferating state their national capacity to quickly detect (through intelligence) the other attributes of a decision to proceed toward nuclear weapons and convince them that proceeding will subject them to swift punitive (perhaps military) responses as a result. The successful application of this strategy has no doubt helped dissuade Iran from crossing the threshold for the past two decades and lured Tehran occasionally into diplomatic arrangements that slowed down its nuclear advances.

Threats alone, however, are unlikely to succeed in continuously deterring threshold states — to remain effective over time they should be coupled with reassurance. If the leaders of a threshold state believe that without nuclear deterrence they would suffer the stipulated consequences regardless of whether or not they exercise restraint, there is little incentive for them to refrain. Deterrence without reassurance could even inadvertently drive a state to cross the threshold. Thus, opposing states need to find credible means of communicating that lack of movement to cross the threshold means that core security interests will be respected and further punishments will be held in abeyance.

The acute challenge presented by Iran’s current nuclear status is clear. Neither diplomacy nor current punitive measures (sanctions) can make it roll it back its nuclear gains. Covert measures have proven strategically ineffective, while the threat of a U.S. and/or Israeli preventive military action against its nuclear infrastructure appears less credible under the current circumstances, especially when Iran enjoys intimate cooperation and backing from Russia. Given what Iran’s leadership values, the more impactful area for the United States and others to apply leverage now might be the Islamic Republic’s stability and security. Threatening the regime’s survival might still dissuade Iran from taking the next step to cross the line. But such a threat should be invoked subtly and be accompanied by reassurances and an extended hand for resumption of diplomacy. This combined approach would avoid feeding the regime’s self-serving view that a certain amount of friction with the United States over its nuclear ambitions helps guarantee regime survival while also providing a useful cover for subversive activities throughout the Middle East. Furthermore, the United States and its European partners would want to avoid legitimizing or rewarding the threshold status or to tolerate a threshold state’s pernicious behavior in other domains.

Managing the Broader Threshold Challenge

Iran foreshadows a potentially bigger problem — not only as a role model but also as a catalyst for other states to seek a threshold capability, in the Middle East (Egypt, Turkey, and Saudi Arabia immediately come to mind) and well beyond (South Korea and Japan). Furthermore, increasing interest in nuclear power for climate and energy security could drive a significant increase in the spread of the equipment, materials, and knowledge required to develop nuclear weapons. This trend is exacerbated by interest in “non-proscribed” military applications such as nuclear-powered submarines, mobile reactors to power military bases, or space propulsion systems. Worse, the rivalry between the major powers in general, and especially the growing rift with Russia (and its nascent but rapidly growing military cooperation with both Iran and North Korea), undermines traditional collaboration to prevent the spread of nuclear weapons.

Drawing from past efforts to deal with nuclear threshold states, some hope still resides in efforts combining deterrence and reassurance. In particular, sustaining and even strengthening credible bilateral or multilateral security guaranteesas a form of assurance will be crucial in incentivizing nuclear restraint among U.S. allies and partners. The Biden administration has pursued this approach with South Korea, Japan, and Australia, and via expanded NATO membership for Sweden and Finland. At the same time, Washington would probably have to make more explicit the contingent relationship between its security commitments and nuclear weapons forbearance among its other allies and partners.

To manage the broader nuclear threshold challenge, however, policymakers also need to adapt existing nonproliferation approaches. Japan and Iran are instructive cases for how to go about this. Both have ample fissile material. Only Iran, so far as is publicly known, has pursued work to design nuclear weapons and construct the missiles and command structure to deliver them. It is these weaponization and militarization activities that are most problematic, not simply (or solely) the ability to make fissile material. Policymakers thus need to establish clearer norms around activities that are oriented toward nuclear weapons and to focus more detection and analytic effort on their pursuit. The International Atomic Energy Agency should be given broader remit to assess these activities, accordingly.

Sharpening the distinction between nuclear energy and weapons and shifting away from the sole focus on production of fissile materials is also important in the context of facilitating an expansion of nuclear energy for climate, energy security, and sustainable development objectives. In particular, it will be indispensable to winning over support for the new norm from countries legitimately interested in producing fuel for their nuclear energy programs. Policymakers could bolster this distinction by enumerating a framework for responsible nuclear power behavior based on adoption of safety, security, nonproliferation, environment, and liability best practices commensurate with peaceful nuclear ambitions. This would endow with credibility commercial nuclear energy pursuits on the basis of application of safeguards and transparency measures, rather than access to the technology itself.

Articulating and incentivizing a stronger normative framework for nuclear energy could thus make seeking a nuclear threshold both less appealing and easier to observe. In turn, it would avail a more moderate Iranian leadership an avenue to credibly reassure about its non-weapons nuclear intentions without in the process singling it out.



Toby Dalton is senior fellow and co-director of the Nuclear Policy Program at the Carnegie Endowment for International Peace.

Ariel (Eli) Levite is a senior fellow at the Carnegie Endowment and author of numerous articles and papers on nuclear hedging.

Image: Wikimedia