Iran war challenges nuclear latency strategy of Japan, South Korea

The Iran war has delivered a verdict on nuclear latency – the strategy of possessing the capability to develop a nuclear weapons program while stopping short of crossing the weaponization threshold.

There are significant implications for Northeast Asia, as South Korea and Japan are pursuing such a strategy amid eroding confidence in US extended deterrence.

Both may have latent capability close enough to alarm adversaries but insufficient to deter them.

However, if nuclear latency proves to catalyze rather than prevent conflict, the foundational assumptions underpinning nonproliferation strategy require a fundamental reassessment.

Both may have latent capability close enough to alarm adversaries but insufficient to deter them.

The failure of nuclear latency in Iran

Nuclear latency isdefined as having the capabilityto rapidly develop nuclear weapons – having the fissile material to make a bomb, the technology to construct a warhead and the means to deliver the weapons – without crossing the threshold. In theory, nuclear latency acts effectively as a deterrent against external attack without ever paying the costs, material or diplomatic, of acquiring nuclear weapons.

As Ankit Panda, the Stanton senior fellow at the Carnegie Endowment for International Peace,writes of Iran:

The theory behind the threshold strategy had a certain appeal: Maintain latent capability as a deterrent, avoid the diplomatic costs of overt weaponization, preserve the Supreme Leader’s fatwa against nuclear weapons, keep the door open to negotiations and explore if you might be able to manifest non-weaponized nuclear deterrence effects of some kind.

But latent deterrence depends on two conditions:

• making your adversary believe you can rapidly cross the threshold, if attacked; and
• offering a diplomatic alternative to war.

Iran constructed what Panda describes as the worst possible nuclear posture: proximate enough to a weapon to justify preventive attack, “yet unwilling to cross the threshold that might have actually prevented one.”

Parallel paths in Seoul and Tokyo

Despite its singular historical experience as the only nation subjected to a nuclear attack, Japan has maintained a deliberate hedging strategy for decades. For example, it has a stockpile of forty-five tons ofweapons-grade plutonium(eight tons of it held in Japan), the capability to enrich uranium, ballistic-missile technologies developed under its satellite-launch programs and advanced fighter aircraft with nuclear delivery potential.

Arecent report by the International Atomic Energy Agencyobserved that Japan’s latency hedges against regional threats while simultaneously functioning as diplomatic leverage against the United States – an implicit signal that weakening security guarantees could trigger rapid proliferation.

South Korea has long sought capabilities similar to Japan’s but has been thwarted by the123 Agreementwith the United States, a civil nuclear pact that restricts Korea’s ability to produce fissile material and develop potential delivery systems.

The Lee Jae Myung administration is negotiating changes to the agreement, specifically regarding uranium enrichment to build nuclear-powered submarines. What was once a predominantly conservative aspirationnow spans the political spectrum.

For both South Korea and Japan, the motivation is not only the threat from China, North Korea and Russia. The unstated driver for nuclear latency is increased doubt about the reliability of the US commitment to provide extended deterrence.

“The US role in the world is changing in fundamental ways as Washington takes deliberate steps away from the rules-based order that it helped build in the years following World War II,” states a joint report from theCenter for Strategic and International Studies and the Asan Institute for Policy Studies, published in February 2026. These shifts “have also yielded new questions regarding the enduring nature of the US commitment to the alliance.”

The erosion of restraint

Historically, two forces constrained South Korean and Japanese nuclear ambitions: a credible US nuclear umbrella and a US non-proliferation policy that actively opposed any steps in this direction. Both of those restraints are now in question.

Extended deterrence “was never completely credible to the Japanese which is why they keep going back to ask for us to give them more assurance,” Richard Samuels, professor at the Massachusetts Institute of Technology who haswritten extensively on this issue, told this writer.

“We would be foolish to imagine that the Japanese are not having second thoughts,” Samuels warned, drawing a direct parallel to conversations in Germany and France.

Former Lieutenant General Noboru Yamaguchi, a senior government advisor, went farther. “It is impossible to prove extended deterrence is valid,” he explained to this writer. “Deterrence is about how we feel. It was questionable during the Cold War. Now I don’t believe in any kind of deterrence.”

If the Donald Trump administration no longer opposes allied proliferation, asTrump himself has signaled at times, then both South Korea and Japan can move more openly down the road of a robust nuclear latency without triggering the consequence of diplomatic isolation.

“Suppose you don’t get the blowback from the Americans,” former Director of Los Alamos National Laboratory Siegfried Hecker said to this writer. “That takes away a lot of the negatives for South Korea, and for Japan.”

The window of vulnerability

The critical question is whether adversaries might act as the United States and Israel have done in Iran and attempt to destroy South Korea and Japan’s nuclear capability before it crosses the threshold.

“At the end of the day, the people you have to convince are not the Japanese or the Koreans,” commented Professor Samuels. “The people you have to convince are the adversaries – the Chinese and the Russians. We might have been looking in the wrong place.”

Nuclear expert Panda writes that China and Russia may initially resort to gray-zone tactics. These could include cyberattacks such as those launched against Iran previously. They might want to stop short of the use of force if the United States maintains a limited nuclear umbrella, he told this writer.

The biggest problem will be what analysts call the “window of vulnerability,” the gap in time between when South Korea and Japan clearly move toward weaponization and when they cross that threshold. That window widens significantly if the goal is tangible deterrence that requires a survivable arsenal with submarine-launched ballistic missiles.

Japan holds a substantial advantage. Hecker, a plutonium metallurgist by training with deep knowledge of the nuclear programs of China, North Korea and Russia, assessed that Japan could direct the assembly of a uranium weapon in six months to a year. A plutonium weapon would likely take up to two years.

South Korea faces a longer timeline, according to the former Los Alamos director – two-plus years to produce fissile material alone.

But Seoul may be farther along in the technical research needed to construct a working weapon, Panda added.

One unconventional scenario includes a South Korea-Japan partnership in which South Korea contributes weaponization expertise and Japan supplies fissile material. “It would be a marriage made in heaven,” said Panda.

Latency an accelerant?

Nuclear latency may prove to be an accelerant to precisely the conflicts it was designed to prevent. If South Korea or Japan or both move toward a weapons threshold, “The North Koreans may pull the Russians in to do something,” suggested Hecker.

“China will be incensed,” he added. “If there is one [thing] that gets the Chinese exercised, it’s Japan and a potential nuclear program.”

But the question is whether policymakers in South Korea and Japan will read Iran as a cautionary tale or conclude that, with eroding alliance guarantees and a deteriorating threat environment, they have no better option.

Daniel C. Sneider is a non-resident distinguished fellow at the Korea Economic Institute of America (KEI) and a lecturer in East Asian studies at Stanford University. This article, originally published by KEI, is republished with permission.