How the War in Iran Reshapes South Korea and Japan’s Nuclear Strategyby Daniel Sneider, lecturer in East Asian studies at Stanford University, non-resident distinguished fellow at the Korea Economic Institute of America, and APP Member.
First Published March 13, 2026 on The Peninsula.
The Iran war has delivered a verdict on nuclear latency, or 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 U.S. extended deterrence. And 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.
The Failure of Nuclear Latency in Iran
Nuclear latency is defined as having the capability to 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, both material and diplomatic, of acquiring nuclear weapons.
“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,” Stanton Senior Fellow at the Carnegie Endowment for International Peace Ankit Panda wrote of Iran.
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. But Iran constructed what Panda described 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 of weapons-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. A recent report by the International Atomic Energy Agency observed 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 the 123 Agreement with 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 aspiration now spans the political spectrum.
For both South Korea and Japan, the unstated driver for nuclear latency is not only the threat from China, North Korea, and Russia, but increasing doubts about the reliability of the U.S. commitment to provide extended deterrence.
“The U.S. 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 the Center 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 U.S. commitment to the alliance.”
The Erosion of Restraint
Historically, two forces constrained South Korean and Japanese nuclear ambitions: a credible U.S. nuclear umbrella and a U.S. 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 has written 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 further. “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, as Trump 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 told 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 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,” comments 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 wrote that China and Russia may initially resort to gray-zone tactics. These could include cyberattacks such as those launched against Iran previously. If the United States maintains a limited nuclear umbrella, he told this writer, they might want to stop short of the use of force.
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 China, North Korea, and Russia’s nuclear programs, 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 they may be further along in the technical research needed to construct a working weapon, Panda added.
One unconventional scenario includes a South Korea-Japan partnership where South Korea contributes weaponization expertise and Japan supplies fissile material. “It would be a marriage made in heaven,” says Panda.
Conclusion
Nuclear latency may prove to be an accelerant to precisely the conflicts it was designed to prevent. “The North Koreans may pull the Russians in to do something,” suggests Hecker, if South Korea and/or Japan move toward a weapons threshold. “China will be incensed…if there is one thing that gets the Chinese exercised, it’s Japan and a potential nuclear program,” he added.
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.
