Iran War Challenges Nuclear Latency Strategy Of Japan, South Korea

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 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 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 US commitment to the alliance.”

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 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 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, 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 said to this writer.“That takes away a lot of the negatives for South Korea, and for Japan.”

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.