Iron sword or invisible sword: Korea’s nuclear future and its implication

Lee Seung-heon

South Korea stands at a strategic inflection point. North Korea has proven tactical nuclear capability, advancing its technology to intercontinental ballistic missiles (ICBMs) and submarine-launched ballistic missiles (SLBM).

The immediate, visceral call for independent nuclear armament is a natural survival instinct. The real question is not if we need an instrument of defense but what kind of instrument should we acquire. Do we grab the blunt, heavy "iron sword" — a paradoxical choice that could fracture alliances and destabilize our economy? Or do we develop the "invisible sword," a quieter form of latent nuclear power that strengthens deterrence without triggering global backlash? The challenge is finding a threshold that ensures security without pushing the region into crisis.

The path favored by many advocates of nuclear armament is “hard nuclearization,” meaning immediate withdrawal from the Nuclear Non-Proliferation Treaty (NPT) and the full manufacture of nuclear weapons. This choice — the iron sword — may provide temporary satisfaction, but the price threatens the survival of a nation that relies heavily on global trade.

For decades, the United States has viewed upholding the NPT regime as a cornerstone of its global hegemony. South Korea’s nuclearization would send an alarming signal, potentially triggering a nuclear domino effect in Taiwan, Japan and the Middle East. Directly defying a core U.S. national interest would risk immediate and devastating fallout, including alliance collapse, diplomatic isolation, the end of extended deterrence and severe economic sanctions.

China would not remain idle. If South Korea went nuclear, Beijing would likely retaliate. What we witnessed during the Terminal High Altitude Area Defense-driven dispute — economic pressure, trade restrictions, consumer boycotts — would pale in comparison to the consequences of a nuclear breakout. Moreover, a nuclear-armed South Korea would hand Japan’s hawks their strongest argument yet for constitutional revision and independent nuclear forces, igniting widespread destabilization across the Indo-Pacific.

In recent years, however, the United States has shifted away from its traditionally rigid stance against nuclear-related technology for allies, signaling a broader recalibration of regional strategy. For instance, Washington has now approved Seoul's plan to build nuclear-powered submarines. This shift is not merely about military capability. It reflects a strategic recalibration, enabling regional partners to share greater defense burdens. As a result, Seoul is gaining access to advanced technologies once considered off-limits — a development and geopolitical circumstances that make the invisible sword more plausible than ever.

How can Korea effectively deploy the invisible sword instead of relying exclusively on the promise of U.S. extended deterrence, especially when Defense Secretary Pete Hegseth has proclaimed that U.S. troops in Korea should prepare for a future conflict with China rather than North Korea? The answer lies in securing soft deterrence through the “Japan model.”

Japan is officially a nonnuclear state. Yet it holds the invisible sword. Japan possesses complete nuclear fuel-cycle technology and massive reprocessing facilities such as the Rokkasho complex. This grants Japan the status of a latent nuclear state recognized by many experts as having the capability to manufacture a nuclear weapon within six months. This latent capability allows Japan to operate within the NPT while still possessing one of the world’s most advanced reprocessing and enrichment infrastructures.

This is the model South Korea should now consider. The first step is straightforward and entirely legal: revise the Republic of Korea-U.S. Civil Nuclear Cooperation Agreement to allow the reprocessing of spent fuel and independent uranium enrichment-producing protonium. It is not about breaking the rules. It is about restoring nuclear energy sovereignty and achieving parity with Japan. South Korea can remain within the boundaries of the NPT while developing technical capacity that stops just short of weaponization by keeping enrichment at roughly the 3 percent threshold for civilian use.

The ultimate goal is not war, but deterrence. To achieve this, South Korea must build the infrastructure capable of rapid weaponization during a crisis. If national leadership makes that decision in an emergency, the facilities and technical readiness for reprocessing and enrichment of nuclear material must be fully prepared to allow a transition to a nuclear weapon state within six months. This six-month deterrent would reshape calculations not only for North Korea, but for every country in the region. It provides the surest insurance against North Korean miscalculation and a more credible form of mutual deterrence than relying solely on the U.S. nuclear umbrella.

South Korea should not rush toward immediate nuclear armament and show off the iron sword. That would provoke the United States and neighboring countries, leading to economic disaster and regional instability. But neither should it accept permanent strategic vulnerability. A six-month deterrent — lawful, restrained and technologically prepared — offers a third path, an invisible path. It preserves the alliance. It avoids proliferation. And, most importantly, it ensures that Seoul is never again forced to gamble its survival on someone else’s promises.

Lee Seung-heon is currently doing research at MIT's Security Studies Program, studying nuclear strategy among power states including China, North Korea and the U.S. Previously, he was at South Korea's National Assembly as a legislative aide. The views expressed here are the author's own.