Japan and South Korea’s Fragile Nuclear Opposition: CSIS Elite Survey Reveals a Contagion Risk

The Contagion Effect: A Rapid Shift If One Country Acts

The survey’s most consequential finding is the "contagion effect." According to the CSIS data, should either South Korea or Japan take the lead in acquiring nuclear weapons, support for a similar move in the other country "could rise rapidly" .

At the survey’s rollout event on June 18, 2026, CSIS experts warned that this contagion could potentially exceed the impact of a reduction in US troop deployments in the region . The study concludes that a decision by one ally to go nuclear could undermine nuclear stability in northeast Asia by triggering a cascading regional response .

Conditional Ambivalence — and the Role of US Deterrence

Cha and Govella argue in a Foreign Affairs article that the risk of Japan or South Korea going nuclear is often exaggerated. Most elites are not in favor "at least for now" . But that ambivalence is conditional on two factors: the severity of the threat and the actions of the other allied country .

The credibility of US extended deterrence — the promise to defend allies with the full range of US military capabilities, including nuclear — appears to be the critical variable. A separate CSIS-Asan Institute workshop from late 2025 to early 2026 assessed extended nuclear deterrence cooperation within the US-ROK alliance, underscoring the importance of US reassurance to keep allies from pursuing national nuclear options .

Broader Context: Rising Public Support, Shifting Politics

While elites remain cautious, political momentum in South Korea has been moving in the other direction. Conservative politicians, including former President Yoon Suk Yeol, have openly endorsed the idea of indigenous nuclear weapons . A February 2024 Gallup Korea poll showed nearly 73% of South Koreans supported their country acquiring nuclear weapons . A 2025 EAI-Hankook Research poll put support at 75.1%, the highest since 2016 .

A June 2026 Asan Institute survey noted that South Korean views toward Japan have improved markedly, with Japan’s favorability reaching net positive for the first time — a shift that could affect alliance dynamics .

Bottom Line: A Fragile Equilibrium

The CSIS survey reveals a delicate balance. Strategic elites in both Japan and South Korea are currently cautious on nuclear armament, but the findings show that equilibrium is fragile. A nuclear move by one country would likely trigger a rapid surge of support in the other, posing a significant challenge to the nonproliferation order in northeast Asia. The credibility of US extended deterrence appears to be the key variable holding that equilibrium in place .