Strategic elites in Japan and South Korea currently show strong resistance to nuclear weapons development, with roughly three-quarters to four-fifths expressing opposition or deep reservations about their nations acquiring such arsenals. Yet this consensus, revealed in a major survey published by the Centre for Strategic and International Studies, masks a fragile equilibrium that could unravel rapidly if geopolitical conditions shift. The research, directed by CSIS president of geopolitics and foreign policy Victor Cha and Japan chair Kristi Govella, surveyed senior figures including government officials, parliamentarians, academics, policy analysts and corporate leaders through late October, providing rare insight into the thinking of decision-makers across the region.

The stability of this status quo depends on a critical assumption that has grown increasingly uncertain. Should either Japan or South Korea pursue nuclear weapons, the other would face intense domestic pressure to follow suit, potentially reshaping the entire strategic architecture of Northeast Asia in ways that dwarf even significant shifts in United States military presence. This cascade effect concerns CSIS experts far more than incremental changes in American troop levels, reflecting how interconnected nuclear decisions have become in a region where alliance arrangements, historical tensions and great power competition create multiple triggers for proliferation. The risk is not that elites today favour such weapons, but that tomorrow's circumstances could reverse their calculations with stunning speed.

A striking disconnect separates the views of strategic elites from broader public opinion in South Korea, highlighting how vulnerable policy consensus remains to shifts in popular sentiment. While roughly 75 per cent of South Korean decision-makers oppose or express uncertainty about nuclear weapons, a 2024 survey commissioned by the Chey Institute for Advanced Studies found that over 72 per cent of ordinary South Koreans supported their country acquiring such weapons. This gap suggests that Korean leaders have resisted public pressure through careful messaging and alliance management, but this political restraint could evaporate if North Korea conducted further weapons tests, or if voters perceived American commitment to Korean security as wavering. The public rationale behind public support focuses overwhelmingly on countering North Korean threats, a motivation that would intensify rather than diminish if Pyongyang accelerated its arsenal expansion.

Japan presents a different picture, with no comparable chasm between elite and public opinion. Surveys indicate that roughly 80 per cent of ordinary Japanese, like their strategic leadership, oppose nuclear weapons development, a consistency that reflects both post-war constitutional and cultural constraints and widespread public anxiety about nuclear risk. However, media narratives have sometimes amplified calls for nuclear weapons within Japanese defence circles beyond their actual prevalence, potentially distorting international perceptions of the momentum behind such proposals. Kristi Govella cautioned against accepting exaggerated reports of pro-nuclear sentiment within Japanese decision-making, emphasising that current institutional and social barriers remain substantial despite rising security anxieties.

Yet the survey findings revealed crucial vulnerabilities in this Japanese consensus. Japanese supporters of nuclear weapons cite concerns about the durability of American security commitments rather than immediate regional threats, a motivation that gains traction during periods of US strategic distraction or policy ambiguity. If the United States reduced its military footprint in East Asia, seemed preoccupied with other regions, or appeared to abandon extended deterrence arrangements, Japanese advocates for nuclear weapons would find more receptive audiences. This psychological factor—confidence in alliance reliability—creates a different pressure point than North Korea's provocations, one that American policymakers directly control through their actions and statements.

The underlying logic driving elite restraint in both nations reveals how dependent stability has become on external reassurance rather than permanent structural solutions. South Korean decision-makers who oppose nuclear weapons do so partly because the United States nuclear umbrella provides deterrence against North Korea, while Japanese elites rely on similar calculations regarding both North Korea and China. Withdraw or credibly downgrade that umbrella, and the entire calculus shifts. This dependency means that Northeast Asian nuclear stability cannot be separated from American willingness to maintain robust extended deterrence, a commitment that transcends any single administration and requires consistent signalling across administrations, defence budgets and military deployments.

Recently, the United States has intensified efforts to reinforce these reassurances through high-level diplomatic engagement. Bilateral meetings held in Seoul this month advanced consultations on nuclear cooperation initiatives with South Korea, while an extended deterrence dialogue took place in Tokyo with Japan, both designed to demonstrate sustained American commitment. These moves aim to prevent the dominoes from toppling by strengthening the confidence that makes elite opposition to nuclear weapons sustainable. Yet such diplomatic gestures, however serious, cannot fully offset deeper anxieties about long-term American strategy in Asia, particularly amid growing uncertainty about future administrations' commitments to the region.

The broader context complicating these calculations involves simultaneous American pressures on Beijing and China's refusal to participate in nuclear arms control negotiations. Brandon Williams, under secretary for nuclear security at the Department of Energy, announced plans for the United States to invest $600 million in artificial intelligence to accelerate nuclear weapons design and production, shortening the timeline from identifying a capability need to deployment from the current 10 to 15 years. Simultaneously, experts at CSIS have argued that the United States should equip hypersonic weapons with nuclear rather than conventional warheads to present adversaries with greater uncertainty about American response options. These moves signal escalating American nuclear modernisation, partly aimed at deterring China, but with potential consequences for how Japan and South Korea perceive their own security requirements.

The survey highlighted revealing differences in motivation between South Korean and Japanese nuclear advocates. South Koreans who support acquiring nuclear weapons do so primarily to address the immediate North Korean threat, a desire rooted in direct experience of periodic provocations and the knowledge that deterrence depends ultimately on punishment capability rather than reliance on others. Japanese supporters emphasise concerns about American staying power, reflecting Japan's greater distance from immediate Korean peninsula flashpoints and its higher dependence on United States naval presence and extended deterrence for security. These distinct motivations matter because they suggest different policy levers could address each nation's underlying anxieties—conventional deterrence improvements for Seoul, alliance commitment reinforcement for Tokyo.

China's repeated accusations that Tokyo seeks remilitarisation, potentially including nuclear weapons, inject an additional layer of complexity into these calculations. Beijing uses such claims partly as diplomatic rhetoric and partly as genuine expression of concern about relative power shifts, but the allegations also carry consequences by raising Japanese public awareness of perceived Chinese threats and thus gradually shifting the bounds of acceptable Japanese security discourse. If such accusations intensify or if China undertakes military actions that Japanese elites interpret as aggressive, the comfort level with nuclear restraint would decline substantially.

The CSIS findings ultimately underscore that Northeast Asian nuclear stability rests on contingent foundations rather than durable institutional or strategic arrangements. Elite consensus against nuclear weapons development exists today, but could reverse within months if triggering events occurred—a North Korean weapons breakthrough, evidence of American withdrawal from the region, a significant military crisis, or adversary nuclear weapons deployment. The cascade risk means that the first nation to break ranks would face far less isolation than historical precedent might suggest, since elites in neighbouring countries would argue that changed circumstances justified changed policy. Building permanent barriers to proliferation requires not just current diplomatic efforts but long-term institutional arrangements, clearer alliance commitments and mechanisms that make nuclear weapons acquisition domestically costly rather than merely strategically undesirable.