Executive Summary

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The world is entering the Third Nuclear Age, marked by multipolarity and a shift in power dynamics. Japan and South Korea perceive a diminishing American nuclear umbrella and a weakening non-proliferation regime, and they are reevaluating the role of nuclear weapons in their security strategies. This report explores how Japan and South Korea perceive strategic stability and how their understanding of the threats they face – and the strategies they employ to deal with those threats – impact the global nuclear order.

Strategic stability lacks a universally agreed-upon definition and has various interpretations. While some focus on preventing nuclear first strikes, others consider a broader range of factors, including non-nuclear capabilities like conventional weapons and emerging technologies. Traditionally, this concept was applied to the relationship between the United States and Soviet. In the Third Nuclear Age however, China is emerging as third nuclear superpower, and there is greater number of smaller nuclear powers as well. But there is also a “small-m” multipolar aspect to the Third Nuclear Age that has received comparatively less attention: the role that non-nuclear armed allies of nuclear-armed powers play in shaping the global nuclear order and strategic stability.

Japanese and South Korean views of strategic stability

While Japanese scholars are familiar with the concept of strategic stability, its application to the modern geopolitical situation in Northeast Asia remains debated. The stability-instability paradox, where stability at the strategic level could lead to instability on the regional level, is a major concern for Japan. Japanese officials are concerned about the United States’ ability to simultaneously deter multiple adversaries, potentially impacting the credibility of extended deterrence. To compensate for perceived US overstretch, Japan aims to create a denial posture through increased conventional capabilities and partnerships with likeminded countries. While recognizing that it must reduce overdependence on the United States, Japan finds it hard to envision regional security without the crucial role of the United States, viewing it as a “linchpin” for meaningful cooperation with other partners.

South Korean views of strategic stability prioritise predictability, viewing it as crucial for economic prosperity. The concept of strategic stability is either used in a broad general way, or in a limited geographical sense, encompassing only the Korean Peninsula. North Korea’s relationship with Russia is an emerging concern, as it could embolden North Korea to take actions perceived to undermine stability on the Korean Peninsula. South Korean experts are particularly concerned about the potential for a North Korean-Russian nuclear alliance and the risk of adventurism being seen as profitable. While South Korea acknowledges the importance of the US-China strategic competition, there is a reluctance to fully support US deterrence efforts against China, particularly regarding the deployment of US Forces Korea (USFK) outside the Korean Peninsula. This reluctance stems from concerns about arms race stability, crisis stability, and the potential impact on South Korea’s relationship with China. For this reason, it remains a subject of debate in South Korea whether it has a deterrence relationship or not with China.

Japan and South Korea face a similar threat environment, but their different views of strategic stability shape their views of their own strategic roles in the region. They both seek to hedge and tie the United States closer to the region, but they place different emphasis on these strategies: South Korea – especially under progressive administrations – is more prone to hedging and envisioning a region without a significant US presence; Japan – even when it is ostensibly hedging – consistently seeks to keep the United States involved in the region at all costs.

Extended deterrence and nuclear salience

These approaches to the United States are particularly pronounced in the ways that they negotiate the nuclear aspects of their respective alliance and extended deterrence relationships with the United States. Extended deterrence can be conceptualised as an ongoing, continuous negotiation between the nuclear patron and the non-nuclear ally over the development, deployment and employment of nuclear capabilities. Introduction of new and more advanced, visible or destructive capabilities are generally more controversial because they change the role of nuclear weapons within the alliance, and they thus generate more nuclear salience, defined here as the general level of intensity and attention that policy elites perceive that they need to dedicate to a given nuclear policy option.

Allies pursue negative or low salience when they are content with the patron’s capability and willingness to protect them. In the case of Japan and South Korea, US capability is evidenced by a favourable military balance against adversaries, a reliable US second strike capability, and relatively strong warfighting ability of US-ally joint forces. They assess US willingness to defend them based on positive political statements to that effect. They may take pro-disarmament positions – like Japan did and continues to do – or make special non-proliferation commitments – like South Korea did in the 1990s. While neither country made any effort to dispense of their nuclear umbrella altogether in the 1990s and early 2000s, the salience of extended deterrence in both alliance relationships diminished briefly.

Allies pursue low to moderate salience when they perceive a negative regional military balance, weakening credibility of extended deterrence commitments, but remain sure that the alliance patron is still politically committed to their defence. In this position, Japan and South Korea argue for strengthened US nuclear capabilities, chiefly to support escalation dominance. They may not necessarily push for higher salience, such as redeployment of tactical nuclear weapons or nuclear sharing because the US political willingness to defend them is still seen as sufficient without such assurances.

Japan and South Korea began to shift towards this position as China’s military build-up and North Korea’s nuclear tests heightened concerns about strategic stability, leading to increased engagement with the United States on extended deterrence. This engagement, manifesting in dialogues and exercises, aimed to strengthen the credibility of US deterrence and security guarantees. While allies welcome US capability enhancements, they also advocate for conventional-nuclear integration to bolster deterrence credibility. In either case, their approaches to the alliance made nuclear weapons more salient in the alliance than before.

Allies pursue moderate to high salience when they perceive the nuclear patron’s deterrent as technically credible, but harbour doubts about the patron’s intention to use that deterrent. In this negotiating position, moderate increases of salience include further integration and institutionalisation to “lock in” the US commitment, offers to share burden of defence to make the political commitment easier for the United States. In this position, they also argue for approaches with higher nuclear salience, such as redeployment of US capabilities, or nuclear sharing, and become more likely to discuss independent nuclear armament as long as it does not come at the expense of the alliance. They may also seek a “technical deterrent” as a hedge against sudden withdrawal of extended deterrence assurances. These negotiating positions characterised Japanese and South Korean approaches to extended deterrence from the first Trump administration until today, and have made nuclear weapons a salient policy issue in both countries.

Finally, allies pursue high salience when they perceive the nuclear patron’s political commitment as uncredible, and no capability enhancement or integration can make it so within a relevant time-period. In this position, Japan and South Korea may abandon negotiation entirely and seek independent nuclear armament or some alternative form of security arrangement. Neither Japan nor South Korea have assumed this negotiating position, but some – especially in South Korea – argue that the US nuclear deterrent is not sufficiently credible to deter North Korea, and do not trust US intentions either. In either case, such moves to permanently high salience of nuclear weapons would only materialise in case of an actual US withdrawal from the region.

Today, policies implemented between the United States and Japan and South Korea display moderate nuclear salience, with South Korea pushing more actively for higher nuclear salience.

Implications

This report notes three implications of Japanese and South Korean efforts to increase nuclear salience for crisis stability, arms race stability and arms control:

Nuclear salience affects deployment of conventional capabilities and undermines crisis stability: Nuclear salience leads to enhanced conventional postures to compensate for perceived shortfalls in nuclear deterrence. While this may in some cases strengthen strategic stability, it could negatively impact crisis stability due to the potential for misinterpretation and rapid response pressures, and difficulty to distinguish conventional from nuclear capabilities.

Nuclear salience begets nuclear salience, justifies proliferation and undermines arms race stability: Extended deterrence, while historically successful in containing proliferation, may become a driver of nuclear proliferation in the Third Nuclear Age because it reinforces the notion that nuclear weapons are the ultimate security guarantee, leading to a reliance on them and potential proliferation. These implications are noticeable both in East Asia and in Europe today.

Nuclear salience undermines arms control efforts and underscores the need for new non-proliferation tools for a multipolar world: In a multipolar world, traditional non-proliferation tools may not be sufficient to prevent nuclear proliferation. Economic interdependence and the collective reaction of states opposed to nuclear weapons could be powerful deterrents. The empowerment of the Global South in the Third Nuclear Age could be a necessary part of a new multipolar approach to non-proliferation.

As the world is entering the Third Nuclear Age, and nuclear weapons are regaining salience, there is an urgent need for policymakers to not just consider how old familiar concepts of previous nuclear eras can be applied to a new and unfamiliar world, but also how these concepts created this world in the first place. Measures taken to enhance strategic stability in the past, such as the practice of extended deterrence, pose new challenges to strategic stability in the present.

About the Author

Joel Petersson Ivre is a Policy Fellow at APLN. His research interest lies in Northeast Asian nuclear issues, as well as South Korean and Chinese foreign policy. At APLN he has researched regional views of the US-China relationship, threat perceptions among states receiving extended deterrence guarantees from the United States, the domestic South Korean nuclear debate, and engagement strategies with North Korea. He received his Master’s degree from Yonsei University’s Graduate School of International Studies in East Asian Studies and International Security and Foreign Policy, and his Bachelor’s degree in Chinese Language and Culture from Stockholm University. He has held internships at East Asia Institute in Seoul and Stockholm International Peace Research Institute.

This report was supported through a grant from the UK Foreign, Commonwealth, and Development Office. The opinions articulated above represent the views of the author(s) and do not necessarily reflect the position of the Asia-Pacific Leadership Network or any of its members, or funders.