Ⅰ. Introduction

 

The global nuclear order is under unprecedented strain. Russia’s repeated nuclear threats during the Ukraine War exposed its vulnerability (Budjeryn 2022). Meanwhile, North Korea’s continued advancement in nuclear weapons has undermined key international regimes, like the Nuclear Nonproliferation Treaty (NPT) (Knopf 2021). Adding to this instability is the intensifying nuclear competition between the United States and China. China has shifted away from its “minimum deterrence” posture, rapidly expanding its nuclear forces in recent years (Talmadge and Rovner 2023). In response, the United States has also intensified its nuclear modernization efforts, upgrading its nuclear triad (Liang 2024).

 

This evolving global nuclear order presents South Korea and Japan with complex and multifaceted challenges. Heightened concerns about North Korea’s advancing nuclear and missile capabilities loom large, compounded by the strategic implications of accelerating nuclear arms race in the region. The two countries are under mounting pressure to navigate these shifting dynamics in the global nuclear order while safeguarding their own security and regional stability. This article examines the two countries’ positions on nuclear weapons and evaluates how they might cooperate in addressing the challenges posed by these transformative developments.

 

Ⅱ. South Korean and Japanese Positions on Nuclear Weapons

 

Despite shared reliance on the U.S. nuclear umbrella, South Korea and Japan have followed different trajectories regarding nuclear weapons. South Korea’s interest in nuclear weapons dates back to the early 1970s under President Park Chung-hee, who sought to develop an independent nuclear capability amid fears of U.S. abandonment. However, U.S. pressure ultimately compelled South Korea to abandon its nuclear ambitions (Debs and Monteiro 2016). Seoul formalized its non-nuclear stance by joining the NPT in 1975. In return, South Korea received assurances of a nuclear umbrella. Beginning in 1978, a joint communique between South Korean and U.S. defense ministers started to include a reference to the commitment (Roehrig 2017, 126). In addition, since 2009, the joint statement following the annual Security Consultative Meeting (SCM) between the two countries has explicitly reaffirmed U.S. extended nuclear deterrence by stating that U.S. extended deterrence encompasses “full range of U.S. military capabilities, including nuclear, conventional, and missile defense” (U.S. Department of Defense 2024).

 

Despite U.S’ reassurances, South Korean public opinion reflects a strong inclination toward nuclearization. Since North Korea’s first nuclear test in 2006, polls have consistently shown majority support for South Korea developing its own nuclear weapons (Jung 2023), though this support tends to waver when potential costs such as economic sanctions are considered (Son and Park 2023; Lee 2024). A poll conducted in 2024, for instance, shows that about 72.8 percent of respondents concurred with the proposition of South Korea advancing its own nuclear capabilities (Yoo 2024).

 

Japan, too, contemplated nuclear weapons development, particularly after China’s first nuclear test in 1964. However, the country ultimately chose to rely on U.S. security guarantee, bolstered by Washington’s strong assurances of protection. U.S. President Johnson provided assurances of extended nuclear deterrence, and Japanese Prime Minister Sato formally declared the “Four Pillars” policy in 1968 (Roehrig 2017, 99). This policy not only affirmed Japan’s reliance on the U.S. for its nuclear deterrence but also introduced the Three Non-Nuclear Principles: no possession, no production, and no introduction of nuclear weapons. Since then, U.S. extended nuclear deterrence has remained an “indispensable” part of Japan’s security strategy (Japanese Government 2013).

 

Public opinion in Japan contrasts significantly with that of South Korea. Shaped by the collective memory of the Hiroshima and Nagasaki bombings, the Japanese public holds a strong nuclear taboo and shows robust support for nuclear disarmament. A public opinion poll found that approximately 75 percent of respondents supported Japan joining the Treaty on the Prohibition of Nuclear Weapons (TPNW) (Baron, Gibbons, and Herzog 2020). Similarly, another study revealed that only 21 percent of respondents favored Japan developing nuclear weapons (Matsumura, Tago, and Grieco 2023). Even under the scenarios of growing nuclear threats from North Korea or weakened U.S. commitments, only 28 percent of respondents supported Japan pursuing a nuclear option (Matsumura, Tago, and Grieco 2023). This consistent low support reflects Japan’s deep-seated aversion to nuclear armament.

 

Although South Korea and Japan have had different nuclear trajectories and domestic preferences, their shared security challenges in the region and reliance on U.S. extended nuclear deterrence provide compelling reasons to explore potential avenues for cooperation. As the nuclear landscape continues to evolve—with rising tensions involving North Korea and intensifying competition between the United States and China—South Korea and Japan will need to consider pragmatic ways to navigate these challenges together. The following section explores key areas where cooperation is both possible and necessary.

 

Ⅲ. Potential Areas of Cooperation

 

1. Dealing with a Nuclear North Korea

 

The North Korean nuclear issue has remained at a persistent stalemate. Following the breakdown of the second Trump-Kim summit in 2019, efforts to address Pyongyang’s nuclear ambitions have largely stagnated. Under the Biden admіnistration, North Korea was deprioritized as a foreign policy issue, taking a back seat to other pressing international and domestic concerns. In the meantime, North Korea has continued to make significant advancements in its nuclear weapons and missile capabilities, further escalating regional and global security concerns.

 

This prolonged deadlock, however, is likely to be broken in one way or another under the second Trump admіnistration. In exchanges with reporters after his inauguration, President Trump referred to North Korea as a nuclear power, stating “I liked him [Kim Jong-un]. We got along very well…He is a nuclear power.” (Parry 2025). Key admіnistration figures, such as Pete Hegseth, U.S. defense secretary, have similarly acknowledged North Korea’s status as “a nuclear power” (Senate Armed Service Committee 2025). Additionally, President Trump also indicated that he would reach out to Kim (Cho 2025), suggesting the possibility of renewed high-profile summit diplomacy.

 

It remains unclear whether Trump’s remark signals that the United States will indeed officially acknowledge North Korea as a nuclear power and engage in arms reduction negotiations. Yet there has been speculation that the admіnistration might prioritize reducing North Korea’s intercontinental ballistic missile (ICBM) capabilities over pursuing full denuclearization. Influential figures in Trump’s policy circles, such as former Deputy Assistant Secretary of Defense Elbridge Colby, have advocated this approach (Kim 2024). If the Trump admіnistration attempts to shift its focus to arms reduction and recognizes North Korea as a nuclear power, such a position would pose a fundamental challenge to South Korea and Japan.

 

On the one hand, the approach could be considered to be a “realistic” response that reflects the stark reality that past efforts—ranging from bilateral agreements like the 1994 Agreed Framework to the Six-Party Talks and even the two Trump-Kim summits—have all failed to halt North Korea’s nuclear advancements. Furthermore, North Korea has firmly declared its nuclear weapons as non-negotiable, asserting that “nuclear deterrence is the only and most correct choice in the current situation” (Choi 2024). In a 2024 speech, Kim Jong-un also underscored his skepticism about another negotiation, mentioning that “We have already gone as far as we can go with the United States in negotiations, and what we were sure as an outcome was not the willingness of the great power to coexist, but a stance of uncompromising strength and that an aggressive and hostile policy toward the DPRK that will not change anytime soon” (Lee 2025).

 

However, this approach also comes with significant drawbacks. It would not only signify the collapse of key non-proliferation frameworks like the NPT, but, more critically, alter regional security dynamics fundamentally, further destabilizing Northeast Asia. In addition to relying on U.S. extended nuclear deterrence, South Korea and Japan have, to some extent, tolerated being North Korea’s “nuclear hostage” based on the hope that denuclearizing North Korea remained as a slim but viable possibility. If this hope were to disappear, it could pave the way for the two countries—in particular South Korea—to consider more extreme policy options. According to a poll conducted by East Asia Institute (EAI) in 2024, 71.4% of respondents supported South Korea developing its own nuclear weapons if North Korea does not abandon its nuclear weapons (EAI 2024). Such a development would not only provoke strong reactions from China and North Korea but also put Japan in a difficult position, forcing Tokyo to weigh its security concerns against its long-standing non-nuclear principles. Additionally, it could also create tensions between South Korea and Japan due to their diverging domestic attitudes toward nuclear armament. Nuclear acquisition enjoys substantial support in South Korea and South Korean public support for Japan’s nuclearization has also grown to 34% (EAI 2024). Yet, only 10% of the Japanese public favors South Korea becoming a nuclear power, while 61.4% oppose it (Genron NPO 2023).

 

The two countries would thus have incentives to work together proactively to persuade the United States to not completely scrap the goal of denuclearization even though Washington engages in arms reduction. In response to Trump’s remark, South Korea has already reiterated that denuclearization remains its primary objective (Reuters 2025-01-20). Japan has also emphasized the need for “close coordination with the United States to address North Korea’s missile and nuclear programs” (Kyodo News 2025-01-21). South Korea and Japan can work closely with Washington to ensure that denuclearization remains a long-term goal, even if arms reduction becomes the immediate focus. This could involve proposing a phased approach where arms reduction agreements are tied to verifiable commitments from North Korea to limit its nuclear arsenal and halt further advancements, with a long-term goal of denuclearization. Additionally, South Korea and Japan could leverage their roles as regional stakeholders to advocate for a more comprehensive strategy that directly addresses their security concerns. Even if arms reduction efforts successfully limit North Korea’s ICBM capabilities, both countries would remain vulnerable to its shorter-range missile threats and coercive strategies due to their geographical proximity. Thus, focusing solely on ICBM reductions would be insufficient to ensure regional stability, as it fails to mitigate the immediate security risks faced by South Korea and Japan. The two countries can call for an approach that tackles the broader spectrum of threats posed by North Korea.

 

2. Nuclear Risk Reduction

 

Another critical area of cooperation for South Korea and Japan is managing the growing risk of nuclear conflict in the region. The intensifying competition between the United States and China has heightened the potential for nuclear escalation, particularly in a regional conflict scenario involving Taiwan. In such a crisis, nuclear weapons “could play multiple roles in the deterrence and warfighting strategies and operations” of both the United States and China (Weaver 2023). China, for instance, might employ nuclear signaling, such as publicly lowering its nuclear threshold, to deter U.S. intervention (Beauchamp-Mustafaga et al. 2024). Nuclear threats or actual use would have immediate and severe consequences for South Korea and Japan due to their geographic proximity and strategic ties to the United States. Both countries would likely face a range of security challenges, including heightened U.S. expectations for regional support and potential disruptions to U.S. extended deterrence commitments for their own country. North Korea’s evolving nuclear posture further adds to the risks of nuclear conflict. The nuclear law North Korea passed in 2022 expanded its justification for nuclear use compared to its 2013 law, stipulating that nuclear weapons can be launched not only in response to an attack by a hostile nuclear power but also preemptively if North Korea determines that such an attack is “imminent” (Davenport 2022). This shift significantly lowers the threshold for nuclear use, increasing the risk of nuclear crises on the Korean Peninsula.

 

Addressing these risks requires enhanced trilateral coordination with the United States to prepare for potential contingencies and establish clear frameworks for crisis management and response. While South Korea and Japan have maintained extended deterrence dialogue with the United States—South Korea established a Nuclear Consultative Group (NCG) following the Washington Declaration in 2023, and Japan has engaged in the Extended Deterrence Dialogue (EDD) since 2010—a trilateral platform specifically designed to address nuclear risks and contingencies would not only be warranted but also ideal for fostering alignment and mutual understanding. This is especially crucial if North Korea is tacitly acknowledged as a nuclear power, despite the preferences of the two countries. In such a scenario, closer coordination with the United States would be essential to ensure that South Korea and Japan’s security concerns are addressed, while also mitigating the broader risks posed by nuclear weapons in the region. The institutional channel established as a follow-up measure after the Camp David Summit in 2023 would be a good place to start. By working together on these fronts, South Korea and Japan can contribute to reducing nuclear risks while reinforcing their shared commitment to global security and stability.

 

Ⅳ. Conclusion

 

Amid the strained global nuclear order, South Korea and Japan have opportunities for cooperation. This article has examined these potential avenues. However, the feasibility of such cooperation is partly contingent on the domestic political dynamics within each country. Security cooperation between the two countries has always been a contentious issue domestically, and changes in leadership—such as the next admіnistration in South Korea or the selection of Japan’s next Prime Minister—could significantly reshape the trajectory of their bilateral relationship.

 

Nevertheless, this does not mean that cooperation between South Korea and Japan is unattainable. As established democracies in the region, the two countries share fundamental values, including a commitment to the rule of law, human rights, and freedom. Additionally, their shared security concerns—particularly regarding North Korea’s nuclear ambitions, and the broader security challenges posed by great-power rivalry—will continue to provide a foundation for cooperation, even amid political or geopolitical uncertainty. In an increasingly volatile and unpredictable environment, the mutual benefits of cooperation should not be underestimated. ■

 

References

 

Baron, Jonathon, Rebecca Davis Gibbons, and Stephen Herzog. 2020. “Japanese Public Opinion, Political Persuasion, and the Treaty on the Prohibition of Nuclear Weapons.” Journal for Peace and Nuclear Disarmament 3, 2: 299-309.

 

Beauchamp-Mustafaga, Nathan, Elliot Ji, Alexis Dale-Huang, Cindy Zheng, Gregory Graff, and Dahlia Anne Goldfeld. 2024. “Denial Without Disaster: Keeping a U.S.-China Conflict Over Taiwan under the Nuclear Threshold.” Rand Corporation.

 

Budjeryn, Mariana. 2022. “Distressing a Systеm in Distress: Global Nuclear Order and Russia’s War against Ukraine.” https://thebulletin.org/premium/2022-11/distressing-a-systеm-in-distress-global-nuclear-order-and-russias-war-against-ukraine/#post-heading (Accessed February 14, 2025)

 

Choe, Sang-Hun. 2025. “Trump Hints at New Talks with Kim Jong-un. Might be Harder This Time.” New York Times. January 24. https://www.nytimes.com/2025/01/24/world/asia/trump-north-korea-nuclear-talks.html (Accessed January 31, 2025)

 

Choi, Eunji. 2024. “North Korea tells international community ‘it will not give up building nuclear arsenal’” Herald Gyungje. November 5. https://mbiz.heraldcorp.com/article/3846971 (Accessed January 31, 2025)

 

Davenport, Kelsey. 2022. “North Korea Passes Nuclear Law.” https://www.armscontrol.org/act/2022-10/news/north-korea-passes-nuclear-law (Accessed January 31, 2025)

 

Debs, Alexandre, and Nuno P. Monteiro. 2016. Nuclear Politics: The Strategic Causes of Proliferation. Cambridge University Press.

 

East Asia Institute. 2024. “[EAI Public Opinion Briefing] 2024 EAI Public Opinion Poll on International Affairs: Overall Perception, United States, China, North Korea.” https://www.eai.or.kr/new/en/project/view.asp?intSeq=22782&code=102 (Accessed January 31, 2025)

 

Genron NPO. 2023. “Analyzing Differences in National Awareness as Japan-South Korea Relations Improve.” https://www.genron-npo.net/en/opinion_polls/archives/5629.html (Accessed January 31, 2025)

 

Jung, Sang Mi. 2024. Analysis on South Korean Public Opinion on the Country’s Nuclearization. Institute of Foreign Affairs and National Security.

 

Kim, Donghyun. 2024. “DPRK Denuclearization is an Unrealistic Goal…US Should Focus on Limiting its ICBM Range.” Yonhap News. May 8. https://www.yna.co.kr/view/AKR20240507088200071 (Accessed January 31, 2025)

 

Knopf, Jeffrey W. 2022. “Not by NPT alone: The Future of the Global Nuclear Order.” Contemporary Security Policy 43, 1: 186-212.

 

Kyodo News. 2025. “Japan to Coordinate with U.S. as Trump Calls North Korea Nuclear Power.” January 21. https://english.kyodonews.net/news/2025/01/fdca5f1da990-japan-to-coordinate-with-us-as-trump-calls-n-korea-nuclear-power.html (Accessed January 31, 2025)

 

Lee, Kyung Suk. 2024. “South Korean Cost Sensitivity and Support for Nuclear Weapons.” International Interactions 50, 3:506-536.

 

Lee, Sanghyun. 2025. “Would Kim Jong-un responds to Trump? Likely to be Cautious.” Yonhap News. January 24. https://www.yna.co.kr/view/AKR20250124128400504 (Accessed January 31, 2025)

 

Liang, Xiaodon. 2024. “U.S. Nuclear Modernization Program.” Arms Control Association. https://www.armscontrol.org/factsheets/us-modernization-2024-update (Accessed January 31, 2025)

 

Matsumura, Naoko, Atsushi Tago, and Joseph M. Grieco. 2023. “External Threats and Public Opinion: The East Asian Security Environment and Japanese Views on the Nuclear Option.” Journal of East Asian Studies 23, 1: 23-44.

 

Park, Eunil. 2019. “Positive or negative? Public perceptions of nuclear energy in South Korea: Evidence from Big Data.” Nuclear Engineering and Technology 51, 2: 626-630.

 

Parry, Richard Lloyd. 2025. “Alarm in Asia After Trump Calls North Korea a Nuclear Power.” The Times. January 22. https://www.thetimes.com/world/asia/article/alarm-in-asia-after-trump-calls-north-korea-a-nuclear-power-vnqmtm757 (Accessed January 31, 2025)

 

Prime Minister’s Office of Japan. 2013. “National Security Strategy.” December 17. https://japan.kantei.go.jp/96_abe/documents/2013/__icsFiles/afieldfile/2013/12/18/NSS.pdf (Accessed January 31, 2025)

 

Reuters. 2025. “South Korea Says Denuclearizing North Korea still Goal After Reports Trump Called it Nuclear Powers.” January 20. https://www.reuters.com/world/asia-pacific/south-korea-says-denuclearising-north-korea-still-goal-after-reports-trump-2025-01-21/ (Accessed January 31, 2025)

 

Roehrig, Terence. 2017. Japan, South Korea, and the United States Nuclear Umbrella: Deterrence after the Cold War. Columbia University Press.

 

Senate Armed Service Committee. 2025. “Advance Policy Questions for Peter “Pete” B. Hegseth Nominee to Serve as Secretary of Defense, January 6, 2025.” https://www.armed-services.senate.gov/imo/media/doc/hegseth_apq_responses.pdf (Accessed January 31, 2025)

 

Son, Sangyong, and Jong Hee Park. 2023. “Nonproliferation Information and Attitude Change: Evidence from South Korea.” Journal of Conflict Resolution 67, 6: 1095-1127.

 

Talmadge, Caitlin, and Joshua Rovner. 2023. “The Meaning of China’s Nuclear Modernization.” Journal of Strategic Studies 46, 6-7: 1116-1148.

 

U.S. Department of Defense. 2024. “56th Security Consultative Meeting Joint Communique.” October 30. https://www.defense.gov/News/Releases/Release/Article/3951794/56th-security-consultative meeting-joint-communique/ (Accessed January 31, 2025)

 

Yoo, Younggyu. 2024. “US Media Eyeing South Korea’s Discussion on Nuclearization. But is It Realistic?” SBS News. August 19. https://news.sbs.co.kr/news/endPage.do?news_id=N1007766655 (Accessed January 31, 2025)

 

Weaver, Greg. 2023. The Role of Nuclear Weapons in a Taiwan Crisis. Atlantic Council, Scowcroft Center for Strategy and Security.

 

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■ Jiyoung Ko is Associate Professor in the Department of Political Science and International Relations at Korea University.

 

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■ Edited by Hansu Park, EAI Research Associate
    For inquiries: 02 2277 1683 (ext. 204) | hspark@eai.or.kr