South Korea’s Nuclear Submarines: Deterrence and Risk in Northeast Asia

As part of his first international trip, United States (US) Under Secretary of War for Policy Elbridge Colby travelled to South Korea on 25 January 2026 to discuss bolstering the US military alliance in the region. At the centre of this discussion was the US-South Korea submarine deal, concluded previously by the presidents of the two nations during their meeting on the sidelines of the Asia-Pacific Economic Cooperation (APEC) Summit in October. One key decision from that meeting centred on the Trump administration’s approval for South Korea to build nuclear-powered attack submarines, a drastic shift from the earlier US-South Korea alliance agreement. Since the announcement, a series of debates and discussions have emerged in the region, with a cautious range of statements from North Korea and Japan, signalling a shifting regional dynamic. While South Korea’s interest in developing nuclear submarines is not new, the decision has raised concerns over its regional implications, spanning other areas, including non-proliferation and the arms race.

South Korea’s Quest for Nuclear Submarines 

South Korea has long perceived North Korea’s evolving nuclear and military modernisation as an existential threat to its security. As a result of this protracted concern, Seoul has, over the last two decades, lobbied the US to allow it to develop certain strategic technologies, including nuclear submarines—reflecting Seoul’s long-standing interest—dating back to the Roh Moo-hyun presidency in 2003. During his 2021 presidential campaign, current President Lee Jae Myung promised to actively pursue US support for such submarines, framing it as a critical step to strengthen South Korea’s defence posture. The US approval to build a nuclear attack submarine should be seen as part of Seoul’s long-pending demands. Nuclear-powered submarines help nuclear-armed states secure stealth and survivability, and thereby, an assured second-strike option. However, for a non-nuclear state like South Korea, nuclear-powered submarines are not especially relevant in operational terms. Amid recent challenges surrounding alliance commitments and fears of abandonment, this move is intended to strengthen non-nuclear, conventional deterrence against North Korea.

Nuclear-powered submarines help nuclear-armed states secure stealth and survivability, and thereby, an assured second-strike option. However, for a non-nuclear state like South Korea, nuclear-powered submarines are not especially relevant in operational terms.

Some of South Korea’s outstanding demands were granted in 2021 during President Joe Biden's tenure, including the signing of the Washington Declaration, the creation of the Nuclear Consultative Group (NCG), the deployment of a nuclear-armed submarine, and the complete removal of restrictions on missile development. Other issues, such as the transfer of Wartime Operational Control, remain under discussion.

The rapid development of Pyongyang’s nuclear and submarine programme adds to the urgency. In the last few years, North Korea has consolidated its nuclear posture and has entered into a comprehensive strategic partnership treaty with Russia. South Korea sees a significant risk from Moscow’s potential assistance to North Korea in developing nuclear-powered submarines, a move that could destabilise the regional balance. Simultaneously, Pyongyang has adopted a strategy of nuclear brinkmanship vis-à-vis the United States to counter its extended deterrence arrangement with South Korea. Against Seoul, Pyongyang follows a strategy of warfighting, despite possessing nuclear weapons, to consolidate territorial gains and the unification of the Korean peninsula. North Korea, in September 2022, passed a new law which legitimises the first use of nuclear weapons in a crisis. To validate its crisis planning, the Kim regime in August 2023 conducted a simulated ‘scorched-earth’ tactical nuclear strike on targets across South Korea in anticipation of a preemptive strike from the United States.

In this context, North and South Korea see the changing regional equations among the US, China, and Russia as an opportune moment to build their capabilities. In this era of great-power contest, both have hedged their bets in an effort to advance their respective strategic interests. US-China strategic competition and Russia’s war in Ukraine have reduced external pressure on North Korea. Similarly, South Korea has seized the opportunity to enhance its deterrence, leveraging its alliance with the US. Building on the 2021 Washington Declaration, which granted South Korea missile sovereignty, the recent move signals a strategic shift in Seoul’s nuclear posture. 

Washington’s Strategic Calculus in Northeast Asia 

For the US, an unmanaged Northeast Asia has implications for its credibility and leverage in other regions. Since the war in Ukraine, North Korea, as a nuclear challenge, cannot be sidelined by the US, especially when considered as part of a strategic nuclear triangle linking North Korea, China, and Russia. However, the two Koreas approach this issue differently.

Multiple factors influence American decision-making. For the US, the geographically distant yet imminent threat of North Korean nuclear missiles to the homeland remains a key factor in its security calculus. These concerns have driven support for strengthening South Korea’s deterrence posture in the region and aligning it with the US’s evolving Indo-Pacific strategy. This has played out in the development and procurement of missiles by US allies in the region, including South Korea and Japan, as well as in initiatives such as the AUKUS (Australia-United Kingdom-United States) partnership.

For the US, an unmanaged Northeast Asia has implications for its credibility and leverage in other regions. Since the war in Ukraine, North Korea, as a nuclear challenge, cannot be sidelined by the US, especially when considered as part of a strategic nuclear triangle linking North Korea, China, and Russia.

However, it would be hard to ignore another factor: the politics behind this deal. Given President Trump’s transactional approach and South Korea’s pledge of a US$350 billion investment as part of a broader trade deal, it would be naïve to underestimate the role of the political leadership on both sides. This is underscored by the fact that the Trump administration, in its first term, was opposed to the proposal to supply South Korea with low-grade nuclear fuel, citing non-proliferation concerns. The same principles of nuclear non-proliferation have now been effectively overruled during President Trump’s second term.

Beyond serving Washington’s own interests, the approval of nuclear-powered submarines is a constructive step vis-à-vis Pyongyang. It helps reassure Seoul of the US commitment to the alliance amid fears of abandonment in the post-Ukraine environment. Building on the Washington Declaration signed between the Biden and Yoon administrations in 2023, the decision to acquire—or the actual acquisition—of latent capabilities such as nuclear-powered submarines may create avenues for constructive engagement towards risk reduction in Northeast Asia.

Regional Implications: Rising Tensions and Proliferation Risks 

The approval of a nuclear-powered submarine carries serious regional implications, affecting security, stability, and political dynamics. First, it incentivises countries such as South Korea to move closer to nuclear proliferation in the near future. Second, an order that encourages nuclear proliferation can have severe consequences, potentially setting in motion a broader regional momentum, including in Japan, which may follow suit. However, as a party to the Treaty on the Non-Proliferation of Nuclear Weapons (NPT), South Korea would face significant constraints in pursuing such a path.

Against the backdrop of an evolving security environment and the region’s increasing nuclearisation, Japan is viewing the prospects of acquiring nuclear-powered submarines more favourably. For instance, in November, Shinjirō Koizumi, the Japanese Defence Minister, expressed support for such submarines, noting that, “the environment surrounding Japan is truly becoming so severe that we must debate whether to continue with diesel as we have, or to opt for nuclear-powered submarines.” The government is facing growing calls within the ruling party and among its allies advocating for the acquisition of nuclear weapons. Even within the government, there is an emerging debate on revisiting Japan’s three non-nuclear principles.

The approval of a nuclear-powered submarine carries serious regional implications, affecting security, stability, and political dynamics. First, it incentivises countries such as South Korea to move closer to nuclear proliferation in the near future. Second, an order that encourages nuclear proliferation can have severe consequences, potentially setting in motion a broader regional momentum, including in Japan, which may follow suit.

Moreover, by implicitly legitimising North Korea’s existing nuclear submarine-launched ballistic missile programmes, this step further provides the Kim regime with an additional strategic rationale to invest in upgrading its conventional and non-conventional arsenal—moves that are likely be announced at the upcoming Ninth Party Congress this year. This development further diminishes the already limited prospects for the denuclearisation of the Korean Peninsula. The modernisation of Pyongyang’s submarine fleet has long been constrained by limited access to technology and resources. As North Korea’s ties with China and Russia strengthen, the prospects for technology cooperation among the three will depend on the extent to which the US allows its allies to develop strategic assets.

In conclusion, the US approval will likely send multiple signals to different stakeholders, including both allies and adversaries. While the premises of such a framework sit well with Trump’s worldview of ‘peace through strength’, the move risks backfiring in a region where the logic of the ‘balance of threat’ looms large. Therefore, whether this departure from the approaches of previous administrations will yield results is contingent on time. As part of the long game, US policy needs to balance the dilemma of reassuring allies without giving away too much— including a setback to the non-proliferation order—while at the same time addressing the threats to its allies and its Indo-Pacific strategy.

Abhishek Sharma is a Junior Fellow with the Strategic Studies Programme at the Observer Research Foundation.

Rahul Rawat is a Research Assistant with the Strategic Studies Programme at Observer Research Foundation.

• International Affairs
• Nuclear Security
• East Asia
• USA and Canada
• nuclear powered submarines south korea
• south korea north korea relations
• south korea nuclear submarine
• south korea submarine fleet
• south korea submarine program

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