Key Takeaways:

- Integration over Autonomy: While South Korea views acquiring nuclear-propelled submarines (SSNs) as a path to strategic autonomy, the U.S. intends to use the deal to deepen industrial integration and leverage Korean shipbuilding capacity to support the American defense base, mirroring the AUKUS model. 

- Expanding Regional Security: The Trump administration views the provision of SSN technology as a "downpayment" that obligates South Korea to expand its military footprint beyond the Peninsula and participate actively in broader Indo-Pacific deterrence against China. 

- Strategic Alignment Gap: Unlike Australia's clear maritime role under AUKUS, South Korea currently lacks a proven strategic framework for deploying SSNs regionally, creating a risk of a significant "expectation gap" between Washington's demands for a global ROK navy and Seoul's traditional focus on North Korea.

Introduction

US President Donald Trump’s promise to assist the Republic of Korea (ROK) in acquiring nuclear-propelled submarines (SSNs) caused much excitement around the region, including in Australia. The ‘deal’ was notable for several reasons, not least because it was announced at the same time as the fate of another SSN deal – that with Australia through the AUKUS pact – hung in the balance. However, now it seems that the tables have turned. While AUKUS is ‘full steam ahead’, there are growing questions about the scope and requirements of a potential US-ROK SSN arrangement, including where such a deal might fit with South Korea’s wider role with respect to Indo-Pacific security.

Rather than providing an industrial roadmap to be emulated, AUKUS instead offers broad lessons for the ROK about the key industrial, political and strategic drivers of US interest in such an arrangement. Indeed, as the White House’s new National Security Strategy makes clear, nothing comes cost-free for allies from the Trump administration. In that respect, AUKUS shows that US assistance with allied SSN aspirations, whatever that may ultimately look like, is about more than allies owning their security burdens. Just as it has with Canberra, Washington will expect Seoul to integrate more deeply with the US defence industrial base to support American military requirements, and to align more deeply on regional defence strategic priorities, meaning that negotiations between the US and the ROK over a potential SSN deal cannot be separated from Washington’s growing pressure to retool the alliance for deterrence vis-à-vis China and the First Island Chain.

Integration over autonomy

Firstly, whatever form the US-ROK submarine deal takes, it will almost certainly require a greater degree of integration between Washington and Seoul on defence industrial policy. This will have clear implications for South Korea’s sense – if not its reality – of ‘strategic autonomy’, essentially a shorthand for greater South Korean agency in managing its own strategic affairs on the Korean Peninsula in a way that reduces the necessity, and therefore influence, of US military and political assistance. In fact, several ROK leaders have already explicitly framed the as a means to enhance Seoul’s strategic autonomy, casting the ‘deal’ as shifting the burden of defence on the Korean Peninsula off of the United States. They have also insisted that the ROK should produce the submarines on its own shipyards with only minimal US support on matters of nuclear fuel, a position at odds with President Trump’s original indication that the SSNs would be built in America.

Yet it’s just as likely that a US-ROK SSN deal will further entwine the two countries on a deeper level. The US increasingly sees Korea as a valuable source of naval maintenance capacity to meet in-region sustainment requirements for US forces, and as a source of capital for expanding America’s own shipbuilding capacity at home. In that respect, it’s no coincidence that there have already been reports of a formal co-production arrangement that would see ROK SSNs co-produced in US and Korean shipyards. This would be consistent with a broader trend whereby defence industrial and technology integration has become ground zero for US alliance modernisation agenda in Asia, including with South Korea (though the US-ROK alliance has lagged compared to others until now), efforts premised on deeper defence industrial integration at the policy and regulatory level to support regional strategic objectives and to offset major weaknesses in the US defence industrial base, including on submarine production.

While the details of a prospective US-ROK deal remain forthcoming, the good news for Seoul is that autonomy and integration are not mutually exclusive. If anything, Australia’s experience through AUKUS suggests that they can be mutually reinforcing. On the one hand, AUKUS has been cast in in terms of bolstering Australia’s capacity to manage its own security affairs, and is consistent with the 2024 National Defence Strategy’s commitment to enhancing Australia’s capacity to deliver independent military effects in its near region. At the same time, Australia recognises that the success of AUKUS is contingent upon deeper defence industrial and technology integration with the United States, including the harmonisation of defence trade controls, foreign investment screening and security clearance processes; workforce sharing and training; large Australian investments in all three AUKUS countries’ defence industrial bases; and a range of other initiatives aimed at strengthening the combined submarine industrial base of all three countries. While all of these measures will strengthen Australia’s own defence capability and industrial capacity, they will also support America’s long-term strategic position in Asia – another core objective of Australian strategy.

Of course, a US-ROK SSN arrangement would not perfectly mirror AUKUS given clear differences between the two alliances, not least Australia’s preexisting status as a part of the legal definition of the US defence industrial base and membership in the Five Eyes intelligence-sharing arrangement, and South Korea’s comparatively mature submarine industrial base and proven defence production capacity. Yet present trends suggest that the United States will be seeking an agreement that contributes to its defence industrial vitality and, ideally, shores up its wider regional position. In that respect, arguments that the ROK will be helping to meet US objectives simply by becoming more ‘autonomous’ are unlikely to gain sufficient traction with an administration seeking to leverage alliance integration to support ‘America First’ policy objectives.

Transforming the alliance and the ROK’s regional role

Relatedly, discussions over a potential SSN deal cannot be separated from ongoing debates within and between the United States and South Korea over how to expand the alliance’s strategic aperture. In basic terms, while South Korean policymakers primarily seem to view the utility of SSNs vis-à-vis priorities on and around the Peninsula, evidence suggests that the Trump administration would see cooperation on these assets as a downpayment on a more active Korean contribution to regional security beyond the confines of Northeast Asia. It's no coincidence that the submarine issue has surfaced in the midst of a wider discussion not only about cost-sharing arrangements for US Forces Korea (USFK) Peninsula and the numbers and types of forces stationed there, but increasingly about how US and South Korean forces stationed on the Peninsula should play a more prominent role in wider regional security. In other words, Seoul’s strategic intent is just as important a factor to Washington as the SSN capability itself.

To be sure, this is not a new debate. In recent years, successive Commanders of USFK have sought to underscore the off-Peninsula utility of USFK and the US-ROK alliance more broadly, yet South Korean policymakers have historically resisted[TC1]  overtures for greater ‘strategic flexibility’ for USFK, let alone Korean forces. However, US military leaders have upped the pressure in the wake of the submarine ‘deal’. In November, US Forces Korea Commander Xavier Brunson published an ‘East-Up Map’ highlighting how allied forces on the Peninsula were already "capable of immediate cost-imposition against multiple adversaries," not just North Korea, and positing the Peninsula as part of "a strategic triangle connecting Korea, Japan, and the Philippines" with respect to the First Island Chain, a geographic construct linking Northeast Asia, the Taiwan Strait, and the South China Sea. That same month, US Chief of Naval Operations Daryl Caudle stated in no uncertain terms that "there will be a responsibility for Korea to deploy those submarines globally and move away from just being a regional navy to a global navy,” and highlighted the “natural” role that prospective ROK SSNs would have in countering China’s region-wide maritime activities.

The basic implication of these statements is that Washington will expect the ROK to employ its naval forces, including SSNs, for missions away from the Korean Peninsula. Yet the ROK’s own operational and strategic logic for doing so remains unclear. Operationally, while ROK SSNs would be useful in countering North Korea’s own emerging subsurface fleet – including nuclear-powered and nuclear-armed submarines– their value in monitoring Chinese activities in the East and West Seas is far less convincing given the shallow depths of those waters. Strategically, successive Korean administrations have pledged to play a more prominent security role across Asia, yet there is relatively little evidence to suggest that South Korea is actually doing so in ways that would benefit from the capabilities that SSNs would provide. Even the relatively modest goal of deploying counter-piracy units to the region articulated under the previous government’s Indo-Pacific Strategy has gone unrealised.

That President Lee Jae-myung reportedly cited both North Korean and Chinese threats in pressing South Korea’s case in his pre-announcement meeting with President Trump was striking, but whether or not his administration means to follow through on those sentiments remains to be seen. Indeed, neither of Seoul’s operational nor strategic cases for SSN acquisition are likely to be sufficient to satisfy Washington’s questions over the remit of the US-ROK alliance and/or the ROK Navy’s regional footprint. This contrasts with AUKUS, where Australia’s operational requirements for SSNs are more widely accepted given the country’s extensive maritime geography, and where Canberra has frequently demonstrated its willingness to employ defence assets in support of collective deterrence operations across the First Island Chain, even before AUKUS was announced in September 2021.

Importantly for South Korean stakeholders, AUKUS is not premised on a political precommitment by any party – including Australia – to any specific regional contingency. Though questions have been raised about specific contingencies where Australian SSNs could be employed against Chinese forces including in prospective Taiwan scenarios, in actuality these are discussions about alliance planning and strategic and operational alignment rather than about the terms and conditions of the AUKUS program itself. Likewise, South Korea should not expect, nor be expected, to pre-commit to hypothetical conflict scenarios over the Taiwan Strait or the South China Sea in exchange for assistance with SSN. But it will almost certainly be expected to contribute more consistently and more visibly to peacetime maritime deterrence missions across the wider region. In any case, resolving these broader alliance debates concurrently with the prospective SSN arrangement will be essential if the allies are to avoid the risk of widening, rather than narrowing, an already sizable expectation gap between the two countries over the mid- to long-term, even if a quick deal would meet short-term industrial and political needs.