US Strategy Shift Opens Door to South Korea’s Nuclear Future.

A U.S. defense analyst says Washington may adopt a more flexible stance on allied nuclear capabilities. The review comes as South Korea debates nuclear-powered submarines and restrictions under the U.S.–ROK 123 Agreement.

Jennifer Kavanagh, senior fellow at the Washington think tank Defense Priorities, confirmed in a recent interview that the U.S. National Defense Strategy is under review in ways that could affect South Korea’s defense posture. Her remarks, published September 22 by Hankyoreh, come as Seoul considers nuclear-powered submarines and a possible renegotiation of its U.S.–ROK 123 Agreement. This matters because Washington’s evolving stance may open the door for allies to expand nuclear-related capabilities, reshaping regional security dynamics.
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The question of U.S. troop levels in South Korea remains central. Kavanagh explained that, despite earlier calls to scale down, a sudden cut is unlikely. The Pentagon still regards South Korea as a crucial partner in Northeast Asia, but doubts have grown about whether Seoul would agree to grant full strategic flexibility in a crisis. She compared the issue to the Middle East, where reluctance from host nations limited the usefulness of U.S. bases when strikes against Iran were under consideration. In her view, if American forces in Korea cannot be freely used for wider contingencies in the region, their strategic value diminishes. Nevertheless, she assessed that troop levels would probably remain stable in the short term, with the possibility of a reduction appearing only within the next decade.

The technical side of the debate is bound up with the role of nuclear forces. The United States has long relied on its own nuclear triad to provide extended deterrence to allies, making clear that South Korea, Japan, and others fell under the American umbrella. The current 123 Agreement with Seoul, revised in 2015, prohibits enrichment above 20 percent and bans any military application of U.S. origin nuclear fuel. This has prevented South Korea from pursuing nuclear powered submarines, even though it possesses a mature civilian nuclear industry and advanced shipbuilding capacity. Cho Hyun, the South Korean foreign minister nominee, has said publicly that revising the agreement is necessary if Seoul is to consider nuclear propulsion, a step that would extend submarine range, increase endurance at sea, and offer survivable second strike potential against North Korea.

Nuclear powered submarines, unlike conventional diesel electric boats, do not require surfacing to recharge batteries. They can stay submerged for months, limited mainly by crew endurance and supply. A compact naval reactor, usually fueled with highly enriched uranium, provides the energy for propulsion and onboard systems. The difference in performance is not marginal. A diesel submarine might manage a few weeks of endurance before needing to snorkel, while a nuclear powered attack submarine can cross oceans at sustained high speeds, shadow adversary surface groups, or patrol silently for extended periods. South Korea already operates the KSS III class, capable of launching submarine launched ballistic missiles, but these remain limited by conventional propulsion. A nuclear option would dramatically alter the regional balance of power at sea.

For the United States, the operational implications are complex. If South Korea were to gain nuclear propulsion, either through indigenous development or cooperation, it would change the way allied undersea forces interact with U.S. Pacific Command. American attack submarines form the backbone of undersea deterrence in the Pacific, capable of tracking Chinese and Russian fleets and supporting carrier strike groups. The addition of allied nuclear powered boats could multiply coverage areas and provide depth, but it could also blur the lines of control and create overlapping missions. Washington would need to coordinate doctrines, communications, and training, while also managing the delicate question of fuel supply. Any move to allow enrichment for naval reactors would require consent under the U.S. Atomic Energy Act and likely congressional review.

The geopolitical consequences are even more far reaching. Kavanagh acknowledged in her remarks that within the Pentagon, some now believe limited allied nuclear proliferation may be acceptable if it compensates for a reduction in U.S. forward presence. This represents a marked departure from decades of non proliferation policy. For China and Russia, such a shift would be provocative. Both have opposed the AUKUS arrangement under which Australia will acquire nuclear powered submarines. Extending a similar tolerance to South Korea would be seen as another escalation, prompting countermeasures and intensifying rivalry. North Korea, meanwhile, has already showcased what it claims to be a nuclear powered ballistic missile submarine. Any American endorsement of allied nuclear propulsion would give Pyongyang fresh justification for its own programs.

Public opinion in South Korea leans strongly toward greater nuclear autonomy, with polls showing over seventy percent support for either nuclear weapons or nuclear powered submarines. Among elites, however, the preference is for nuclear latency, the ability to develop weapons if needed without crossing the threshold immediately. From Washington’s perspective, this hedging position may appear convenient, but it introduces risk. If the U.S. guarantees are questioned and allies begin to move toward independent deterrents, the fabric of the non proliferation regime could fray. Other countries, such as Japan or even Taiwan, might re evaluate their options, triggering a chain reaction.

The broader context is the global repositioning of U.S. power. The forthcoming National Defense Strategy is expected to place China as the pacing threat, with Europe relegated to third priority and the Middle East losing relative importance. The logic is simple: the United States has finite resources and must prepare for one major war, assumed to be against China. This pushes allies to shoulder more responsibility. In Asia, that may mean greater tolerance for allied nuclear capabilities, whether in propulsion or in latent weapons programs. Yet it remains a gamble. By loosening the long standing opposition to allied proliferation, Washington could shift some of the burden but also set off unintended consequences. The debate is only beginning, but Kavanagh’s comments highlight how much has changed since the era when American officials considered any allied nuclear activity unacceptable.