Asia Defense | Security | East Asia

Washington’s approval for Seoul’s SSNs was more signal than substance – but it reveals the shifting balance of trust and dependence shaping the alliance.

The Republic of Korea Navy submarine Na Dae-Yong (SS-069) prepares to moor alongside the Emory S. Land-class submarine tender USS Frank Cable (AS 40) at the ROK Navy base in Jeju-Do, Oct. 11, 2024.

Credit: U.S. Navy photo by Mass Communication Specialist 2nd Karri Langerman

U.S. President Donald Trump’s late October visit to South Korea, framed as a historic breakthrough, came at a moment when both allies are redefining the balance between trust and autonomy. Washington’s long-awaited decision allowing Seoul to develop nuclear-powered submarines made headlines as a sign of growing confidence. However, beneath the surface, the story was more complex.

No contracts have been signed, no fuel-cycle waivers have been granted, and Seoul’s industry ministry stated that it had not been briefed on the details. The “approval” was a political gesture – a signal of unity presented as strategic progress, rather than the beginning of an actual program. It reflected Trump’s tendency to connect commercial and security interests, transforming defense policy into a stage where both leaders could claim success at home.

For South Korea, the moment was significant but also brought uncertainty about the future. Nuclear propulsion promises to expand the Republic of Korea Navy’s reach from the Yellow Sea to the broader Indo-Pacific, enabling patrols near Taiwan and the Luzon Strait alongside U.S. and Japanese partners. Yet by agreeing to build the first vessel in an American shipyard, Seoul tied part of its modernization to U.S. industrial and political cycles – gaining capability but within certain limits.

This reflects the alliance’s evolving logic: autonomy achieved through partnership, where each side continually reevaluates what it considers crucial to security. Order at sea, and within the alliance itself, now relies as much on shared perception as on tangible power.

Symbolism vs. Substance

Trump’s “green light” was less an operational approval than a calculated signal meant to project confidence and reinforce unity after rounds of trade and energy talks. Those meetings made the announcement appear to be part of a broader exchange, though evidence of a direct trade-off remains unclear.

Trump offered endorsement without process – more symbolism than policy. The United States has yet to modify the restrictions in the 123 Agreement, which provides the “the legal framework for significant nuclear cooperation” with South Korea, or finalize any technical details. For Washington, the gesture cost little but conveyed decisiveness; for Seoul, it brought both opportunity and obligation, requiring years of negotiation, investment, and patience.

Like all political gestures, it simplified complexity, turning a technical issue into a symbol of direction. Such clarity helps with coordination, but can obscure the fact that the agreement remains tentative. However, beneath that symbolism was a practical calculation, linking strategic reassurance to economic exchange.

The Political Transaction

Trump’s approval highlighted his deal-oriented view of alliances, where strategic favors often come with economic concessions. In Seoul, this pattern was evident: as Trump supported nuclear propulsion, President Lee Jae-myung promised increased energy imports, investments in U.S. shipbuilding, and collaboration on semiconductors. The announcement strengthened the link between deterrence and trade. By proposing construction at the Philadelphia Shipyard, recently acquired by South Korea’s Hanwha conglomerate, Trump redefined industrial policy as defense cooperation – proof, he asserted, that “America’s allies are investing in America’s security.”

For Lee, the deal offered both advantages and visibility. It responded to public demands for stronger deterrence while showcasing leadership abroad, but it also increased South Korea’s exposure to U.S. politics. By aligning his modernization efforts with an unpredictable partner, Lee took a calculated risk that transactional diplomacy would enhance, rather than limit, national autonomy. This tension – seeking recognition while striving for independence – shapes the nature of asymmetrical alliances. Every call for reassurance risks increasing dependence even as it tries to break free from it.

Strategic Maritime Relevance

Why does this matter? Because propulsion determines presence – and presence, in maritime strategy, equals power. For South Korea, the main benefit of nuclear propulsion is endurance: how long its submarines can stay deployed across the vital sea lanes of the Indo-Pacific. Diesel-electric submarines can defend coastal waters but lack the range for extended patrols. Nuclear propulsion, on the other hand, would allow continuous operations through the Miyako and Luzon Straits and the eastern approaches to Taiwan – routes vital to China’s access to the Pacific.

Nuclear-powered submarines will expand Seoul’s operational reach, bringing its navy closer to U.S. and Japanese allies. The capability also provides a subtler advantage: a constant presence at sea amid regional uncertainty. This is a form of strategic vigilance that endures even as policies shift on shore.

Greater reach entails greater responsibility, however. Long-range patrols increase expectations from allies and draw scrutiny from rivals. Beijing’s reminder that Seoul must “fulfill its non-proliferation obligations” suggests concern that endurance may bring strategic ambiguity. Meanwhile, Tokyo is watching closely. Japan’s conventionally powered Soryu and Taigei submarines are still advanced, but South Korea’s move into nuclear propulsion could quietly restart debates about parity – similar to what AUKUS did in 2021. The waters connecting the Korean Peninsula, Japan, and Taiwan might shift from shared geography to a zone of adaptive competition, where deterrence becomes more distributed and interdependent.

Normative and Diplomatic Fallout

But the decision also has repercussions far beyond the peninsula. For decades, the United States has limited naval nuclear propulsion to a small group of trusted partners, reinforcing the authority of the Non-Proliferation Treaty. AUKUS, which will provide Australia with nuclear-powered submarines, had already stretched that limit; granting similar flexibility to South Korea could further weaken it.

While the law clearly separates nuclear weapons from propulsion, perceptions often influence proliferation incentives. If Seoul interprets the 123 Agreement more broadly, other capable nations – such as Japan, Canada, or Brazil – might seek similar arrangements. The result could normalize nuclear propulsion as a symbol of major power status, eroding the moral distinction that once surrounded it.

Each precedent changes not only the balance of power but also the meaning of restraint itself. When norms evolve faster than understanding, diplomacy becomes a pursuit of both coherence and stability. Beijing views the move as another step in a U.S.-led undersea coalition; Pyongyang will condemn it as hypocrisy. Even allies responded cautiously: Australia supports a stronger South Korea but worries about diluting AUKUS, while Japan faces renewed debate over its conventional limits.

To maintain legitimacy, Seoul must show transparency. A trilateral framework with Washington and the International Atomic Energy Agency (IAEA) could confirm responsible stewardship; without it, the initiative risks appearing political rather than principled.

From Announcement to Execution

If Trump’s declaration was meant as a dramatic start, the reality is much more complex. The ROK Navy estimates that, even with U.S. help, developing a nuclear-powered vessel would take at least a decade – requiring new reactor technology, trained personnel, and reliable funding. None of these essentials currently exists. The engineering challenge matches the diplomatic one: only a few navies have mastered compact reactors and the safety protocols they require. Maintaining that commitment across administrations will be vital; each election will test whether the project remains a national priority or becomes a partisan symbol.

The United States faces similar constraints. Any transfer of nuclear propulsion technology requires congressional approval and interagency review, and the Philadelphia Shipyard is built for commercial rather than nuclear construction. History warns against optimism: Japan’s F-3 fighter and India’s Arihant submarine both experienced decade-long gaps between political announcement and operational reality. Without a clear roadmap, the project may stay aspirational. Success will rely less on rhetoric than on consistent governance.

Conclusion

Trump’s visit will be remembered more for what it revealed about the changing logic of alliances than for the ceremony. By granting Seoul a symbolic “green light” to pursue nuclear propulsion, Washington redefined deterrence as a partnership that combines security and economics. The gesture showed trust but also increased reliance – empowerment balanced with alignment.

For South Korea, the challenge now is to turn conditional approval into lasting capability, ensuring that autonomy gained through the alliance is not limited by it. Whether submarines are built or not, the episode has already altered regional expectations, where technology, prestige, and strategy converge beneath the surface.

The new South Korea-U.S. understanding is not a final settlement but an ongoing act of alignment – an effort to find direction amid uncertain seas. Its success will depend not on confidence or show, but on humility, patience, and steadiness when navigating the unknown. Even in politics, navigation is more important than speed.