From coastal defense to ocean power: Why South Korea needs nuclear-powered submarines
Moon Keun-sik
The focus of national security is shifting from territorial defense to maritime capabilities.
Factors like sea lines of communication, energy transport routes, raw material supply chains, and, crucially, who controls and manages the seas are increasingly becoming the key elements of national competitiveness and security. In this changing security environment, naval power — especially nuclear-powered submarines capable of long, covert operations — has emerged not merely as a military asset but also as a diplomatic one.
Today, naval power is less about fighting and more about preventing conflicts. The sea is no longer a space to be defended only after conflict erupts; it has become a part of national infrastructure that must be managed even in peacetime to prevent conflicts from arising at all. States capable of maintaining this infrastructure steadily earn the trust of neighboring countries, and that trust translates into diplomatic influence. Naval power, therefore, has moved beyond the realm of defense and become a language of diplomacy.
Asian countries are witnessing this shift. China has placed underwater forces at the center of its military modernization, continuously expanding its fleet of both nuclear-powered attack submarines and ballistic missile submarines.
Japan, despite Constitutional constraints and non-nuclear principles, has built one of the world’s most advanced diesel-electric submarine fleets and strengthened its ability to sustain operations in the open sea.
Recently, discussions have even emerged within Japan on the necessity of nuclear-powered submarines as the security environment has become more volatile. North Korea, regardless of its technical maturity, maintains a large submarine force and is extending its threats in the maritime domain. What all these countries share in common is their renewed focus on their navies’ underwater capabilities.
The strategic significance of submarines — especially nuclear-powered ones — cannot be overstated. Nuclear-powered submarines are free from the constraints of fuel replenishment and battery limits. They can remain submerged for extended periods, combining high speed with stealth. This does not merely mean superior combat capability; it also means an overwhelmingly higher probability of being present in the right waters at the right time. From a diplomatic perspective, this translates into the national capability to provide a reliable safety net in times of crises.
For South Korea, this transformation is particularly consequential. Due to its geographical characteristics, it is a quintessentially maritime-dependent nation. It relies on imports for most of its energy, and the overwhelming majority of its trade is by sea. When sea lines are disrupted, economic and industrial crises will erupt.
Yet Korea’s maritime strategy has long focused on coastal defense. While this was a rational choice in the past, it is no longer sustainable in today’s security environment.
Building nuclear-powered submarines (K-SSN) is no longer an option for South Korea. It is a must.
The K-SSN project is not simply about adding one more powerful weapon. It symbolizes the transition of the Republic of Korea Navy from a coastal force to an ocean-oriented navy, providing a foundation for South Korea to establish itself as a responsible maritime player. Nuclear-powered submarines offer deterrence in times of crisis, while in peace they are critical to building trust with allies and partners through intelligence gathering, surveillance and protection of lines of communication.
Moreover, nuclear-powered submarines fundamentally change the nature of defense diplomacy. Submarine cooperation does not end with the sale of equipment. It entails comprehensive collaboration encompassing design, construction, maintenance, crew training, safety management and long-term operational concepts. In effect, it is a declaration of a strategic relationship lasting at least 30 years. For this reason, political trust and institutional stability are evaluated before price or performance in submarine cooperation.
China’s maritime buildup adds urgency for building nuclear-powered submarines. While adding to the military pressure on Taiwan, China has fortified the South China Sea with artificial islands and military bases, advancing a strategy of fait accompli. The challenge is that this approach does not rely on full-scale war but on incremental occupation and growing China’s presence in the region. States without robust underwater and naval capabilities risk losing control of their maritime domains. This is precisely why Southeast Asian and Indo-Pacific countries are upgrading their submarine forces and naval capacities. If not well-prepared, these nations have no other options but back down and concede their maritime rights to China.
Ultimately, safeguarding the seas takes the form of diplomacy. A nation capable of possessing and operating nuclear-powered submarines is not seen merely as a military power, but as a partner capable of jointly managing and preventing maritime disputes. South Korea’s quest for nuclear-powered submarines is not about being a powerful maritime state. The key questions the nation should ask itself is whether it is fully prepared to shoulder the responsibility as a reliable security partner.
Competition among Asian countries is underway to rebuild and upgrade their naval capabilities.
What kinds of roles South Korea is seeking to play in the region with its maritime power? Now is the time to answer this question. How its naval power is addressed in its national strategy will offer a clue to South Korea’s choice.
Ret. Navy Capt. Moon Keun-sik is special professor at Hanyang University Graduate School of Public Policy.