Rethinking South Korea's Three-Axis System

South Korea’s Three-Axis System—comprising the Kill Chain, Korea Air and Missile Defense (KAMD), and Korea Massive Punishment and Retaliation (KMPR)—was established as a comprehensive response to North Korea’s evolving missile and nuclear threats. It has served as a critical element in reinforcing South Korea’s deterrence posture, especially in the wake of Pyongyang’s rapid advancements in ballistic missile technology and nuclear capabilities. However, despite its strategic significance, the system remains overly reliant on land-based infrastructure and assets. This structural imbalance is becoming increasingly problematic in the face of North Korea’s strategic evolution—particularly its shift toward mobile, survivable, and sea-based nuclear delivery systems.

KMPR, the third axis, was introduced as a doctrine of punitive retaliation in response to a nuclear or strategic attack. While it carries symbolic weight, the credibility of massive retaliation—especially through decapitation strikes—is increasingly uncertain. North Korea has taken significant measures to harden and decentralize its command-and-control systems, dispersing them underground or even to potential maritime platforms. KMPR’s current dependence on fixed land-based strike systems does not provide the survivable second-strike capacity necessary to deter a rational adversary armed with nuclear weapons.

The current Three-Axis System therefore suffers from an underappreciated vulnerability: its overwhelming dependence on land-based assets, while neglecting the critical maritime dimension of deterrence. North Korea’s pursuit of nuclear-armed submarines and underwater delivery platforms—combined with its recent test of what it claimed to be nuclear-capable underwater drones—highlights the urgency of enhancing South Korea’s sea-based capabilities. A credible deterrence posture in the 2020s and beyond cannot ignore the growing relevance of the maritime domain, particularly in a region where undersea competition is intensifying.

To address this imbalance, South Korea must integrate maritime and undersea capabilities into the strategic core of its deterrence architecture. The Navy should be equipped with long-range precision strike weapons that can be launched from Aegis destroyers and advanced submarines. The deployment of ship-based missile defense systems, including next-generation interceptors, will add much-needed flexibility and redundancy to South Korea’s air and missile defense network. Crucially, the development of submarine-launched cruise missiles and—eventually—ballistic missiles will enable South Korea to possess a survivable second-strike capability, which is currently absent from its strategic toolkit.

In this context, the acquisition of nuclear-powered submarines (SSNs) must be considered not only a naval modernization initiative but a core national security priority. Unlike conventional diesel-electric submarines, which are limited in endurance and operational range, SSNs provide the ability to patrol farther and remain submerged for extended periods. This endurance is essential to monitor and counter North Korea’s expanding submarine fleet and ensure persistent deterrence across the vast maritime expanse surrounding the Korean Peninsula. Additionally, SSNs are significantly harder to detect, increasing the survivability of South Korea’s second-strike capability and thereby enhancing strategic stability.

Moreover, SSNs can serve as flexible, mobile launch platforms for advanced cruise or ballistic missiles, thereby correcting the asymmetry created by North Korea’s own undersea nuclear aspirations. They also provide a powerful tool for intelligence gathering, anti-submarine warfare (ASW), and maritime domain awareness—functions that are increasingly central to multidomain deterrence. In strategic terms, SSNs give South Korea the operational depth and stealth needed to maintain escalation dominance in a crisis scenario.

South Korea must also invest in enhancing maritime domain awareness through advanced ISR platforms, unmanned undersea vehicles, and seabed surveillance infrastructure to monitor and counter North Korea’s expanding underwater threat. Anti-submarine warfare capabilities, including platforms like the P-8 Poseidon and indigenous ASW assets, must be prioritized alongside a broader naval modernization effort. At the same time, strategic coordination with the United States should evolve to fully incorporate naval and undersea assets into combined deterrence planning.

Ultimately, South Korea’s Three-Axis System must evolve into a more balanced, multidomain framework that incorporates not only land, air, and missile components but also maritime, cyber, and space-based capabilities. This transformation is not merely a technical upgrade; it is a strategic imperative. As North Korea increasingly disperses and diversifies its nuclear arsenal, South Korea cannot afford to rely on a deterrence structure rooted in outdated assumptions about where and how future conflicts will unfold.

Deterrence in the 21st century requires flexibility, survivability, and integration across all domains. A system that neglects the sea—where North Korea is rapidly expanding its nuclear footprint—risks strategic obsolescence. To maintain peace and stability on the Korean Peninsula, South Korea must reshape its deterrence posture into one that is resilient, adaptive, and capable of meeting the full spectrum of emerging threats. The introduction of nuclear-powered submarines is not a luxury, but a strategic necessity in this broader evolution.

Jihoon Yu is a research fellow and the director of external cooperation at the Korea Institute for Defense Analyses. Jihoon was the member of Task Force for South Korea’s light aircraft carrier project and Jangbogo-III submarine project. He is the main author of the ROK Navy’s Navy Vision 2045. His area of expertise includes the ROK-US alliance, the ROK-Europe security cooperation, inter-Korean relations, national security, maritime security, and maritime strategy. He earned his MA in National Security Affairs from the US Naval Postgraduate School and PhD in Political Science from Syracuse University.