Dividing the Sea: A Realistic Japan-ROK Naval Strategy for a Dual Contingency
The MOC
By
Dr. Ju Hyung Kim
November 14, 2025
As tensions rise both in the Taiwan Strait and on the Korean Peninsula, East Asia is facing probably the most unstable maritime security environment in decades. If China’s invasion of Taiwan and North Korea’s full-scale attack occur simultaneously, U.S. naval power would be seriously stretched out, and this would necessitate U.S. capable allies in East Asia—namely, Japan and South Korea—to assume greater responsibility.
Although the Japan Maritime Self-Defense Force (JMSDF) and the Republic of Korea Navy (ROKN) field advanced Aegis destroyers, submarines, and C2 systems, deep operational integration is limited due to political and legal constraints. Even under such restraints, Tokyo and Seoul should adopt a division of labor within a shared maritime framework—rather than symbolic cooperation—in order to deter multifaceted attacks and maintain maritime control.
A Shared Sea with Finite Resources
A dual contingency would expose the limitations of the U.S. Navy’s spectrum of responses, especially in the initial couple of weeks of the conflict. By coordinating within their existing legal and command systems, Japan and South Korea could alleviate such vulnerability through a three-belt maritime architecture that assigns the two countries’ complementary roles.
The Inner Belt would include the South Korean littoral and the Sea of Japan. Here, South Korea should lead coastal defense, mine-sweeping, and anti-submarine warfare (ASW), while Japan offers surveillance and logistical support. The Mid Belt—which includes the Korea Strait and adjacent waters of the Sea of Japan—is where the two countries jointly manage operations, and ASW and convoy-protection operations should be conducted by both countries, eventually linking South Korea’s ports to Japan’s sea lines of communication (SLOCs). Meanwhile, the Outer Belt covers the Ryukyu Islands to the Philippine Sea, where Japan would take primary responsibility for long-range missile and air defense through JMSDF’s Aegis destroyers and F-35B-capable aircraft carriers; South Korea would provide anti-submarine assets and mine-sweepers as supplements.
Such a multilayered structure would complement, not substitute, U.S. command authority, enabling Japan and South Korea to center their focus on comparative strengths. In that way, South Korea could concentrate on littoral defense and logistics, while Japan focuses on broader maritime control.
Transforming Concept into Capability
To make such a structure operational, the two navies should align their missions and procedures. In the field of ballistic-missile defense (BMD), Japan’s Atago-class and Maya-class destroyers paired with the new Aegis System Equipped Vessels (ASEVs) could defend the Ryukyu Islands as well as the Sea of Japan. South Korea’s KDX-III Batch II destroyers could defend industrial centers and air bases on the peninsula. If realized, they could create a continuous BMD corridor from Busan to Okinawa under a U.S.-supported data-sharing system.
In terms of anti-submarine warfare, JMSDF’s P-1 patrol aircraft, Taigei-class submarines, and Mogami-class frigates would defend deep-water chokepoints, while ROKN’s KSS-III submarines and corvettes could patrol the East and Yellow Seas. The sharing of sonar and acoustic data through the Japan-Korea GSOMIA could create a unified common operational picture.
Cooperation in the mine-warfare domain is equally important. North Korea’s sea mines—estimated to be more than 50,000—could paralyze regional ports. In that sense, the South Korean Navy’s mine countermeasures (MCM) forces should sweep the South Korean littoral, while JMSDF’s airborne and unmanned minesweeping systems should secure the Korea Strait and the Ryukyu Islands. By pre-positioning MCM equipment in Busan and Sasebo, an immediate joint response would be possible.
In the domain of convoy protection, ROKN could conduct short-distance shuttle convoys between Korean ports and Sasebo, while JMSDF could manage long-range convoys that are headed toward Guam and Hawaii. Standardized communication and handoff procedures could smoothly render the transition. Above them, the Izumo and Kaga carriers—expected to be F-35B-capable by 2027—could expand air-defense range in and around the southern Ryukyu Islands, while South Korean destroyers would protect coastal resupply routes.
To coordinate these missions, Tokyo and Seoul should establish a Bilateral Maritime Coordination Cell (BMCC). It would be established both in Yokosuka and Busan, interconnected with the U.S. 7th Fleet and the U.S.–ROK Combined Forces Command. The BMCC would manage priorities regarding anti-submarine and anti-mine warfare, convoy schedules, and data integration through Link-16 and Link-22. Such efforts would turn the trilateral cooperation commitment agreed at the 2023 Camp David Summit into a standing operational mechanism.
From Planning to Implementation
Tokyo and Seoul could actually practice the aforementioned system even without a new treaty or massive budget. Both countries should co-author a chokepoint playbook on the Korea Strait, standardize convoy handoff protocols, and pre-position identical MCM equipment in shared hubs. Furthermore, the participation of South Korean officers in Izumo-class carriers’ F-35B takeoff and landing exercises, and liaison exchanges with the U.S. 7th Fleet, would harmonize doctrine and tactics.
Such measures respect each country’s domestic political limitations, including South Korea’s Peninsula defense concept and Japan’s 2015 legislation on collective self-defense. Operating within such boundaries increases the chances of cooperation.
Among various immediate operational risks, mine warfare is the most important. Since swift port mining could paralyze allied logistics supply within a couple of hours, a pre-arranged MCM posture would be one of the most urgent joint tasks.
Meanwhile, the U.S. should play the role of a facilitator, not a commander. By pre-positioning anti-submarine and MCM equipment in Sasebo and Busan, sending liaison officers to each branch of the BMCC, and providing minimal logistical support, the U.S. could reinforce its allied initiatives while signaling U.S. confidence in regional leadership.
The three-belt maritime architecture and a standing BMCC would transform Japan-ROK political alignment into a practical operational capability. Even if U.S. carrier assets are dispersed across two frontlines, coordinated Japanese and South Korean naval activities would enable them to maintain regional maritime control to a certain extent. If Japanese and South Korean naval vessels could escort convoys along the Korea Strait, exchange real-time submarine-contact data, and clear mine-laden sea lanes within a couple of days by the end of this decade, such achievement would indicate the realization of a functional maritime entente—that goes beyond mere cooperation—based on trust and common security.
Dr. Ju Hyung Kimcurrently serves as a President at the Security Management Institute, a defense think tank affiliated with the South Korean National Assembly. He has been involved in numerous defense projects and has provided consultation to several key organizations, including the Republic of Korea Joint Chiefs of Staff, the Defense Acquisition Program Administration, the Ministry of National Defense, the Korea Institute for Defense Analysis, the Agency for Defense Development, and the Korea Research Institute for Defense Technology Planning and Advancement. He holds a doctoral degree in international relations from the National Graduate Institute for Policy Studies (GRIPS) in Japan, a master’s degree in conflict management from the Johns Hopkins University School of Advanced International Studies (SAIS), and a degree in public policy from Seoul National University’s Graduate School of Public Administration (GSPA).
The views expressed in this piece are the sole opinions of the author and do not necessarily reflect those of the Center for Maritime Strategy or other institutions listed.
Center for Maritime Strategy
By Dr. Ju Hyung Kim
As tensions rise both in the Taiwan Strait and on the Korean Peninsula, East Asia is facing probably the most unstable maritime security environment in decades. If China’s invasion of Taiwan and North Korea’s full-scale attack occur simultaneously, U.S. naval power would be seriously stretched out, and this would necessitate U.S. capable allies in East Asia—namely, Japan and South Korea—to assume greater responsibility.
Although the Japan Maritime Self-Defense Force (JMSDF) and the Republic of Korea Navy (ROKN) field advanced Aegis destroyers, submarines, and C2 systems, deep operational integration is limited due to political and legal constraints. Even under such restraints, Tokyo and Seoul should adopt a division of labor within a shared maritime framework—rather than symbolic cooperation—in order to deter multifaceted attacks and maintain maritime control.
A Shared Sea with Finite Resources
A dual contingency would expose the limitations of the U.S. Navy’s spectrum of responses, especially in the initial couple of weeks of the conflict. By coordinating within their existing legal and command systems, Japan and South Korea could alleviate such vulnerability through a three-belt maritime architecture that assigns the two countries’ complementary roles.
The Inner Belt would include the South Korean littoral and the Sea of Japan. Here, South Korea should lead coastal defense, mine-sweeping, and anti-submarine warfare (ASW), while Japan offers surveillance and logistical support. The Mid Belt—which includes the Korea Strait and adjacent waters of the Sea of Japan—is where the two countries jointly manage operations, and ASW and convoy-protection operations should be conducted by both countries, eventually linking South Korea’s ports to Japan’s sea lines of communication (SLOCs). Meanwhile, the Outer Belt covers the Ryukyu Islands to the Philippine Sea, where Japan would take primary responsibility for long-range missile and air defense through JMSDF’s Aegis destroyers and F-35B-capable aircraft carriers; South Korea would provide anti-submarine assets and mine-sweepers as supplements.
Such a multilayered structure would complement, not substitute, U.S. command authority, enabling Japan and South Korea to center their focus on comparative strengths. In that way, South Korea could concentrate on littoral defense and logistics, while Japan focuses on broader maritime control.
Transforming Concept into Capability
To make such a structure operational, the two navies should align their missions and procedures. In the field of ballistic-missile defense (BMD), Japan’s Atago-class and Maya-class destroyers paired with the new Aegis System Equipped Vessels (ASEVs) could defend the Ryukyu Islands as well as the Sea of Japan. South Korea’s KDX-III Batch II destroyers could defend industrial centers and air bases on the peninsula. If realized, they could create a continuous BMD corridor from Busan to Okinawa under a U.S.-supported data-sharing system.
In terms of anti-submarine warfare, JMSDF’s P-1 patrol aircraft, Taigei-class submarines, and Mogami-class frigates would defend deep-water chokepoints, while ROKN’s KSS-III submarines and corvettes could patrol the East and Yellow Seas. The sharing of sonar and acoustic data through the Japan-Korea GSOMIA could create a unified common operational picture.
Cooperation in the mine-warfare domain is equally important. North Korea’s sea mines—estimated to be more than 50,000—could paralyze regional ports. In that sense, the South Korean Navy’s mine countermeasures (MCM) forces should sweep the South Korean littoral, while JMSDF’s airborne and unmanned minesweeping systems should secure the Korea Strait and the Ryukyu Islands. By pre-positioning MCM equipment in Busan and Sasebo, an immediate joint response would be possible.
In the domain of convoy protection, ROKN could conduct short-distance shuttle convoys between Korean ports and Sasebo, while JMSDF could manage long-range convoys that are headed toward Guam and Hawaii. Standardized communication and handoff procedures could smoothly render the transition. Above them, the Izumo and Kaga carriers—expected to be F-35B-capable by 2027—could expand air-defense range in and around the southern Ryukyu Islands, while South Korean destroyers would protect coastal resupply routes.
To coordinate these missions, Tokyo and Seoul should establish a Bilateral Maritime Coordination Cell (BMCC). It would be established both in Yokosuka and Busan, interconnected with the U.S. 7th Fleet and the U.S.–ROK Combined Forces Command. The BMCC would manage priorities regarding anti-submarine and anti-mine warfare, convoy schedules, and data integration through Link-16 and Link-22. Such efforts would turn the trilateral cooperation commitment agreed at the 2023 Camp David Summit into a standing operational mechanism.
From Planning to Implementation
Tokyo and Seoul could actually practice the aforementioned system even without a new treaty or massive budget. Both countries should co-author a chokepoint playbook on the Korea Strait, standardize convoy handoff protocols, and pre-position identical MCM equipment in shared hubs. Furthermore, the participation of South Korean officers in Izumo-class carriers’ F-35B takeoff and landing exercises, and liaison exchanges with the U.S. 7th Fleet, would harmonize doctrine and tactics.
Such measures respect each country’s domestic political limitations, including South Korea’s Peninsula defense concept and Japan’s 2015 legislation on collective self-defense. Operating within such boundaries increases the chances of cooperation.
Among various immediate operational risks, mine warfare is the most important. Since swift port mining could paralyze allied logistics supply within a couple of hours, a pre-arranged MCM posture would be one of the most urgent joint tasks.
Meanwhile, the U.S. should play the role of a facilitator, not a commander. By pre-positioning anti-submarine and MCM equipment in Sasebo and Busan, sending liaison officers to each branch of the BMCC, and providing minimal logistical support, the U.S. could reinforce its allied initiatives while signaling U.S. confidence in regional leadership.
Toward a Functional Maritime Entente
All such measures would be achievable under existing modernization plans. All three of South Korea’s KDX-III Batch II destroyers are expected to be fully operational by 2027, Japan’s ASEV Aegis destroyers would be deployed by 2028, while Izumo-class carrier conversion is expected to be completed by 2027. The 2023 Camp David framework has already institutionalized trilateral missile-warning exchanges and annual exercises. What remains is an operational blueprint that could connect such capabilities to specific maritime zones and task assignments.
The three-belt maritime architecture and a standing BMCC would transform Japan-ROK political alignment into a practical operational capability. Even if U.S. carrier assets are dispersed across two frontlines, coordinated Japanese and South Korean naval activities would enable them to maintain regional maritime control to a certain extent. If Japanese and South Korean naval vessels could escort convoys along the Korea Strait, exchange real-time submarine-contact data, and clear mine-laden sea lanes within a couple of days by the end of this decade, such achievement would indicate the realization of a functional maritime entente—that goes beyond mere cooperation—based on trust and common security.
Dr. Ju Hyung Kim currently serves as a President at the Security Management Institute, a defense think tank affiliated with the South Korean National Assembly. He has been involved in numerous defense projects and has provided consultation to several key organizations, including the Republic of Korea Joint Chiefs of Staff, the Defense Acquisition Program Administration, the Ministry of National Defense, the Korea Institute for Defense Analysis, the Agency for Defense Development, and the Korea Research Institute for Defense Technology Planning and Advancement. He holds a doctoral degree in international relations from the National Graduate Institute for Policy Studies (GRIPS) in Japan, a master’s degree in conflict management from the Johns Hopkins University School of Advanced International Studies (SAIS), and a degree in public policy from Seoul National University’s Graduate School of Public Administration (GSPA).
The views expressed in this piece are the sole opinions of the author and do not necessarily reflect those of the Center for Maritime Strategy or other institutions listed.