South Korea and Japan are inextricably linked by their shared geographical and security concerns, as well as their separate alliances with the United States.
In the event of a full-scale war on the Korean Peninsula, the South Korea-U.S. combined forces would engage North Korean troops following Operations Plan 5015, an updated version of Operations Plan 5027. While the majority of U.S. reinforcements are expected to arrive from the mainland within 90 days, United States Forces Japan (USFJ) would play a pivotal role as the immediate response force, deploying from Japan ahead of those reinforcements. In such a scenario, Japan would provide support to U.S. warfighting efforts in line with Operations Plan 5055.
U.S. bases in Japan would be essential for staging and logistics support in a Korean contingency, making Japan’s cooperation and stability vital to any military response to North Korean aggression.
Japan’s proximity and its function as a logistical and operational hub – further linked through the United Nation Command and the United Nations Command Rear – create an inherent connection between Japan and South Korea in terms of their security outcomes in the event of an all-out war on the Korean Peninsula. The coordinated defense between U.S., South Korean, and Japanese forces highlights the importance of trilateral security cooperation, as any breakdown in this relationship could severely hamper efforts to respond to North Korea’s military actions swiftly.
Consequently, the security dynamics of Japan and South Korea are closely tied in the face of potential threats from North Korea, further underscoring the need for deepened cooperation between these two neighbors.
North Korean Missile Threat and Japan-South Korea’s Divergent Responses
North Korea’s missile and nuclear capabilities present a significant threat not only to South Korea but also to Japan. While South Korea has long been under direct threat from North Korean artillery and missiles due to its proximity, Japan was rudely awakened to this reality when North Korea launched the Taepodong missile over Japanese airspace in 1998. This event marked the first time that Japan was confronted with the stark reality of North Korea’s missile threat, as the Taepodong demonstrated the North Korea’s ability to reach Japanese territory.
Since then, North Korea has continued to develop and improve its missile technology, coupling it with nuclear warheads that have made the situation more perilous. The Hwasong series of intercontinental ballistic missiles (ICBMs) has extended North Korea’s strike range, putting much of Japan, including critical U.S. military bases, at risk. As a result, Japan is no longer free from the North Korean military threat, with Pyongyang’s continued provocations keeping the Japanese government on high alert. This shared threat makes the security of South Korea and Japan interdependent and heightens the necessity for collaboration in missile defense and broader security measures.
Japan has developed a sophisticated missile defense structure to counter the growing missile threat from North Korea. Japan’s ballistic missile defense (BMD) system is centered on a multi-layered defense strategy involving Aegis destroyers equipped with SM-3 interceptors and ground-based Patriot Advanced Capability-3 (PAC-3) systems. The Aegis Ballistic Missile Defense System, deployed on Maritime Self-Defense Force ships, is capable of intercepting missiles during their mid-course phase, while the PAC-3 batteries are designed to intercept short-range ballistic missiles in their terminal phase.
Japan has also been working closely with the United States on missile defense, including the co-development of the SM-3 Block IIA interceptor missile, which is designed to improve interception capabilities against intermediate-range ballistic missiles. Japan’s missile defense architecture has been further reinforced with radar systems like the X-band radar, which enhances early detection and tracking of incoming missile threats.
In response to the increasing missile threat from North Korea, South Korea has established its own comprehensive missile defense system, the “Three Axis” system, after declining the U.S. offer for joint development of a missile defense structure in 2002. This system comprises three key components: the kill chain, the Korean air and missile defense (KAMD) system, and the Korea massive punishment and retaliation (KMPR) plan. The kill chain is intended to detect and carry out preemptive strikes on North Korean missile launch sites and military assets before a missile launch occurs, while KAMD serves as a missile defense system designed to intercept North Korean ballistic missiles. Including Patriot and Terminal High Altitude Area Defense (THAAD) systems, KAMD operates in a manner similar to Japan’s BMD system. Finally, the KMPR is a retaliatory strategy that focuses on the destruction of North Korean leadership and key military assets in the event of a significant provocation or attack. Together, these three pillars aim to provide a layered and comprehensive defense against North Korean aggression.
Despite the sophistication of both Japan’s missile defense and South Korea’s Three Axis system, significant challenges remain. A key challenge lies in intercepting North Korea’s newer missiles, such as the Iskander-class ballistic missiles (KN-23), which can execute unpredictable maneuvers during flight. This makes them harder to intercept using conventional missile defense systems. While reports indicate that Patriot batteries may have intercepted Russia’s Kinzhal missiles, which possess an unpredictable flight path, during the Russia-Ukraine war, the overall effectiveness of current missile defense systems against such erratically maneuvering missiles remains uncertain.
Furthermore, missile defense systems can be overwhelmed by a saturation attack, where multiple missiles are launched simultaneously to exhaust the defense’s interception capabilities. North Korea has been developing its ability to launch large numbers of missiles in a short period, making it more difficult for Japan and South Korea to defend against such an attack effectively.
Additionally, North Korea’s advancements in solid-fuel missiles increase the speed of deployment, reducing the time for detection and interception. Beyond these factors, if North Korea were to launch a nuclear-tipped missile, even a single failed interception could result in catastrophic consequences for either Japan or South Korea. These vulnerabilities highlight the limitations of current missile defense systems and underscore the need for enhanced defensive strategies and potentially new capabilities.
Japan’s Introduction of Counterstrike Capability and Its Possible Integration with South Korea’s Three Axis System
Recognizing the limitations of purely defensive systems, Japan introduced a significant shift in its defense policy with the 2022 National Security Strategy, which includes the acquisition of counterstrike or “enemy base strike” capability. This new capability would allow Japan to launch preemptive strikes against enemy missile launch sites and command centers, aiming to neutralize threats before they can materialize.
Japan plans to enhance its long-range strike capabilities, aiming to deploy Tomahawk cruise missiles by 2027 and develop its own hypersonic missiles and glide vehicles by the early 2030s. These platforms will enable Japan to strike targets deep within North Korean territory, should the need arise. The counterstrike capability represents a substantial expansion of Japan’s military posture, moving beyond the traditional defense-oriented approach. While the specifics of Japan’s counterstrike capability are still being developed, its introduction marks a critical evolution in Japan’s ability to respond to missile threats from North Korea and other regional adversaries.
In this context, integrating Japan’s counterstrike capability with South Korea’s Three Axis system could strengthen the security of both countries. For instance, Japan’s counterstrike capability could complement South Korea’s kill chain by adding an extra layer of firepower during a Korean contingency. This cooperation could allow for a division of labor, where South Korean missiles focus on mobile North Korean targets, while Japan’s counterstrike missiles are used against fixed targets like command centers. This strategic distribution would enhance the efficiency and effectiveness of both countries’ defense systems by leveraging their respective capabilities. Furthermore, intelligence sharing between Japan and South Korea could be expanded to include real-time data on missile launches and targeting information, improving the effectiveness of both countries’ preemptive and retaliatory strikes. Such collaboration would require deeper military coordination and perhaps the establishment of joint operational command structures during a crisis.
Japan’s counterstrike capabilities could be effectively linked and reinforce South Korea’s Three Axis system through several strategic alignments, addressing shared threats from North Korea. They could complement each other in several ways.
First, an integration of ISR (intelligence, surveillance, reconnaissance) capabilities. Japan’s ISR systems, such as its intelligence gathering satellites and reconnaissance aircraft, can be integrated with South Korea’s reconnaissance network. Sharing real-time intelligence and targeting data would enhance both nations’ ability to detect North Korean missile launches and other military threats early on, increasing the accuracy of preemptive strikes or counterstrikes.
Second, via coordinated strikes and targeting. Japan’s counterstrike capability, especially once equipped with Tomahawk and hypersonic missiles, could be synchronized with South Korea’s kill chain (preemptive strike) system. This would allow both nations to execute simultaneous or complementary strikes on North Korean missile launch sites, command centers, or other critical infrastructure, multiplying the effectiveness of their responses.
Third, through enhanced coordination for response timing. By aligning Japan’s counterstrike doctrine with South Korea’s kill chain, both nations can better coordinate the timing of their responses. While Japan’s doctrine emphasizes defense after an attack begins, integrating with South Korea’s kill chain could offer flexibility, allowing earlier intervention in situations where preemptive action is deemed necessary.
Finally, Japan’s counterstrike capabilities and South Korea’s Three Axis system could complement each other through combined command and control. Establishing a joint command and control system between Japan and South Korea under the broader U.S. security framework would enable more synchronized decision-making during crises. This would ensure that Japan’s counterstrike capabilities and South Korea’s Three Axis system operate in tandem, reducing the risk of miscommunication and maximizing the overall effectiveness of their defense strategies.
Advantages and Challenges of Linkage
Linking Japan’s counterstrike capability with South Korea’s Three Axis system presents both advantages and challenges. On the one hand, linking Japan’s counterstrike capability with South Korea’s defense system would significantly strengthen their overall deterrence posture. By coordinating their responses, North Korea would be confronted with a unified and formidable defense, capable of delivering a coordinated and overwhelming reaction to both conventional and nuclear missile attacks. A joint system would improve response time, provide more comprehensive coverage of potential launch sites, and reduce the likelihood of a successful missile strike on either country.
On the other hand, the political and diplomatic challenges of such a partnership cannot be ignored. Historical tensions between Japan and South Korea, coupled with lingering mistrust, could make deep military cooperation difficult. While it’s possible that the recently established Trilateral Security Cooperation Framework includes plans to integrate Japan’s counterstrike capability with South Korea’s Three Axis system (though details remain undisclosed), public sentiment – particularly in South Korea – could significantly hinder deeper security cooperation between the two nations. Thus both nations must carefully address these concerns and ensure that any integration of their defense systems is positioned within the framework of shared security interests.
Conclusion
North Korea perceives Japan and South Korea as a unified theater of operations in the event of a conflict and may seek to weaken U.S. alliances with both nations, creating divisions between them.
This strategy could significantly compromise the security of both countries, exposing them to North Korea’s escalating missile and nuclear threats. In this context, it is critical that Japan and South Korea, along with the United States, pursue greater trilateral security cooperation.
By linking Japan’s counterstrike capability with South Korea’s Three Axis system, the two countries can present a united front against North Korean provocations and ensure that they are prepared for any potential conflict.
While political and historical challenges remain, the shared threat from North Korea makes it worth the effort to deepen security cooperation and integrate their defense capabilities. A coordinated defense strategy will not only enhance deterrence but also ensure that both nations are better equipped to respond to the complex and evolving missile threats posed by North Korea.