A Challenging but Essential Task: Security Cooperation Between South Korea and Japan
Security cooperation between the Republic of Korea (ROK or South Korea) and Japan may appear to be a relatively recent development, but over the past 70 years—from the onset of the Korean War to the present day—Japan has contributed to South Korean security in various ways. Although the likelihood of Japan providing financial and technological assistance has significantly decreased, its capacity to offer logistical and operational assistance in a Korean contingency has notably increased in recent years. These findings are not solely based on my own observations; they are the culmination of interviews with over 60 security experts from the United States, Japan and South Korea.
In this context, South Korea and Japan should establish a formal framework for discussing the scope of logistical and operational support Japan could provide in the event of a crisis on the Korean Peninsula. Additionally, the United States, South Korea and Japan should collaboratively formulate a strategy to manage a potential two-front war scenario, encompassing contingencies in both Taiwan and the Korean Peninsula. Addressing these issues should be a central aspect of South Korea-Japan security cooperation moving forward.
How Far Could South Korea-Japan’s Security Cooperation Evolve?
The Korean War of 1950 was initiated by North Korea, with backing from the Soviet Union and China. To prevent the further spread of communism in the region, the United States intervened militarily, primarily using Japan as a logistics base. This event underscored the importance of Japan’s role in a Korean contingency, leading to the establishment of the UN Command-Rear in Japan in 1957, solidifying its function as a logistical hub. Aware of this security link between South Korea and Japan, the leaders of both countries managed to preserve the existing security framework despite occasional negative public sentiments, often triggered by sensitive matters, such as textbook controversies, comfort women and the Yasukuni Shrine.
North Korea’s expanding WMD capabilities, along with rising concerns about a potential Chinese military invasion of Taiwan in recent years, are heightening the importance of security cooperation between South Korea and Japan. While North Korea’s nuclear-tipped missiles capable of reaching Japan could potentially lead to a nuclear decoupling between the United States and Japan, South Korea’s role in a Taiwan contingency remains speculative. Regardless of the scenario, a closely aligned partnership between the United States, South Korea and Japan will be the crucial foundation for ensuring South Korea’s security. Given these conditions, robust security cooperation between South Korea and Japan, within the context of the US-ROK-Japan trilateral relations, is considered vital for South Korea’s national interests.
Recent positive developments in trilateral security cooperation include the institutionalization of efforts through the Trilateral Security Cooperation Framework (TSCF). While the specific details of the TSCF remain confidential, and it is uncertain whether it will evolve into something akin to an Asian version of the North Atlantic Treaty Organization (NATO), it is evident that interoperability among the trilateral partners will be strengthened. This progress has paved the way for enhanced security cooperation between South Korea and Japan, including agreements such as the Acquisition and Cross-Servicing Agreements.
Despite these achievements, however, doubts remain among both Koreans and Japanese about whether the current bilateral security cooperation can be sustained over the long term.
To better understand the nature of this challenge, it is crucial to examine the modern history (1950 to 2024) of security contributions Japan has made to South Korea to see how security and political triggers have transformed it over time. This retrospective analysis could provide insights for future bilateral security cooperation.
The period under review could be segmented into five distinct phases, detailed below. Japan’s security contributions made during each of those phases can be categorized into three types: logistical support, operational support and financial and technological support. Such distinction in timing was established after consultations with over 20 Japanese security experts, including Akiyama Masahiro, a former Vice Defense Minister of Japan.
In Phase I (1950-1953), Japan primarily focused on providing logistical support to South Korea.[1] This approach was influenced by the use of Japan as a strategic base and primary supply hub by the United Nations (UN) forces, primarily made up of US troops, during the Korean War. Financial and technological support from Japan was minimal, as the majority of such assistance came from the United States. The lack of diplomatic normalization between Japan and South Korea at that time also hindered significant financial and technological contributions from Japan. Although Japan did contribute operational support by deploying minesweepers in Korean waters, it did not send combat units to the Korean Peninsula due to both domestic and international constraints. Consequently, Japan’s operational support during this period was moderate.
In Phase II (1954-1968), following the Korean Armistice Agreement in 1953, Japan’s operational support naturally waned.[2] Although diplomatic relations between South Korea and Japan normalized in 1965, US aid remained pivotal in addressing South Korea’s economic needs, reducing the demand for financial and technological support from Japan. While logistical support saw a reduction compared to Phase I, it still played a significant role among the three types of contributions throughout this period. Japan continued to be essential in supporting US bases in Japan in the event of a contingency on the Korean Peninsula, a role reinforced by the signing of a secret agreement between the United States and Japan in 1960.
In Phase III (1969-1995), the levels of logistical and operational support from Japan stayed relatively consistent with those of Phase II, influenced by similar conditions.[3] During this period, South Korea actively pursued financial and technological assistance from Japan, particularly for constructing the Pohang steel mill in the late 1960s, as restrictions on US aid for heavy industries and a scarcity of international financing posed significant challenges. In the early 1980s, South Korea sought defense burden-sharing. However, due to domestic political constraints, Japan was unable to supply military hardware and instead provided a $4 billion loan.
In Phase IV (1996-2014), Japan’s financial and technological support decreased as South Korea’s economic advancement rendered such assistance redundant.[4] Meanwhile, operational support escalated from minimal in Phase III to moderate in Phase IV, primarily due to the implementation of the 1997 Defense Guidelines and the 1999 Act on Measures to Ensure the Peace and Security of Japan in Perilous Situations in Areas Surrounding Japan (SIASJ Law). Logistic support experienced a significant rise, prompted by the launch of a North Korean ballistic missile over Japan in 1998, which led Japan to intensify its efforts in safeguarding US bases against various threats.
In Phase V (2015-2024), Japan’s financial and technological support continued to be minimal, reflecting South Korea’s robust economic growth.[5] However, logistical and operational support saw an increase, driven by Japan’s implementation of further measures to protect US bases and an expansion of the Japan Self-Defense Forces’ (JSDF) role in potential Korean contingencies. This enhancement was markedly influenced by the 2015 Defense Guidelines and Japan’s 2015 security legislation.
Japan as an Active Participant in a Korean Contingency
Based on the elements discussed, two major implications emerge.
First, considering the size of South Korea’s economy, it is unlikely that Japan would revert to some level of financial and technological support for South Korea’s security in the near future. This type of contribution occurred primarily during Phase III, when South Korea required significant funding for full-fledged industrialization in the 1970s and the development of social infrastructure in the 1980s, during a time when other foreign funding sources, particularly from the United States, were inadequate. However, given South Korea’s current economic status, this type of cooperation is simply not necessary anymore.
Second, the current level of logistical and operational support is likely to persist indefinitely, regardless of political regime changes in Tokyo or Seoul, due to irreversible structural developments. Since no all-out war similar to the Korean War has occurred since the 1953 Korean Armistice, the extent of Japan’s potential contribution in such a scenario remains uncertain. However, it is clear that Japan has significantly enhanced and expanded its capacity to support US forces operationally in a Korean contingency, largely due to the 2015 security legislation.[6] This legislation marked a shift from Japan’s previous policy of acting solely in self-defense, allowing the JSDF to engage in foreign conflicts.
It is also evident that Japan would take every measure to protect US bases within its territory in accordance with Operation Plan 5055, which is crucial in the initial phase of a Korean contingency. During this phase, US forces stationed in Japan would be deployed from these bases to the Korean Peninsula before the main reinforcements arrived from the US mainland. Many Japanese defense experts and officials I have interviewed believe that if a full-scale war were to erupt on the Korean Peninsula, Japan would likely provide logistical and operational support to the US, at least equivalent to what was provided during the Korean War period.
Recommendations
Given these factors, challenges to sustaining or even enhancing ROK-Japan security relations on a longer-term basis exist. In this context, two policy recommendations emerge.
Policy Recommendation Number One: South Korea and Japan should establish a Military Committee Meeting (MCM)-like mechanism to discuss the extent of logistical and operational support Japan could offer in the event of a Korean contingency, such as the allocation of North Korean ground targets.[7] To be sure, such a bilateral mechanism should operate within the framework of the Trilateral Security Cooperation Framework.
After discussions with both South Korean and Japanese defense officials, there will predictably be issues where the two sides do not see eye to eye. For instance, in the case of a North Korean invasion of South Korea, the South Korean military firmly rejects any potential deployment of the Japanese Ground Self-Defense Force to the Korean Peninsula for any reason. However, the Japanese side has indicated that the Self-Defense Forces might be deployed to evacuate Japanese citizens from major South Korean cities and that Japan could exercise its right to self-defense if attacked by North Korean forces while carrying out evacuations within South Korean territory.[8]
Another point of disagreement is likely to be around the potential circumstances under which Japan might use its recently adopted (2022) “counterstrike capability.” Some South Korean and Japanese security experts believe in integrating Japan’s enhanced capabilities with South Korea’s three-axis system, although numerous challenges must be addressed to do so, including matters of sharing target information and allocating targets.
Prioritizing these issues should be a key focus of future South Korea-Japan security cooperation, with discussions occurring regularly rather than sporadically. To address the concerns mentioned above (such as the potential deployment of the JSDF on the Korean Peninsula and the alignment of Japan’s enemy base strike capability with South Korea’s three-axis system) in greater detail, South Korea and Japan should review their respective contingency plans (South Korea’s OPLAN 5015 and Japan’s OPLAN 5055) and discuss the aspects that may involve both countries using the MCM-like mechanism.
Policy Recommendation Number Two: The US, South Korea and Japan should develop a coordinated plan to address a potential two-front war scenario.
As tensions have risen in the Taiwan Strait in recent years, the US and Japan have engaged in detailed discussions about their potential responses to a Taiwan contingency. The nature of these responses could vary, depending on the extent of China’s military actions towards Taiwan.
However, South Korean defense experts have expressed concerns that such a scenario could diminish the capabilities of US Forces Japan and Japan itself in responding to a crisis on the Korean Peninsula. They worry that if Kim Jong Un views the situation in Taiwan as an opportunity, he might initiate military provocations on the Korean Peninsula. In this kind of multi-front scenario, many South Koreans believe that the US and Japan might be unable to assist South Korea as their resources could be maxed out defending Taiwan.
In such a case, while Operation Plan 5015 (detailing actions by US Forces Korea and South Korean forces in a Korean contingency) would be activated, Operation Plan 5055 (outlining the response of US Forces Japan and Japan in a Korean contingency) might not be fully implementable due to the diversion of resources to Taiwan. One Japanese SDF official explained in an interview, for instance, that in a scenario involving conflicts on two fronts, Japan would likely be preoccupied with protecting Taiwan, deploying most of its available aircraft and naval vessels there, except those needed for the defense of Japan, since securing the Taiwan Strait is a top priority for Japan.
Although the likelihood of such a critical two-front war scenario is not high, it is imperative that the United States, South Korea and Japan devise measures that could deter Kim Jong In from misjudging the potential for North Korean success in such a situation. These include: 1) reaffirming that OPLAN 5015 will be fully implemented under any circumstances if North Korea decides to invade South Korea; 2) ensuring that nuclear decoupling among the United States, South Korea and Japan will not occur; and 3) confirming that the US extended nuclear deterrence remains intact.
Conclusion
Historical issues have consistently affected the relationship between South Korea and Japan since normalizing relations in 1965. However, recent developments have made reliable security cooperation between the two countries essential within the context of the Trilateral Security Cooperation Framework. With North Korea’s nuclear-tipped missiles capable of targeting both Seoul and Tokyo, a potential crisis in the Taiwan Strait could prompt Kim Jong Un to take provocative actions on the Korean Peninsula as well.
Kim is well aware that a united front among the US, South Korea and Japan would greatly reduce the likelihood of success for any North Korean military aggression against South Korea. Consequently, he is likely to attempt to exploit the weakest link within this tripartite structure: the relationship between South Korea and Japan.
The upcoming US election in November could introduce further complications. If Trump wins and seeks to significantly raise the costs of stationing US forces in both South Korea and Japan, the two countries may respond differently. Such varied responses could lead to a rift within the alliance, potentially convincing Kim Jong Un of a lack of cohesion among the US, South Korea, and Japan. It is crucial for the tripartite to work diligently to prevent this, as a division would leave them ill-prepared to handle a crisis in Taiwan, on the Korean Peninsula or both.
- [1]
The categorization into distinct phases, driven by noteworthy international and/or domestic events affecting both South Korea and Japan, ultimately influenced the formulation or reformulation of Japan’s security policy. For instance, the Korean War, which took place from 1950 to 1953, turned Japan into a logistical base for US military operations on the Korean Peninsula. This development marks 1950 to 1953 as a distinct phase in Japan’s security policy.
- [2]
- [3]
- [4]
- [5]
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- [7]
In the context of the US-ROK alliance, a Military Committee Meeting (MCM) refers to a high-level, strategic meeting between military leaders from both nations. This meeting is part of the broader bilateral defense coordination framework and plays a key role in ensuring close cooperation and readiness in military operations between the United States and South Korea.
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