Table of Contents


For decades, Japanese and South Korean national security has been inextricably linked by common threats and both countries’ alliances with the United States. But lingering animosities between Seoul and Tokyo dating back to before World War II have long made cooperation uneasy. Those tensions have burst back into the open in recent years, threatening to erode the basis of cooperation even as common rivals in North Korea and China are becoming more formidable. But policymakers in Tokyo and Seoul must remain mindful of the enduring need for a common defense, because any weakening of cooperation could have severe ramifications if a sudden crisis or outright war were to test the limits of their relationship.

At the start of the Korean War, when the North Korean military crossed the thirty-eighth parallel and invaded South Korea on June 25, 1950, no foreign troops were on the peninsula to aid the Republic of Korea’s (ROK) fledgling military. In the spring of 1949, the few remaining U.S. troops had withdrawn at the direction of U.S. president Harry Truman, despite warnings from numerous intelligence and defense institutions that withdrawal would trigger an invasion by North Korea.1 Still, even with only 500 U.S. advisers physically on the peninsula, the United States was able to respond more quickly than any other nation due to its military presence in Japan supporting the U.S. occupation there. Truman committed air and naval forces to the defense of South Korea on June 27, the very day the United Nations (UN) Security Council passed Resolution 83 recommending member states provide assistance to South Korea. The first foreign ground troops, U.S. Task Force Smith, arrived from Japan on July 1, 1950.2

Just as during the Korean War, U.S. and ROK readiness for conflict on the peninsula today is partially a function of Japanese contributions. During the Korean War, Japan’s geostrategic location made it vital to the U.S. response. Today, Japan’s ability to respond defensively and of its own volition in support of U.S. and ROK efforts is important to the defense of South Korea and Japan alike. Trilateral cooperation is more than a force multiplier—it is integral to ensuring the United States, South Korea, and Japan can prevent catastrophic conflict, loss of life, and widespread destruction on all sides.

Kathryn Botto
Kathryn Botto was a senior research analyst in the Asia Program at the Carnegie Endowment for International Peace. Her research focuses on Asian security issues, with particular emphasis on the Korean Peninsula and U.S. defense policy towards East Asia.
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But South Korea’s and Japan’s negative perceptions of each other inhibit closer trilateral cooperation. Differing positions on Japan’s colonial past, Japan’s military goals, and appropriate approaches to China and North Korea often prevent the two countries from coming together over their common interest in promoting peace in the region. More than just disagreements, these issues make the two countries view one another as unreliable security partners. This divergence was acutely felt in 2019 when Seoul and Tokyo’s disagreements over historical issues snowballed to impact their economic and security relationship, leaving ROK-Japan relations at their lowest point in decades.

A major catalyst of this deterioration occurred in October 2018, when the South Korean Supreme Court ordered the Nippon Steel Corporation to pay compensation to South Koreans forced to work in its factories during Japanese colonization. A similar verdict was handed down to Mitsubishi Heavy Industries of Japan in November. Amid this escalating legal battle, Japan removed South Korea from its trade whitelist in August 2019, although Tokyo claimed that it did so due to national security concerns over South Korean exports of highly sensitive materials rather than in retaliation for the Supreme Court decisions. Seoul responded in kind by removing Japan from its own trade whitelist and subsequently threatened to withdraw from the General Security of Military Information Agreement (GSOMIA), an intelligence-sharing pact with Tokyo. These disagreements were punctuated by a December 2018 dispute over whether or not an ROK Navy destroyer directed its fire-control radar at a Japanese Maritime Self-Defense Force patrol aircraft.3 South Korea then accused a Japanese surveillance plane of making “provocative” flights over its naval vessels.4 Unable to resolve their differences, the two countries suspended port calls and canceled senior-level defense exchange programs.

Throughout 2019, the lines between Japanese and Korean history, security, and economic issues were entangled. This marks a departure from the two countries’ approach to one another over the past decade. Though hostile domestic political rhetoric has always persisted, both nations largely have allowed security and economic cooperation to increase incrementally but substantially over the past few decades without resolving lingering historical issues. This is not to say that these issues have never intersected. But since the normalization of relations in 1965, neither country had previously allowed domestic politics to seep into the security realm to the extent they did in 2019.

South Korea and Japan’s ability to cooperate remains dependent on the United States as a conduit and impeded by bilateral strategic mistrust

While tensions remain high in early 2020, Japan and South Korea both have incentives to cooperate with the United States and one another to prepare for contingencies on the Korean Peninsula. All three countries want to mitigate the North Korean threat and hedge against China’s rise. But to ensure a coordinated response to such challenges, the United States, South Korea, and Japan need to improve their interoperability, which refers to their ability to conduct joint operations. Enhancing interoperability requires attention on three dimensions: human, procedural, and technical. In each area, South Korea and Japan’s ability to cooperate remains dependent on the United States as a conduit and impeded by bilateral strategic mistrust. Given the stakes, developing interoperability despite differences is a far better outcome than allowing any fundamental drawdown in trilateral security cooperation—and the accompanying security risks—to emerge.

Strategic Mistrust and Other Barriers to South Korean–Japanese Cooperation

Policymakers and experts largely accept that Japan and South Korea both have an interest in closer bilateral cooperation with each other and trilateral cooperation with the United States. In some respects, Tokyo’s and Seoul’s security concerns align even more closely than either party’s views do with Washington’s. While the United States is primarily concerned with North Korea’s long-range and intercontinental ballistic missiles, Japan and South Korea see North Korea’s short- and medium-range missiles as a common threat as well.

Pyongyang has driven this point home by launching missiles not only into the East Sea (or the Sea of Japan) but also over Japanese territory, most recently over the northern island of Hokkaido in 2017.5 Like South Korea, Japan is a potential target for North Korea’s nuclear arsenal too. The centrality of the East Sea (Sea of Japan) in any potential conflict, Japan’s role in facilitating the transit of allied reinforcements to South Korea and evacuating civilians in such a conflict, and Japan’s economic dependence on China and South Korea, as well as its involvement in global and regional supply chains and international organizations are among the many other, well-documented issues that would make a crisis on the Korean Peninsula a major concern for Tokyo.

Although these commonalities make Japan and South Korea seem like natural security partners, they perceive one another as unreliable security partners. This inhibits them from improving the interoperability that their aligned security interests would otherwise seem to encourage. Differing interpretations of Japan’s colonial history are one contributing factor. Transitioning from a colonizer-colonized relationship to one of cooperative partnership is never natural. In addition, the two countries also have increasingly divergent perceptions of their two biggest security threats: North Korea and China. While Tokyo and Seoul share common threats, their divergent perceptions of those threats undermine cooperation and prevent closer alignment on how to approach security issues.

Moreover, the modern South Korea–Japan relationship was not initially borne of shared national security concerns—until the 1990s, the relationship was approached overwhelmingly in economic terms. In the 1960s, then South Korean president Park Chung-hee desperately required more aid to stimulate the country’s economy, particularly in the face of U.S. threats to reduce aid at the time. Despite strong domestic protests, he sought to normalize diplomatic relations with Japan to secure economic assistance.6 Japan provided South Korea with $800 million (about $6.5 billion today) in grants and loans, and in exchange Japanese firms got access to the developing South Korean market.

In the 1990s, the security environment in Northeast Asia began to change in ways that compelled Japan and South Korea to explore deeper security cooperation. Tokyo’s relationships with friends and foes alike changed in remarkable ways. Japan’s primary security threat shifted from being the Soviet Union to North Korea following the end of the Cold War, as Pyongyang’s nascent nuclear weapons program began to grow. With the Cold War over, Japan’s utility to the United States as a proxy for Communist containment was also diminished.7 With the rationale for the U.S.-Japan alliance already tenuous, suspicions had taken hold among some U.S. observers that Japanese technological superiority posed a threat to U.S. national security.8 Meanwhile, South Korea shared Japan’s concerns over the North Korean threat and the future of the U.S. role in Northeast Asia. Amid this newfound volatility in their regional security environment, the two countries began high-level exchanges between their defense establishments.9

Yet while economic links have expanded and high-level defense exchanges between the two countries have taken place, South Korea still mistrusts Japan’s military intentions. This has been particularly true since Prime Minister Shinzo Abe’s cabinet announced a 2014 reinterpretation of Article 9 of the country’s pacifist constitution to allow for limited collective self- defense.10 Japan’s exercise of collective self-defense could be beneficial to South Korea in the event of a conflict, as it would allow Japan to militarily help another country if it were attacked in a way that “threatens Japan’s survival and poses a clear danger to fundamentally overturn [Japanese] people’s right to life, liberty and pursuit of happiness.”11

Yet many South Koreans nonetheless view Japan’s constitutional reinterpretation as a return to Japanese militarism. The Abe government has reinvigorated efforts to not only reinterpret but outright amend Article 9 and other parts of the constitution to allow the Japan Self-Defense Forces to function as a traditional military. The increased frequency with which Abe and other conservative Japanese political figures visit the controversial Yasukuni Shrine and heightened tensions over the disputed Dokdo Islands (also called Takeshima or the Liancourt Rocks) have lent some credence to the concerns of many South Koreans. In Japan, South Korean anxieties over its potential remilitarization are mostly viewed as unfounded and create a perception that South Korean perspectives on security matters are unreasonable and imprudent.

Tokyo’s and Seoul’s views on North Korea are diverging as well. Although Abe has sought a summit with North Korean Supreme Leader Kim Jong Un, on the whole Japan has maintained a far more hardline stance on North Korea than the Moon administration. But Abe’s seemingly conciliatory shift was less due to a change in Japan’s fundamental view of North Korea and more a reaction to the United States’ exclusion of vital Japanese national interests in its talks with North Korea. Tokyo felt increasingly sidelined as U.S. President Donald Trump continually deferred any chance to bring up North Korea’s human rights record in his meetings with Kim, particularly in regard to the Japanese abductee issue.12 Additionally, Trump has made clear that he is not concerned about the North Korean short- and medium-range missiles that threaten Japan and South Korea, but rather only the country’s intercontinental ballistic missiles capable of reaching the United States.13

Under Moon, Seoul has been far softer on Pyongyang. The Moon administration has advocated for sanctions exemptions that would allow various inter-Korean projects to proceed, such as the reopening of the Kaesong Industrial Complex or the Mount Kumgang tourist site.14 The relationship between sanctions relief and denuclearization presents somewhat of a chicken-and-egg dilemma. South Korean conservatives and U.S. and Japanese government officials insist that there can be no sanctions relief until North Korea denuclearizes (or at least shows tangible progress toward denuclearization), whereas progressives like Moon in South Korea maintain that sanctions relief is critical to confidence building in hopes of persuading North Korea to denuclearize. This fundamental mismatch of perceptions creates more skepticism of one another’s’ judgment and intentions.

Increasingly, South Korea’s and Japan’s perceptions of China are also divergent. In 2019, Japan’s annual defense white paper placed China over North Korea as Japan’s most serious security threat for the first time.15 Japanese Defense Minister Taro Kono cited China’s rapid increases in military spending and increased deployment of assets in waters surrounding Japan as the reasoning behind this decision.16 Japan’s cooperation in the Free and Open Indo-Pacific strategy and the Quadrilateral Security Dialogue, two initiatives born of strategic competition with China, also underscore Japan’s perception of the China threat.While deeply concerned about China’s regional influence, particularly after its retaliation to the deployment of the Terminal High Altitude Area Defense (THAAD) system, Seoul feels constrained in its options for challenging Beijing.17 China remains the key to mitigating the threat from North Korea, Seoul’s utmost priority. South Korea therefore has been reluctant to endorse the Free and Open Indo-Pacific strategy or to oppose the Chinese-led push to fund worldwide infrastructure projects through the Belt and Road Initiative, decisions that would invite retaliation or jeopardize the success of its North Korea policy. Many in Tokyo perceive Seoul as ambivalent on the threat China poses, once again deepening the skepticism Japan and South Korea harbor over one another’s strategic goals.

These issues are deep-seated and unlikely to be resolved in the near term, yet the incentives for Japanese–South Korean cooperation remain and are growing more pressing as North Korea continues to expand its nuclear and ballistic missile programs. If Tokyo and Seoul allow bilateral and trilateral cooperation (with the United States) to weaken, the opportunity costs will be significant. The limited level of cooperation the two countries have today has taken decades to build and is still lacking. The shaky foundations of cooperation in Japan-ROK relations means that if engagements or agreements are terminated, the two countries will be left with few institutionalized processes to fall back on, and previous progress made would likely be lost and difficult to recover.

The trilateral alliance would then be left with little recourse other than to rely on ad hoc mechanisms of cooperation if conflict broke out, mechanisms that would not be coherent or efficient enough to facilitate genuine interoperability. The vulnerabilities this creates in the relationships between Tokyo, Seoul, and Washington could lead to miscalculation, wasted resources, and greater loss of life. For these reasons, it is imperative that the three nations work to insulate security cooperation from historical grievances, even as the thorny underlying issues remain unresolved.

Trilateral Cooperation, Bifurcated Interoperability

Interoperability is often discussed in the context of trilateral cooperation, but the term tends to be inconsistently and vaguely defined. The North Atlantic Treaty Organization (NATO), arguably the best example of multinational cooperation, defines it as “the ability for allies to act together coherently, effectively and efficiently to achieve tactical, operational and strategic objectives.”18 Interoperability has three dimensions: human, procedural, and technical. The human element of interoperability includes the education and training of actual personnel as well as the degree of mutual understanding, communication, and respect between them. Procedural interoperability describes the compatibility of partners’ doctrines, policies, and procedures, whereas technical interoperability refers to the connectivity of allies’ hardware, communications and information systems, and other equipment. Using this definition is not to say that the trilateral alliance between Seoul, Tokyo, and Washington will ever amount to an Asian NATO, but NATO’s definition does provide a gold standard for understanding the nuances of interoperability and for facilitating effective cooperation.

While the United States has achieved a high level of bilateral interoperability with Japan and South Korea individually in all three dimensions, true trilateral interoperability remains elusive. Trilateral interoperability with the United States can satisfy the first condition for interoperability in the NATO definition of enabling the three countries to simply “act together.”19 Currently, interoperability between Japan and South Korea tends to rely on the United States as an intermediary. But working through Washington does not make for coherent, effective, or efficient interoperability—the second condition of the NATO definition. Decisions must be made efficiently and quickly in the fog of war, and any lack of these three qualities could prevent the United States, South Korea, and Japan from achieving their desired outcomes.

Despite ongoing differences in the bilateral relationship between South Korea and Japan, tabletop exercises have consistently reaffirmed that trilateral cooperation can readily arise when a major crisis unfolds, including in ways that did not previously exist.20 Importantly, coordination mechanisms between the countries’ foreign and defense ministers and cooperation on ballistic missile defense, threat monitoring, and other critical areas emerge on an ad hoc basis. However, while many areas of cooperation arose naturally at the strategic level, the lack of prior consultation, planning, and exercise of these functions at the operational and tactical levels degraded the quality and effectiveness of cooperation between the two partners. As the radar lock-on dispute demonstrated, even where prior consultation and practice occurred, cooperation is often undermined by the tone of the bilateral relationship. The dispute occurred while South Korea was attempting to rescue a North Korean fishing boat in distress, and though Japan and South Korea have frequently exercised coordination in naval search and rescue operations, deep mistrust turned this potential opportunity for utilizing existing avenues of cooperation into a crisis.21

Tabletop exercises have also demonstrated that uneasy relations between Seoul and Tokyo can impede the effectiveness of a trilateral response. As one account of a Sasakawa Peace Foundation tabletop exercise described:

Seoul-Tokyo relations limited trilateral policy effectiveness. The absence of a genuinely trusting relationship between Tokyo and Seoul impacted trilateral approaches. Japan was willing to share information with the ROK, but the interactions were mainly passive (communicating each other’s actions/goals rather than a discussion on coordinated courses of action). Japan and South Korea found it easier to coordinate in trilateral (with the United States) rather than bilateral settings. This impeded their ability to work together in concert.22

To some extent, the United States can successfully help its two allies communicate better; exchange data and intelligence; align training priorities; and adopt shared terminology, procedures, and doctrine. This facilitates the first condition of the NATO definition of interoperability in that it enables allies to act together to deter and defend against the North Korean threat. Still, bifurcated cooperation is not as coherent, effective, or efficient as it should be. Using the United States as a facilitator is cumbersome, wastes valuable time (even more so in a conflict scenario), risks miscommunication, and makes coordination between the three militaries more difficult. Greater interoperability can help ensure that Japan and South Korea are not forced into a scenario where they must cooperate without experience working together, formalized relationships, or knowledge of each other’s systems and procedures. The limits of current Japanese–South Korean interoperability and room for future improvement apply to the human, procedural, and technical elements of interoperability alike.

Human Interaction Versus Human Interoperability

For any military, mere interactions with a partner do not necessarily deepen human interoperability in meaningful ways. In human terms, interoperability requires that multinational partners have a mutual understanding of how to communicate and operate together so as to smoothly facilitate technical and procedural interoperability between forces and systems. This helps ensure that allied efforts are deconflicted and in sync and that resources are efficiently used. But while Japan and South Korea are interacting significantly more at the strategic level, most of the activities they are undertaking to facilitate this interoperability—particularly exercises, education, and training—are vulnerable to fluctuations in the two countries’ political relationship.

The human element shapes every aspect of interoperability. As detailed by the Army Sustainment Command, “of the three dimensions, the human dimension is most closely connected to interoperability effectiveness and is the most likely to determine system effectiveness.”23 This dimension contains a multitude of facets “rang[ing] from communication at the individual level” to the coalition’s ability to employ standardized and executable capabilities that maximize national contributions.”24 This element of interoperability requires more than interactions through meetings and engagements—it requires that those interactions lead to actions that enhance the ability of multinational defense establishments to work together efficiently and effectively. Having established channels and modes of communication enables every aspect of an allied response. The human dimension in Korean-Japanese cooperation has occurred mostly through interactions at the strategic rather than through sustained and shared operational experience. Japan and South Korea have greatly improved the level and quality of the interactions between their defense establishments over the past thirty years, particularly at senior levels. But institutionalized channels for communication remain insufficient.


Face-to-face meetings are an especially necessary ingredient for closer coordination between partners. In 1994, Lee Byung-tae became the first South Korean defense minister to visit Japan, a major step toward greater high-level engagement between the two countries.25 Since that time, such interactions have expanded to include a number of regular bilateral, trilateral, and multilateral meetings. In particular, the annual Defense Trilateral Talks provide a forum for dialogue between senior U.S., Japanese, and South Korean defense officials. The three countries have also had defense ministerial meetings on the sidelines of the Shangri-La Dialogue annually in most of the past twelve years.26

By establishing familiar channels between high-level officials, these meetings give the partners a degree of consistency and a forum to set priorities for lower-level engagement. Consistency is crucial for monitoring trends and setting shared goals. It is also vital for strengthening existing cooperation by helping policymakers and officials develop institutionalized familiarity with their counterparts and clear lines of communication.

Though the quality of these encounters has greatly improved since the inception of the first of the Defense Trilateral Talks, many of these forms of engagement remain ad hoc or depend on the tone of Japan–South Korea relations at a given time. To some extent, ad hoc engagements are an asset—at both the working and senior levels—as a way for counterparts to make contact and respond to current events in realtime. For instance, when the need arises, the Joint Chiefs of Staff and other high-level defense officials hold video teleconferences, often in response to North Korean missile tests and other provocations, even at times when Japan–South Korea relations have been strained.27 That being said, human interoperability is still undermined when political tensions between Japan and South Korea interfere with the regularity and quality of engagement. When political issues disrupt such interactions, that signals that mutual respect and understanding between the two parties are not sufficiently institutionalized to the extent that they constitute human interoperability.

Exercises and Training

To some extent, senior-level meetings will always be prone to political pressures, as leaders often use them to send a public message on the state of a given relationship for domestic consumption. A critical focus at the senior level should be furthering trilateral education and training, which are less public and less sensitive to changing political winds. In this regard, military exercises are a foundational component of education and training. They enhance human-level interoperability by establishing and familiarizing participants with channels and methods of communication, helping participants navigate language barriers, and reconciling differing doctrines and procedures. In doing so, exercises provide an opportunity to gauge whether human cooperation can successfully facilitate the procedural and technical aspects of interoperability. Though military exercises touch on all three dimensions of interoperability, the technical and procedural dimensions of an exercise are nearly impossible to carry out without the foundation of human interoperability.

To cite one set of examples, since at least 2012, the South Korean, Japanese, and U.S. navies have participated in trilateral exercises to improve interoperability and communication. The scope and scale of these exercises and this training has also expanded over the past few years, particularly since 2015. In that year, the United States and Japan revised their defense guidelines to allow the Self-Defense Forces to support the U.S. response to regional contingencies without a direct attack on Japan. Tokyo, Seoul, and Washington also signed the Trilateral Information Sharing Arrangement in December 2014 to allow the three nations to share classified information, albeit with the United States as a go-between.28 This allowed the three countries to conduct trilateral missile defense exercises in 2016 and 2017, the first of their kind outside a major multinational exercise.29 The exercises focused on facilitating communication between each country’s Aegis systems, which can detect, track, and intercept North Korean missiles—a critical need in any potential contingency.

Japan and South Korea also participate in larger-scale multinational exercises with the United States. They have participated alongside one another in all twenty-six iterations of the biennial Rim of the Pacific (RIMPAC) exercises since 1971.30 Meanwhile, the Proliferation Security Initiative (PSI) has also been invaluable in terms of facilitating multinational cooperation to prevent the transfer of weapons of mass destruction. South Korea and Japan have both hosted PSI exercises, including the Eastern Endeavor exercise hosted by the ROK in 2019 and the Pacific Shield iteration hosted by Japan in 2018.31 Additionally, the two countries cooperate closely in an U.S. Indo-Pacific Command sanctions enforcement coordination cell hosted at Yokosuka. Under this initiative, South Korea and Japan join the United States and its Five Eyes partners (Australia, Canada, New Zealand, and the United Kingdom) to coordinate efforts from a single headquarters to enforce UN Security Council sanctions and prevent illicit oil and ship-to-ship transfers to North Korea.32 The ability of personnel and equipment to share facilities is a vital ingredient of efficient cooperation and other aspects of interoperability.33

Like other trilateral forms of engagement, exercises have often been inconsistent and subject to the ebbs and flows of political tensions between Seoul and Tokyo. While missile defense has been an important aspect of multilateral PSI exercises, no trilateral missile defense exercises have occurred since 2017. And although major multilateral exercises like RIMPAC are consistent and can reveal areas for improving interoperability, they do not necessarily train participating countries to confront the North Korean threat specifically.

Beyond tensions between Japan and South Korea, other diplomatic considerations have also impeded attempts to enhance trilateral training, particularly the ROK’s fears that doing so could disrupt South Korean relations with China or North Korea. Nikkei Asian Review reported that, in November 2017, the United States proposed that Japan be involved in a naval exercise with South Korea, but Seoul reportedly objected because it could upset the South Korean public and antagonize Beijing while South Korea was trying to mend ties with China over the THAAD deployment controversy.34

Military exercises inherently have a political dimension, as the United States has underlined in its recent attempts to create diplomatic space with North Korea by downsizing, postponing, and canceling exercises. Unlike trilateral engagement, the bilateral U.S. and ROK military relationship is institutionalized to the extent that the allies have options for adjusting their normal routines. The two countries have the flexibility to compensate by conducting computer-simulated exercises, scaling exercises down, or rescheduling exercises to mitigate reductions in readiness in the near term. But trilateral engagement has not been routinized enough to make such flexibility possible. If a trilateral exercise does not occur, there is often nothing to take its place and trilateral readiness is lost.

Force Flow and Procedural Interoperability

As important and foundational as the human element of interoperability is, there are other important facets of keeping allied military personnel and national security policymakers in sync, including procedural interoperability. The procedural dimension of interoperability is achieved through “standardization agreements, standardized communication, and agreed upon terminology, tactics, techniques, and procedures that minimize doctrinal differences.”35 These mechanisms prevent militaries from having conflicting or misunderstood procedures that can slow—or even halt—critical aspects of the mission, such as the transit of troops and equipment or approval of operational plans.

As with other aspects of interoperability, the United States serves as a go-between to facilitate procedures for Japanese support of a contingency on the Korean Peninsula, but procedural coordination directly between Japan and South Korea lacks even basic established command relationships. Japan does have a role in Korean contingencies through multinational mechanisms like the UN Command–Rear (UNC–Rear), but this role is seldom exercised on a trilateral basis. Most procedural interoperability between Japan and South Korea is bifurcated, derived from each country’s respective bilateral relationship with the United States.

Between the United States and Japan, the impetus for Japanese support of Korean contingencies and the procedures needed to facilitate such support emerged in the 1990s. At that time, the two countries began to revise the architecture of their alliance to address the new post–Cold War security environment, including the North Korean threat. In 1997, Tokyo and Washington revised the Guidelines for Japan-U.S. Defense Cooperation to reflect a commitment not only to planning for an armed attack against Japan but also to “mutual cooperation planning in situations in areas surrounding Japan.”36 This departure from the earlier 1978 guidelines marked Japan’s willingness to support U.S. military activities in response to Korean contingencies, particularly in areas such as rear-area support, intelligence gathering and surveillance, noncombatant evacuation operations, and minesweeping.37

Rear-support functions include command, control, communications, computers, and intelligence (C4I); wartime host nation support; reception, staging, onward movement, and integration (RSOI); and noncombatant evacuations. Japanese bases are critical to the United States’ ability to execute all of these functions, but when it comes to the procedural dimension of interoperability RSOI is particularly important. RSOI refers to the processes by which personnel and equipment arriving in a new military theater transition into forces ready and capable of meeting operational requirements.”38

In the event of a crisis on the Korean Peninsula, U.S. forces and equipment would transit to the peninsula from the United States (including Guam) and Japan. Coordinating this movement is a major logistical challenge that requires coordination and approval at many levels, including from all service branches, national authorities, and multiple commands. (The relevant military commands include those of individual allied countries like the U.S. Indo-Pacific Command, United States Forces Japan [USFJ], United States Forces Korea [USFK], the Japan Self-Defense Forces, and the ROK Armed Forces, as well as the binational Combined Forces Command and multinational UNC.) Having familiar and established relationships and communication channels is essential to ensuring that every aspect of an operation happens smoothly and efficiently.

Japan’s chief utility for procedural interoperability with respect to the Korean Peninsula is a function of geographic proximity, the U.S. defense posture in the region, and Japan’s role in UNC as the host country of UNC–Rear. Any crisis on the peninsula would require significant augmentation from U.S. forces off the peninsula, the most well-positioned of whom are in Japan.

U.S. forces stationed in South Korea are predominantly ground forces, while U.S. troops in Japan include far more naval and air force personnel. USFK is comprised of 19,500 soldiers and 7,800 airmen on the peninsula. Only about 470 personnel from the U.S. Navy and the U.S. Marine Corps are stationed in South Korea.39 The ROK military is also largely a ground force. Whereas USFK lacks naval and air resources, the U.S. military has 20,250 sailors, 18,800 marines, and 12,500 airmen in Japan (see figure 5).40

Support of U.S. naval forces on the peninsula is the responsibility of the U.S. Seventh Fleet, the navy’s largest forward-deployed armada, which is based at Yokosuka. In addition to personnel, Japan is home to a high concentration of U.S. naval and air assets that USFK lacks, including the permanently deployed USS Ronald Reagan aircraft carrier, Aegis destroyers, antisubmarine warfare assets, and a significant number of combat-capable aircraft.41 Outside of the forces already in the theater, the United States will also require its bases in Japan, as well as the logistics, support, and transport capabilities there, to be used for the transiting of U.S. Indo-Pacific Command forces to the peninsula.42

U.S. bases in Japan also would serve as UNC–Rear bases for the U.S. military and a subset of UNC sending states who have an agreement in place with the Japanese government to use such facilities to send personnel to the theater in response to a contingency on the Korean Peninsula. According to this standing agreement, the Agreement Regarding the Status of United Nations Forces in Japan (more commonly referred to as the UN-GOJ SOFA), the United States and eight other sending state signatories are permitted to exercise the use of seven UN/U.S. bases in Japan to “rehearse the procedures necessary to conduct missions during a contingency and to enable USFJ to enhance their preparedness to support Sending State forces which would either transit through or operate from Japan.”43 Specifically, Camp Zama, Yokota Air Base, Yokosuka Naval Base, Sasebo Naval Base, Kadena Air Base, White Beach Naval Facility, and Futenma Marine Corps Air Station are used for this purpose.

In addition, although the agreement with Tokyo is technically the approval authority for such transit, the Japanese government has made explicitly clear that the United States should consult with it before using Japanese bases for any emergency deployments, making Japanese support and cooperation essential to exercising the function of UNC–Rear bases.

U.S. procedure for facilitating cooperation with its allies in such contingencies is well established, but links between Japan and South Korea are less developed. When it comes to the bilateral U.S.-Japan relationship, these roles are standardized through agreements and common procedures. The aforementioned UN-GOJ SOFA and the U.S.-Japan Defense Guidelines commit Japan to aiding in the U.S. response, directly or indirectly, to a crisis on the Korean Peninsula.

But the only formally established command relationship between the Japan Self-Defense Forces and either USFK or the ROK military is through UNC, a tenuous connection that has seldom been exercised. In a crisis, the shared interests of both countries would likely compel them to cooperate should the need arise, but the procedures and mechanisms for this cooperation are not clearly delineated.44 Japan has a number of valuable roles to play in addition to enabling the movement of U.S. forces, including on missile defense, minesweeping, noncombatant evacuation, search and rescue, humanitarian assistance, intelligence gathering, and other functions. Agreements with the United States can facilitate these functions to an extent, but ad hoc connections that arise are likely to be incoherent and insufficient.

Connectivity, Information Sharing, and Technical Interoperability

While the human and procedural elements of interoperability are undoubtedly vital, there is also a technical dimension, which refers to connectivity and compatibility between allies’ hardware, equipment, armaments, and other military systems. Technical interoperability does not necessarily require that partners share the same military equipment; rather, the most important factor is “that the equipment can share common facilities, and is able to interact, connect and communicate, [and] exchange data and services with other equipment.”45

South Korea and Japan share several capabilities, such as Aegis missile defense systems, as both countries’ defense imports are predominantly from the United States. There are a number of capability-related interoperability issues pertaining to both Japan’s and South Korea’s respective bilateral alliances with the United States and related to the trilateral relationship, but those issues fall beyond the scope of this publication. The more pressing need in the trilateral relationship at this moment is for agreements that establish information exchanges and a common operational picture, without which all other aspects of technical interoperability are without value or even useless.46

Information Sharing

The need for Japan-Korean information sharing has been acutely apparent amid ongoing tensions over the GSOMIA intelligence-sharing arrangement. It is increasingly clear that South Korea needs greater communication with Japan to adequately monitor and address North Korea’s missile threat. In October 2019, Chong Jong-sup, an opposition lawmaker in the Liberty Korea Party, revealed ROK Navy data showing that Seoul had failed to detect and track at least five North Korean missile launches between May and September 2019.47 South Korea’s chief of naval operations commented that South Korean radar failed to detect the missiles because they were out of range.48 Closer coordination with Japan could have prevented these gaps in South Korea’s detection capabilities.

Japan and South Korea have the interoperable capability to detect, track, and engage North Korean missiles with their Aegis ballistic missile defense systems. Seoul has long held that it would not integrate trilateral missile defense and reiterated that position in 2017 in response to China’s retaliation over the deployment of THAAD. But, at the same time, South Korea and Japan have demonstrated the interoperable potential of their missile defense systems in numerous exercises. Moreover, sharing information on North Korea’s programs is in the best interests of Tokyo and Seoul.49

Given this need, the near disintegration of GSOMIA in late 2019 is highly concerning. If the GSOMIA mechanism had been dissolved, Japan and South Korea could still swap classified information on North Korea’s nuclear and missile programs through the Trilateral Information Sharing Agreement with the United States serving as an intermediary.50 GSOMIA, however, allows the two countries to share intelligence directly, a far more streamlined process that would improve response time and reduce the chances of miscommunication in the event of a crisis. Timeliness and efficient communication is highly crucial, as a North Korean missile can reach Tokyo in ten minutes and takes even less time to reach Seoul.51

GSOMIAs, for their part, are a very basic form of intelligence-sharing agreement. Memorandums of understanding or GSOMIAs are often the first foundational document in an intelligence-sharing arrangement. These agreements are not legally binding, but they set parameters for and establish channels for intelligence sharing. The United States has hundreds of such agreements with foreign governments.52 Prior to the signing of the Japan-ROK GSOMIA, Seoul had such agreements with thirty-two countries and NATO.53 South Korea even signed a 2001 GSOMIA with Russia, a country whose security interests are far less aligned with Seoul’s.54 Yet while basic in form, this agreement marked a major step in the trilateral relationship, given that no other bilateral intelligence sharing agreement exists.

A Common Operational Picture

The limits of current information sharing and exercises have hampered efforts to determine whether allied hardware and other systems can adequately communicate and have prevented the United States, South Korea, and Japan from establishing a common operational picture to work from in a conflict scenario. The Joint Chiefs of Staff define a common operational picture as “a single identical display of relevant information shared by more than one command that facilitates collaborative planning and assists all echelons to achieve situational awareness.”55 By establishing a common operational picture, the United States, Japan, and South Korea can assure they are making decisions based on the same information.

The management of C4I systems from bases in Japan is critically important to the United States’ ability to attain and maintain a common operational picture, particularly as command functions are now predominately concentrated in a single location at Camp Humphreys, making them more vulnerable. But these functions are not well coordinated at the trilateral level. The GSOMIA and other intelligence-sharing arrangements can help accomplish this. With proper permissions, all three countries would be able to share information and establish a common operational picture by way of common systems like Link 16, a tactical data information exchange system used by the United States and NATO allies, which all three countries already operate. Japan is particularly important to the United States’ ability to attain a common operational picture as Japanese bases are tasked with C4I as part of their rear-area support functions.

However, it is unclear whether existing systems would be able to establish a common operational picture quickly and effectively at the tactical level in the event of a conflict without prior coordination. Particularly, Japanese defensive maritime and air support would be active in the East Sea (Sea of Japan), where allied U.S., Japanese, and South Korean vessels and submarines would be operating in the same waters as rival North Korean, Russian, and Chinese vessels.56 The high concentration of maritime assets increases the risk of incidence, making it highly important that the United States, South Korea, and Japan are working from a common operational picture to accurately track threats and communicate in the tight confines of that operating area. Without prior consultation and planning, the quality and level of detail contained in any common operational picture at the tactical level would be compromised. Beyond the increased risk of a military incidence, the absence of a common operational picture makes accurate tracking of North Korean missiles, vessels, and other threats extremely difficult. But to even establish this base level of coordination, the two countries require mutual understanding of the parameters of their information sharing relationship. Both sides need to have a common understanding of what information they are permitted to share and when they can do so. Otherwise, the coherence and efficiency required of interoperability will be undermined.


While no short-term diplomatic or political fixes will fully bridge the lingering animosities and differences that hamper deeper Japanese-ROK cooperation, the two countries’ security concerns continue to coincide to a considerable degree. Beyond that, the fact remains that if they do not make adequate preparations to cooperate seamlessly in peacetime, fragile, ad hoc arrangements will almost certainly fail in a crisis.

South Korea and Japan are far from being able to commit to long-term participation in regular trilateral exercises. That said, both Tokyo and Seoul have managed to incrementally but substantially improve their security cooperation since diplomatic ties normalized, despite the fact that their historical issues remain unresolved. U.S. leadership should continue to encourage both countries to build on existing education and training opportunities to establish more routinized, large-scale trilateral engagement insulated from historical issues.

Small, near-term steps can pave the way for further cooperation. All three countries should expand the scale and scope of existing tabletop exercises, particularly with scenarios to specifically address the North Korean threat. Various tabletop exercises have found that all three countries were readily willing to establish coordination mechanisms such as a 2+2+2 forum at the foreign and defense minister–level during crises.57 Establishing a coordination forum of this sort in peacetime would contribute greatly toward a dedicated mechanism for coordinating and prioritizing how to develop a trilateral strategic response.

It is imperative that the three nations work to make security cooperation independent from historical issues, even as these issues remain unresolved.

Formalizing and exercising direct command relationships in advance will be essential to an effective response. Particularly, the three countries should invest more time and effort in exercising the role of Japan in UNC–Rear operations and build on this role to formalize command relationships between the ROK military and the Japanese Self-Defense Forces. To facilitate greater technical interoperability, GSOMIA should not only be maintained but used often. Since the agreement was signed, it has been infrequently used, including after North Korean missile tests that threaten both countries.58 The two countries must go further to actually exercise existing procedures to ensure their efforts to enhance security cooperation can coherently, effectively, and efficiently facilitate interoperability.

The level of cooperation the two countries have today has taken decades to build and is still lacking. The shaky foundation of cooperation in the bilateral relationship means that if engagements or agreements are terminated, the two countries would be left with few institutionalized processes to fall back on, and progress made would be lost and difficult to repair. The trilateral alliance would then be left with little recourse other than relying on ad hoc cooperation mechanisms in conflict, which lack the coherence and efficiency to facilitate interoperability. The vulnerabilities this would create in the relationship could lead to miscalculation, wasted resources, and a greater loss of life. For that reason, it is imperative that the three nations work to make security cooperation independent from historical issues, even as these issues remain unresolved.

About the Author

Kathryn Botto is a research analyst in the Asia Program at the Carnegie Endowment for International Peace. Her research focuses on Asian security issues, with particular emphasis on the Korean Peninsula and U.S. defense policy towards East Asia.


1 Central Intelligence Agency, “Consequences of US Troop Withdrawal From Korea in Spring, 1949,” February 28, 1949,

2 U.S. Army Center for Military History, “The Korean War: PHASE 1: 27 June–15 September 1950 (UN Defensive),” January 30, 2001,

3 Reiji Yoshida, “Tokyo Terminates Working Level Talks With Seoul After Revealing New Evidence on Radar Lock-on Dispute,” Japan Times, January 21, 2019,

4 Steve Miller, “No End in Sight for Radar Dispute Between Seoul and Tokyo,” Voice of America, January 28, 2019,

5 “North Korea Fires Second Ballistic Missile Over Japan,” BBC, September 15, 2017,

6 Victor Cha, “Bridging the Gap: The Strategic Context of the 1965 Korea-Japan Normalization Treaty,” Korean Studies 20 (1996): 123–160; Yaechan Lee, “What Brought Them Together? Comparative Analysis of the Normalization Processes of North Korea-Japan and South Korea-Japan,” Korean Journal of International Studies 16, no. 3 (December 2018): 411–433; and the Japan and Republic of Korea Treaty on Basic Relations (signed in Tokyo), June 22, 1965,

7 James Schoff, Uncommon Alliance for the Common Good, Carnegie Endowment for International Peace, January 23, 2017, 22–24,

8 Ibid.

9 Young-June Park, “South Korea’s Diplomacy and the Evolution of Korea-Japan Security Relations, 1965–2015,” Seoul Journal of Japanese Studies 2, no.1 (2016): 119,

10 Japanese Ministry of Foreign Affairs, “Cabinet Decision on Development of Seamless Security Legislation to Ensure Japan’s Survival and Protect Its People,” July 1, 2014,

11 Japanese Ministry of Defense, “Overview and Fundamental Concepts of National Defense,”; and Kawasaki Akira and Céline Nahory, “Japan’s Decision on Collective Self-Defense in Context,” Diplomat, October 3, 2014,

12 In the 1970s and 1980s, at least seventeen Japanese citizens were abducted from Japan by North Korea, and the Japanese government has since made their repatriation (or the return of remains of any deceased abductees) a top priority in its North Korea policy. See Japanese Ministry of Foreign Affairs, “Japan-North Korea Relations: Abductions of Japanese Citizens by North Korea,” December 28, 2018,

13 David Alexander, “Trump Says He Is Not Worried About Short-Range Missiles Fired by North Korea,” Reuters, August 1, 2019,

14 Hyung-Jin Kim, “S Korea’s Moon Could Seek Exemption of UN Sanctions on NKorea,” AP News, January 14, 2020,; and Dasl Yoon and Laurence Norman, “Moon’s Push to Ease North Korea Sanctions Falls Flat,” Wall Street Journal, October 19, 2018,

15 Tim Kelly, “Japan Lists China as Bigger Threat Than Nuclear-Armed North Korea,” Reuters, September 26, 2019,

16 Ibid.

17 “South Koreans and Their Neighbors 2019,” Asan Institute for Policy Studies, April 26, 2019,

18 North Atlantic Treaty Organization, “Interoperability: Connecting NATO Forces,” June 6, 2017,

19 Ibid.

20 Relevant tabletop exercises (among many others) include those conducted by the Sasakawa Peace Foundation and the Pacific Forum. See Brad Glosserman, “Sustaining Deterrence in a Time of Change and Uncertainty,” Pacific Forum 19, January 22, 2019,; and Michael McDevitt, “Testing the U.S.-Japan and U.S.-ROK Security Alliances: Tabletop Exercise Pacific Trident II,” Sasakawa Peace Foundation, October 16–18, 2018,

21 “Japan, South Korea, U.S. Begin Search and Rescue Exercise,” Japan Times, July 22, 2014,

22 McDevitt, “Testing the U.S.-Japan and U.S.-ROK Security Alliances: Tabletop Exercise Pacific Trident II.”

23 Major General Duane A. Gamble and Colonel Michelle M.T. Letcher, “The Three Dimensions of Interoperability for Multinational Training at the JMRC,” Army Sustainment, September–October 2016, 18,

24 Ibid.

25 Ibid, 121.

26 U.S. Department of Defense, “Japan-Republic of Korea-United States Defense Ministerial Meeting Joint Press Statement,” (press release), June 2, 2019,

27 Philip Stewart and Joyce Lee, “Pentagon’s Esper Says It Is Crucial South Korea Pays More for U.S. Troops,” Reuters, November 15, 2019,

28 U.S. Department of Defense, “Signing of Trilateral Information Sharing Arrangement Concerning the Nuclear and Missile Threats Posed by North Korea,” December 28, 2014,

29 K.J. Kwon and Dugald McConnell, “South Korea, Japan to Join U.S. for Missile-Defense Exercise,” CNN, May 17, 2019,; and Ankit Panda, “US, Japan, South Korea to Hold Missile Tracking Exercises,” Diplomat, December 11, 2017,

30 “Rim of the Pacific Exercise (RIMPAC),” Global Security,

31 “Proliferation Security Initiative Asia-Pacific Exercise Rotation Australian-Hosted Exercise Pacific Protector 2017 APER Joint Statement (Australia, Japan, New Zealand, Republic of Korea, Singapore, United States),” Proliferation Security Initiative, (press release), November 22, 2017,

32 U.S. Department of Defense, Indo-Pacific Strategy Report: Preparedness, Partnerships, and Promoting a Networked Region, June 1, 2019, 44,

33 North Atlantic Treaty Organization, “Backgrounder: Interoperability for Joint Operations,” July 2006,

34 Hiroshi Minegishi, “South Korea Rejected Japan Involvement in Joint US Military Drills,” Asian Nikkei Review, November 13, 2017,

35 Gamble and Letcher, “The Three Dimensions of Interoperability for Multinational Training at the JMRC,” 17.

36 Japanese Ministry of Defense, “The Guidelines for Japan-U.S. Defense Cooperation,” September 23, 1997,

37 Ibid, 6–8.

38 United States Joint Chiefs of Staff, “DOD Dictionary of Military and Associated Terms,” January 2020,

39 “Factbox: U.S. and South Korea’s Security Arrangement, Cost of Troops,” Reuters, November 12, 2019,

40 Emma Chanlett-Avery, Caitlin Campbell, and Joshua A. Williams, “The U.S.- Japan Alliance,” Congressional Research Service, June 13, 2019, 25,

41 IISS, The Military Balance 2019, 60–61.

42 Emma Chanlett-Avery, Caitlin Campbell, and Joshua A. Williams, “The U.S.- Japan Alliance,” Congressional Research Service, June 13, 2019,

43 There are sixteen sending states, but only Australia, Canada, France, New Zealand, Philippines, Thailand, Turkey, the United Kingdom, and the United States are signatories to the UN-GOJ SOFA. See HQ United Nations Command-Rear, “United Nations Command-Rear Fact Sheet,”

44 McDevitt and Kato, “Testing Trilateral, U.S.-Japan, and U.S.-ROK Responses to North Korean Provocations: Tabletop Exercise Pacific Trident.”

45 “Interoperability: Connecting NATO Forces.”

46 James Derleth, “Enhancing Interoperability: the Foundation for Effective NATO Operations,” NATO Review, June 16, 2015,

47 Matthew Ha, “South Korea’s Missile Detection Failure Shows Need for Cooperation With Japan,” Defense News, October 24, 2019,

48 Ibid.

49 “Navy Ballistic Missile Defense (BMD) Program: Background and Issues for Congress,” Congressional Research Service, December 17, 2019, 19,

50 U.S. Department of Defense, “Trilateral Information Sharing Arrangement Concerning the Nuclear Missile Threats Posed by North Korea Among the Ministry of National Defense of the Republic of Korea, the Ministry of Defense of Japan, and the Department of Defense of the United States of America,” December 29, 2014,

51 Anna Fifield, “Japan Warns Citizens They Might Have Only 10 Minutes to Prepare for a North Korean Missile,” Washington Post, April 25, 2017,

52 “United States Foreign Intelligence Relationships: Background, Policy and Legal

Authorities, Risks, Benefits,” Congressional Research Service, May 15, 2019, 8,

53 Sangbo Park, “Implications of the General Security of Military Information Agreement for South Korea,” Stimson Center, December 16, 2016,

54 South Korean Ministry of National Defense, “han·il gunsajeongbobohohyeobjeong 4nyeon man-e non-ui jaegae”[South Korea-Japan military intelligence sharing agreement resumes discussion after four years], Defense News, October 28, 2016,

55 U.S. Department of Defense, “DoD Dictionary of Military and Associated Terms.”

56 McDevitt and Kato, “Testing Trilateral, U.S.-Japan, and U.S.-ROK Responses to North Korean Provocations: Tabletop Exercise Pacific Trident,” 30.

57 Michael McDevitt and Yoichi Kato, “Testing Trilateral, U.S.-Japan, and U.S.-ROK Responses to North Korean Provocations: Tabletop Exercise Pacific Trident,” Sasakawa Peace Foundation, February 14–16, 2018,

58 “No Info Shared Between Seoul, Tokyo Via GSOMIA Over N. Korea’s Missile Test: Officials,” Korea Herald, November 29, 2019,