Building Security Network Under Japan's "Quasi-Alliance Strategy"

Kitae Lee 

ktleekorea@sejong.org

Senior Research Fellow

Sejong Institute

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Japan is introducing gradual changes to its security policy that has been maintained under the constraints of Article 9 of the postwar Peace Constitution and the principle of exclusive defense (專守防衛). In the past, Japan’s external security strategy did not move far beyond defense based on reliance on the United States. Recently Japan has accelerated efforts to build a more active and diversified security cooperation network. A significant factor behind this strategic change is the possibility of a shift in the level of U.S. military and political engagement in the Indo-Pacific region.

Japan’s tradition of comprehensive diplomacy created a foundation for forming multilayered security networks with diverse states and regions while reducing dependence on the U.S.-Japan alliance. Traditionally, Japan pursued comprehensive diplomacy through the three principles of Japanese diplomacy announced in 1957 (United Nations centered policy, cooperation with liberal states, and maintaining Japan’s position as an Asian state). The National Security Strategy released in 2013 presented the goal of “strengthening diplomatic and security cooperation with partners for peace and stability in the international community (国際社会の平和と安定のためのパートナーとの外交・安全保障協力の強化),” subsequently expanding into a diversified and multilayered security network, and this later developed into an expansion of multidirectional and multilayered security networks. The revised National Security Strategy of 2022 presented the strengthening of the U.S.-Japan alliance along with deeper coordination with allied and like-minded states (同志国) as a diplomatic approach to actively create a peaceful and stable international environment and to reinforce a free and open international order.

In this way, Japan is not limiting itself to inducing U.S. engagement but is expanding quasi-alliance relations that complement the bilateral framework of the U.S.-Japan alliance and can substitute it when necessary. A quasi-alliance refers to a relationship that reaches a practical level of cooperation equivalent to an alliance in areas such as military exercises, information sharing, and defense industry cooperation, even without a legally binding alliance treaty. From an international politics perspective, Japan’s strategy to expand quasi-alliances can be interpreted as part of a hedging strategy. Hedging is a strategy that prepares for uncertainty in great power competition by avoiding complete dependence on one side and forming multiple cooperation networks.

Based on this background, this paper examines Japan’s strategy of expanding quasi-alliances in a systematic manner. It first outlines the structure of Japan’s security cooperation network and the concept of the quasi-alliance. It then identifies the specific strategic features through case studies of major countries. Through this analysis, it shows that Japan’s quasi-alliance strategy expands its security cooperation network in a way that continually encourages U.S. engagement while supplementing the strengthening of the U.S.-Japan alliance.

In international relations, a “quasi-alliance” refers to a form in which two states achieve alliance-level closeness and coordination through substantive security cooperation (joint exercises, operation of strategic assets, and defense industry cooperation) and through information and policy coordination, without a collective defense treaty or legally binding defense obligations. Unlike a formal alliance, a quasi-alliance allows states to bypass some of their legal and political constraints while pursuing strategic objectives (strengthened deterrence and threat response) in a flexible way. It also offers the advantage of building a high level of interoperability and trust.

Japan’s quasi-alliance allows institutional flexibility and strategic adaptation under the constraints of its constitution. In an environment where Article 9 of the Peace Constitution (No War clause) and domestic public opinion limit the expansion of formal alliances, Japan uses quasi-alliance arrangements to build deterrence and security networks that resemble those of an alliance in a gradual way. This can be viewed as a case of “constraint adaptation,” a strategy that maximizes security interests in a rational manner under structural constraints.

The Japanese government does not officially use the term “quasi-alliance,” yet among the “like-minded countries” with which it strengthens cooperation, it perceives those that complement the U.S.–Japan alliance as quasi-alliance partners. Japan’s quasi-alliance concept aims to expand a structural framework that restrains China’s maritime expansion, and Japan responds through two approaches: strengthening the U.S.–Japan alliance and deepening linkages with like-minded states around Japan. In doing so, Japan signals to the U.S. that it shares responsibilities with like-minded countries, and it seeks to maintain U.S. engagement.

Japan’s quasi-alliance strategy represents an innovative security cooperation model that transcends the limitations of formal alliance systems, embodying institutional flexibility, multilayered threat balancing, network-based cooperation, and normative order formation. This quasi-alliance strategy contributes to Japan’s proactive construction of a new international security order and enables it to emerge as a central actor in regional and extra-regional multilateral security communities despite domestic legal and political constraints.

Next, this study examines the United Kingdom, France, Australia, and the Philippines as major cases of Japan’s “quasi-alliance” security cooperation.

Japan and the UK institutionalized the 2+2 foreign and defense ministers’ meeting in 2017 and expanded defense industry cooperation and joint military training. The two countries concluded the Agreement on the Transfer of Defense Equipment and Technology (effective 2013), the General Security of Military Information Agreement (GSOMIA, effective 2014 and revised in 2015), the Acquisition and Cross-Servicing Agreement (ACSA, effective 2017), and the Reciprocal Access Agreement (RAA, effective 2023). The UK–Japan RAA is the first military agreement Japan has signed with a European state, and it links to the rotational deployment of the British carrier strike group to the Indo-Pacific, which marks the institutionalization of maritime security cooperation.

The UK conducted “port-call diplomacy” with Japan as it deployed its carrier strike groups to the Indo-Pacific in 2021 and 2025. The carrier strike group that included the aircraft carrier Prince of Wales made a port call in Japan in 2025 and carried out joint training in Okinawa. A Maritime Self-Defense Force escort ship provided protection for the British carrier as it moved, which marked the first “protection of weapons and other equipment” based on Japan’s security legislation conducted with a partner other than the U.S. and Australia. Through “protection of weapons and other equipment,” Japan and the UK carried out an advanced military operation that Japan had originally conducted only with allies, and demonstrated an ability to respond together in a contingency.

Japan is now advancing joint development of a sixth-generation fighter aircraft through participation in the “GCAP” program with the UK and Italy, which allows it to share capabilities in defense modernization. Japan is working closely with the UK and Italy in advanced defense industry fields through the joint development of the sixth-generation fighter (GCAP). To counter China and Russia, which are expanding their military capabilities, Japan needs linkage between Europe and Asia, positioned to its west and east. In this way, joint weapons development with Europe goes beyond the goal of sustaining the defense industry and is now connected to Japan’s broader security strategy.

France holds overseas territories in the South Pacific, including New Caledonia and Polynesia, and on this basis it participates actively in the Indo-Pacific strategy. Japan and France have held regular 2+2 ministerial meetings since 2015 and have expanded cooperation in maritime security, logistical support, and defense technology exchange. Their cooperation has grown in multinational maritime exercises such as the La Pérouse exercise, and they are widening coordination in the security of French territories in the region, including New Caledonia, and in the broader Indo-Pacific.

In 2024 Japan and France formally launched negotiations on an RAA that simplifies procedures for reciprocal troop deployments and joint exercises. This move widens the basis for rapid military cooperation and mutual visits in a contingency. The two countries have also concluded agreements on the export and transfer of weapons and defense technology and have strengthened their practical security and defense-industry partnership. Japan and France have concluded the defense equipment and technology transfer agreement (effective 2016), GSOMIA (effective 2011), and ACSA (effective 2019), and they opened negotiations on the RAA in 2024.

Strategically, Australia is a key location that links the Indian and Pacific Oceans, and its interests align with Japan’s in balancing China. Japan has deepened military and diplomatic cooperation with the U.S., Australia, and India through the Quad and has diversified its consultative mechanisms with Australia, New Zealand, and Korea through the Indo-Pacific initiative (IP4). The Japan–Australia RAA, effective in January 2022, provides a legal basis for the two countries’ forces to train and deploy on each other’s territory, and it carries strategic significance as Japan’s first troop-deployment agreement with a country other than the U.S. Japan and Australia concluded the defense equipment and technology transfer agreement (effective 2014), GSOMIA (effective 2013), ACSA (effective 2017), and the RAA (effective 2023).

In August 2025, Japan was selected as the preferred bidder for the export of eleven next-generation frigates to Australia, a project worth about 10 trillion won and the first export of its kind. Normally, defense equipment with high offensive capability, such as frigates, cannot be exported in their original form, but such exports become possible when they take the form of international joint development with a friendly state and when the case is judged to hold value for Japan’s security. The selection of the Mogami-class frigate reflects the strengthening of Japan–Australia military cooperation. It also extends beyond sharing munitions and equipment, since both countries would be able to repair damaged vessels for each other in an emergency, which is expected to significantly increase strategic agility and interoperability in a contingency.

Between 2023 and 2024, Japan and the Philippines elevated their security cooperation to the quasi-alliance level, citing the need to counter China. Prime Minister Kishida officially declared that the Philippines is an “indispensable partner” and initiated negotiations for rapid agreements covering military and intelligence cooperation and joint maritime security responses (OSA, RAA, ACSA). Japan and the Philippines concluded the Defense Equipment and Technology Transfer Agreement (effective 2016) and signed the RAA in 2024. Discussions on GSOMIA are underway, and ACSA negotiations were agreed to commence in 2025. Japan also provided the Philippines with defense equipment such as radar systems at no cost, applying the OSA framework for the first time.

In July 2024, Japan and the Philippines signed the RAA, establishing the institutional foundation for their quasi-alliance. The Japan-Philippines RAA significantly reduces legal and procedural barriers in cases of mutual troop deployment, providing visa exemptions, securing the status of forces, ensuring legal standing, and facilitating joint training and disaster relief operations. However, it is closer to a Visiting Forces Agreement (VFA) than a full-status SOFA, as it does not provide for permanent bases. By concluding an agreement of the same nature with a major Southeast Asian country as with Australia (2023) and the UK (2023), Japan accelerates the establishment of a maritime belt to counter China in the region.

In this way, this paper analyzes how Japan’s security policy has evolved from a traditional U.S.–Japan alliance–centered approach into a multilayered and diversified “quasi-alliance” network strategy. Even under the institutional constraint of Article 9 of the Peace Constitution after the Cold War, Japan expanded the scope of its overseas security engagement through the International Peace Cooperation Act, the reinterpretation of the right of collective self-defense, and the Peace and Security Legislation. In particular, since the Abe administration, these changes have been institutionalized under national strategies such as “Proactive Contribution to Peace” and “expansion of international contributions,” and Japan is moving toward strengthening various “quasi-alliances” to secure distributed deterrence in the Indo-Pacific region.

Japan seeks the strategic meaning of complementarity with U.S. strategy through the expansion of its quasi-alliance diplomacy. The U.S. Indo-Pacific Strategy aligns with the core principles of Japan’s FOIP, yet long-term engagement remains uncertain due to America First tendencies and domestic political factors following the launch of the second Trump administration. Japan’s expansion of “quasi-alliances” functions as a supplementary strategy to fill potential security gaps created by such uncertainty in the U.S.

Above all, the expansion of Japan’s quasi-alliance diplomacy means the multi-layered structuring of China deterrence. Japan builds a maritime security deterrence network with partners other than the U.S. to respond to China’s military expansion and gray-zone tactics. Through this, Japan moves toward optimizing its ability to respond to China’s maritime activities by region and by issue.

On the other hand, the expansion of Japan’s quasi-alliance diplomacy moves in the direction of strengthening Japan’s strategic autonomy. Amid the intensifying US–China strategic competition, Japan enhances its strategic autonomy by expanding its diplomatic and military options through diversified security relations that do not rely entirely on any single country.

Ultimately, Japan’s strategy to expand quasi-alliances emphasizes diversifying its military and diplomatic networks without excessive reliance on the US alliance and pursuing a collective security order capable of responding swiftly and flexibly to regional security threats such as China and North Korea. In particular, by integrating legal and institutional agreements (RAA, GSOMIA, ACSA), Japan has effectively bypassed the constraints of the postwar peace constitution, increasing its freedom for extraterritorial engagement and military operations. Against this backdrop, Japan is gradually positioning itself as an “Asian NATO” through a multidimensional alliance policy that encompasses advanced technology and defense cooperation, joint exercises and arms collaboration, and mutual support systems for military materiel.

Japan’s quasi-alliance strategy has become a core pillar of its Indo-Pacific vision, but several challenges remain. First, China’s backlash is a concern, as the more the strategy emphasizes “containing China,” the higher the risk of economic and diplomatic retaliation. Japan seeks to maintain its traditional approach of “omnidirectional diplomacy” and “balanced diplomacy.” Second, political changes in security partners pose a challenge. As seen in the case of the Philippines, a change of government creates uncertainty in foreign policy, which constrains the continuity of quasi-alliance diplomacy. Third, domestic debate over the interpretation of the peace constitution and changes in military policy, along with shifts in public opinion, constitute another challenge.

With these challenges in mind, it is important to consider the interconnections and differences between future U.S.-Japan-South Korea trilateral security cooperation and Japan’s quasi-alliance security network. The U.S.-Japan-South Korea trilateral cooperation is an alliance-based triangular security system with the U.S. as the central linking axis, whereas Japan’s quasi-alliance network builds on this foundation to function as a multilayered cooperation framework with expanded scope and functions. Both share the common goals of “responding to China” and maintaining regional stability and are based on practical joint response mechanisms such as information sharing, missile warning coordination, and cooperation on North Korean nuclear issues. Japan’s quasi-alliance vision also treats the U.S.-Japan-South Korea cooperation as one axis while simultaneously expanding a peripheral layer of partnerships with countries such as the UK, Australia, and the Philippines to establish a multilayered deterrence system.

However, the U.S.-Japan-South Korea security cooperation differs in nature from the quasi-alliance network, as it is an institutionalized cooperation structure built on treaty-based alliances (ROK-U.S. and U.S.-Japan) that establish a trilateral coordination mechanism. The quasi-alliance network, by contrast, is not legally binding and functions as trust-based cooperation through practical military and defense-industrial collaboration. The former is centered on a deterrence system against North Korea, while the latter has a functional focus on cooperative security, the defense industry, and defense technology cooperation. In other words, Japan’s quasi-alliance is a global security network with the U.S.-Japan alliance as its hub, whereas U.S.-Japan-South Korea cooperation is a “strategic triangular framework” centered on the Korean Peninsula and Northeast Asia. Japan’s quasi-alliance diplomacy also differs from the defensive deterrence-centered frame of the U.S.-Japan-South Korea cooperation, as it encompasses security, economic, and technological domains and seeks to maximize global policy influence through Japan-led regional partnerships.

In this context, the following policy recommendations can be presented for the Korean government.

First, just as Japan’s quasi-alliance diplomacy complements the U.S.-Japan alliance, South Korea should also expand diversified and multilayered cooperation networks while keeping the U.S.-ROK alliance as its central pillar. South Korea should contribute to the formation of regional norms led by Seoul by expanding linkages with existing multilateral frameworks such as the Quad, AUKUS, and the CPTPP, and by connecting them with quasi-alliance networks. Beyond the existing U.S.-ROK-Japan security cooperation, the framework can extend into small-group formats such as U.S.-ROK-Australia or U.S.-ROK-Japan-Philippines cooperation. Building a “U.S.-ROK alliance plus quasi-alliance security network” model strengthens distributed deterrence in regional security. Through this, South Korea can establish a “balanced alliance model” that enhances its diplomatic and security autonomy while preparing for uncertainties in the U.S. Indo-Pacific strategy.

Second, South Korea needs to perceive Japan’s quasi-alliance diplomacy not as “competition” but as a framework of “strategic complementarity.” Japan’s quasi-alliance diplomacy can be seen as a strategic evolution that complements the existing U.S.-Japan alliance and forms a multilayered and diversified security network. South Korea should strengthen security cooperation with Japan in a practical manner within this shift, while building both the complementarity of the U.S.-ROK alliance and South Korea’s own “quasi-alliance network” strategy. This requires deeper cooperation across multiple domains. In other words, South Korea should advance quasi-alliance relations in a comprehensive way by strengthening cooperation in non-military fields such as cyber security, space security, electronic warfare, and supply chains, beyond military exercises and information sharing.

Third, South Korea and Japan should first pursue a Foreign and Defense Ministers’ Meeting (2+2) and in the medium to long term, advance security cooperation in stages through ACSA and then RAA. Regularizing the 2+2 meeting institutionalizes a strategic communication structure between the two countries and creates a basis for mutual trust. The 2+2 meeting should deepen working-level consultations on shared issues such as North Korea’s nuclear and missile threats, maritime security, and cyber security. South Korea and Japan should also expand cooperation beyond simple information sharing under GSOMIA and move toward substantive military logistics and technology cooperation. A South Korea–Japan ACSA would enable mutual support in supplies, fuel, and maintenance during training, disaster response, and humanitarian assistance, and it could serve as a basis for expanded defense industrial cooperation in areas such as maritime patrol, missile defense, and unmanned systems. A South Korea–Japan RAA would institutionalize reciprocal access between the two militaries and ensure access to each other’s bases and training facilities along with their legal status. This would strengthen coordination in maritime combined exercises and multilateral operations in the Indo-Pacific. They could also explore linking multilateral RAAs with India, Australia, and the Philippines to develop a “South Korea–Japan-centered minilateral network.” In this way, the two countries should expand the conclusion of institutionalized agreements such as RAA, ACSA, and defense industry cooperation agreements to avoid limiting cooperation to one-off security initiatives.

Fourth, institutionalizing a South Korea–Japan ACSA and RAA can serve as an “alignment-strengthening mechanism” that links Japan’s quasi-alliance network with South Korea–U.S.–Japan security cooperation in a complementary manner, yet without political and societal acceptance the risk of backlash is high both domestically and internationally. The ACSA and RAA create an institutional foundation for logistics and operational support cooperation between South Korea and Japan, enabling efficient linkage between combined South Korea–U.S. and U.S.–Japan operations in contingencies. Despite progress in trilateral information-sharing, actual systems for mutual supply, maintenance, and logistics support remain insufficient, and the conclusion of an ACSA and RAA can complement this gap and promote the integration of the trilateral logistics network. Given that the Japan Self-Defense Forces are expanding rear-area support and transport capabilities, and that most UN Command rear bases are located in Japan, the strategic necessity is significant for ensuring logistical efficiency and operational sustainability in contingencies. In the long term, the ACSA and RAA can become a pathway through which South Korea secures a position as an “essential strategic partner” within Japan’s quasi-alliance network.

However, the most significant constraint is the domestic political and historical sensitivity surrounding the possibility of the Japan Self-Defense Forces legally entering the Korean Peninsula. As long as memories of past invasion and occupation persist, “wartime logistics cooperation” carries the risk of provoking domestic public backlash and political conflict among parties. If the conclusion of an RAA is perceived as a “quasi-military alliance,” it could also trigger strong reactions from China and North Korea and heighten regional military tensions. Under a potential second Trump administration, where reduced U.S. involvement is being discussed, the more South Korea and Japan intensify “cooperation without the U.S.,” the more neighboring states may become wary. Legal and operational issues likewise remain, including insufficiently developed mechanisms for defining the scope of mutual logistics support, identifying the responsible authority for material management, and ensuring security procedures for military classified information. Securing practical benefits—such as enhanced deterrence and improved operational efficiency—requires transparent political communication, public explanation procedures, and coordinated role-sharing among South Korea, the U.S., and Japan before concluding the agreements. It is also necessary to redesign the agreements not as simple military cooperation but as an institutional infrastructure for international coordination, peacekeeping, and logistics security cooperation.