Japan, Philippines Boost Defense Amid China Tensions

This shift marks a significant evolution from previously limited maritime awareness initiatives to a more comprehensive and complex defense relationship. The collaboration includes increased integration of military exercises and intelligence sharing to address common security challenges.

At the center of this transformation is the Japan Philippines Reciprocal Access Agreement (RAA), signed in July 2024 and officially entering into force on January 11, 2025. This agreement provides a crucial legal framework that facilitates troop operations in each other's countries, removing legal and administrative obstacles for joint training. Its first application was observed during the Dagsian Bayan exercise in October 2025.

The Japan Philippine Acquisition and Cross-Servicing Agreement (ACSA) entered into force on August 22, 2026, establishing a formal framework for rapid exchange of supplies and services. This agreement allows the Japanese Self-Defense Forces and the Armed Forces of the Philippines to quickly provide mutual support. ACSA complements the RAA by strengthening the logistical foundation necessary for sustained joint operations and exercises.

Professor Ken Jimbo of Keio University, an international security specialist, argues that Tokyo increasingly views events in the West Philippine Sea and East China Sea as interconnected security challenges. Both Japan and the Philippines face 'gray-zone pressure' tactics, which involve the use of coast guards and maritime militia to assert claims without triggering conventional military conflict. This unified perspective emphasizes that security threats in one area can quickly impact the other.

This approach involves a Chinese vessel first encountering a Japanese Coast Guard ship, with the understanding that the Maritime Self-Defense Force stands behind it, and further, Japan's alliance with the United States. Tokyo wants Beijing to recognize the escalating levels of response, dissuading further aggressive actions.

The Philippines' 1951 mutual defense treaty with the United States remains a central component of its security equation. This treaty specifically applies to armed attacks on Philippine public vessels or aircraft, providing a critical security guarantee. These assurances were reaffirmed by then-Defense Secretary Gilbert Teodoro under the Trump administration in 2026, underscoring the enduring commitment of the U.S. to Philippine defense.

A significant challenge lies in determining the threshold between harassment and an armed attack, especially given China's use of Coast Guard ships, water cannons, and blocking maneuvers. These methods can inflict damage without necessarily triggering conventional warfare, creating ambiguity in international law. Establishing clear strategic communication and red lines is crucial to navigate this gray-zone ambiguity and prevent unintended escalation.

While the RAA and ACSA dramatically deepen cooperation, they do not establish an automatic Japanese military obligation if Philippine vessels or territory come under attack. Japan lacks a formal mutual defense treaty with the Philippines, meaning these agreements do not mandate automatic military intervention. However, Tokyo increasingly views Philippine security as inseparable from its own national security, suggesting a strong strategic alignment even without a direct defense pact.

A conflict in the Taiwan Strait could directly threaten Japanese territorial waters and U.S. forces stationed in Japan, making it a critical security concern. Former Prime Minister Sonai Takai suggested in November 2025 that Japan might respond militarily to a Taiwan crisis, underscoring the deep impact such an event would have on Japanese security.

The Philippines' defense calculations involve carefully weighing the benefits of its strategic alliance with the U.S. against the inherent risk of becoming a target in a broader conflict. The U.S. has access to key military sites in northern Luzon, which enhances regional security but also potentially exposes the Philippines to greater danger. Professor Jimbo emphasized that these critical strategic decisions ultimately rest with the Philippine government, highlighting the country's sovereign choice in managing its security posture.

Japan, the Philippines, and the U.S. are expanding trilateral cooperation to address shared security concerns. Australia has also increased its involvement in maritime activities with this group, further solidifying the network. The focus of these partnerships is on enhancing interoperability and collectively maintaining freedom of navigation in critical waterways.

Negotiations between ASEAN and China over a Code of Conduct have been stalled for years, while Beijing continues to expand military facilities in disputed areas. Professor Ken Jimbo argues that creating a physical power balance is a priority to make countries more resilient against any coercion China might exert in the South China Sea, emphasizing tangible capabilities over stalled diplomatic efforts.

The Philippines, for instance, actively uses publicized videos and photos to expose China's gray-zone tactics to a global audience. This transparency serves to shape international opinion and public understanding, demonstrating that military conflict now encompasses the information space as much as the physical domain. Professor Jimbo highlights that war is no longer confined to kinetic actions on the ground but extends into how information is controlled and presented.

Japan is actively helping to protect vital sea lanes by building the Philippines' resilience against external threats. This partnership aims to facilitate rapid coordination and a unified response in the event of escalation, creating a more robust defense against regional coercion.