Flashpoints | Security | East Asia | Southeast Asia

Deterring China: The Growing Japan–Philippines Security Partnership

The growing partnership between Japan and the Philippines is not an actual alliance – but it could affect the balance of power in the South China Sea.

Philippine Foreign Secretary Theresa Lazaro and Japanese Foreign Minister Motegi Toshimitsu in Manila pose for a photo after signing the Acquisition and Cross-Servicing Agreement (ACSA) in Manila, Philippines, Jan. 15, 2026.

A significant shift is underway in Asia’s security landscape. As China expands its maritime presence, countries across the region are tightening their defense ties. The growing partnership between Japan and the Philippines is one of the clearest examples – and one that could affect the balance of power in the South China Sea.

Two recent agreements mark an important step in that direction. The Reciprocal Access Agreement, which entered into force in September 2025, and the Acquisition and Cross-Servicing Agreement signed in January, allow Japanese and Philippine forces to operate more easily on each other’s territory and share logistics support. In practical terms, the deals deepen military cooperation and improve interoperability between the two countries.

Against the backdrop of territorial disputes with China, the message is clear. Tokyo and Manila are strengthening their partnership to help deter Beijing’s growing assertiveness. By working more closely together, they hope to make coercion more costly for China.

But how much difference will these agreements really make?

The stakes are high in the South China Sea. China claims almost the entire body of water within its self-declared “nine-dash line,” a sweeping claim that clashes with those of several Southeast Asian countries. The Philippines insists that features such as Scarborough Shoal and parts of the Kalayaan Island Group – including Mischief Reef and Second Thomas Shoal – fall within its jurisdiction.

Beijing has sought to consolidate control through a strategy of gradual pressure. Rather than launching outright attacks, it relies on coast guard vessels and maritime militia ships to harass rival claimants. These operations – blocking resupply missions, ramming ships, or blasting vessels with water cannons – apply steady pressure without crossing the threshold of open conflict.

This approach has been visible for decades. China effectively seized Mischief Reef in 1994 by building installations and presenting Manila with a fait accompli. In 2012, Chinese and Philippine vessels confronted each other around Scarborough Shoal in a tense standoff, leaving China in effective control of the feature. In recent years, dangerous encounters have become increasingly frequent.

Yet Beijing has carefully avoided actions that would trigger the Philippines-U.S. Mutual Defense Treaty. Since 2019, Washington has reiterated that attacks on Philippine forces – including coast guard vessels – in the South China Sea would invoke mutual defense obligations. That treaty remains the Philippines’ strongest deterrent. But as long as Chinese pressure stays below the level of armed attack, Beijing retains room to maneuver.

The new Japan–Philippines agreements aim to narrow that space. 

First, they will strengthen Philippine maritime forces by deepening cooperation in equipment and training. Japan has already become one of Manila’s most important security partners. Through loans from the Japan International Cooperation Agency, Tokyo has financed the transfer of patrol vessels to the Philippine Coast Guard, including Parola-class ships delivered between 2016 and 2018 and the larger Teresa Magbanua-class vessels that followed.

The two countries have also expanded joint exercises. Their navies have participated in Maritime Cooperative Activities alongside the United States and other partners, while the coast guards of Japan, the Philippines, and the United States conducted their first trilateral exercise in 2023. In 2024, Japan and the Philippines held their first bilateral maritime exercise in the South China Sea.

The new agreements will make such cooperation easier and more frequent. They will also help Philippine forces operate more effectively with Japanese-supplied equipment.

Better-trained and better-equipped Philippine forces would make it harder for China to rely on harassment tactics. If Manila can resist these pressures more effectively, Beijing may face a choice: escalate further and risk triggering U.S. intervention, or pull back.

Still, the balance of power remains uneven. The China Coast Guard is far larger than its Philippine counterpart and operates some of the world’s biggest patrol vessels. Its massive Zhaotou-class cutters – 165 meters long and displacing about 12,000 tons – dwarf anything in the Philippine fleet.

The agreements could also allow Japan to maintain a more sustained presence in the region. By improving logistics and operational access, they will make it easier for the Japan Maritime Self-Defense Force and the Japan Coast Guard to operate alongside regional partners in the South China Sea.

But this should not be mistaken for a formal alliance. Unlike the Philippines-U.S. treaty, the new framework does not oblige Japan to defend the Philippines in the event of an armed attack. Nor does it dramatically increase the likelihood that Japan would be drawn into a conflict there.

Instead, Tokyo’s role is more indirect. By strengthening cooperation and increasing its presence, Japan is reinforcing its vision of a “Free and Open Indo-Pacific” – a regional order based on rules rather than coercion. The partnership also signals to Beijing that continued pressure on the Philippines risks further damaging China’s relations with Japan.

On their own, the agreements will not transform the strategic landscape. But they form part of a broader pattern. Across the Indo-Pacific, networks of security cooperation involving Japan, the United States, Australia, and other partners are steadily expanding.

Each new agreement adds another layer to this emerging architecture. Taken together, these partnerships may gradually raise the costs of coercion – and make Beijing think twice before escalating its actions.

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A significant shift is underway in Asia’s security landscape. As China expands its maritime presence, countries across the region are tightening their defense ties. The growing partnership between Japan and the Philippines is one of the clearest examples – and one that could affect the balance of power in the South China Sea.

Two recent agreements mark an important step in that direction. The Reciprocal Access Agreement, which entered into force in September 2025, and the Acquisition and Cross-Servicing Agreement signed in January, allow Japanese and Philippine forces to operate more easily on each other’s territory and share logistics support. In practical terms, the deals deepen military cooperation and improve interoperability between the two countries.

Against the backdrop of territorial disputes with China, the message is clear. Tokyo and Manila are strengthening their partnership to help deter Beijing’s growing assertiveness. By working more closely together, they hope to make coercion more costly for China.

But how much difference will these agreements really make?

The stakes are high in the South China Sea. China claims almost the entire body of water within its self-declared “nine-dash line,” a sweeping claim that clashes with those of several Southeast Asian countries. The Philippines insists that features such as Scarborough Shoal and parts of the Kalayaan Island Group – including Mischief Reef and Second Thomas Shoal – fall within its jurisdiction.

Beijing has sought to consolidate control through a strategy of gradual pressure. Rather than launching outright attacks, it relies on coast guard vessels and maritime militia ships to harass rival claimants. These operations – blocking resupply missions, ramming ships, or blasting vessels with water cannons – apply steady pressure without crossing the threshold of open conflict.

This approach has been visible for decades. China effectively seized Mischief Reef in 1994 by building installations and presenting Manila with a fait accompli. In 2012, Chinese and Philippine vessels confronted each other around Scarborough Shoal in a tense standoff, leaving China in effective control of the feature. In recent years, dangerous encounters have become increasingly frequent.

Yet Beijing has carefully avoided actions that would trigger the Philippines-U.S. Mutual Defense Treaty. Since 2019, Washington has reiterated that attacks on Philippine forces – including coast guard vessels – in the South China Sea would invoke mutual defense obligations. That treaty remains the Philippines’ strongest deterrent. But as long as Chinese pressure stays below the level of armed attack, Beijing retains room to maneuver.

The new Japan–Philippines agreements aim to narrow that space. 

First, they will strengthen Philippine maritime forces by deepening cooperation in equipment and training. Japan has already become one of Manila’s most important security partners. Through loans from the Japan International Cooperation Agency, Tokyo has financed the transfer of patrol vessels to the Philippine Coast Guard, including Parola-class ships delivered between 2016 and 2018 and the larger Teresa Magbanua-class vessels that followed.

The two countries have also expanded joint exercises. Their navies have participated in Maritime Cooperative Activities alongside the United States and other partners, while the coast guards of Japan, the Philippines, and the United States conducted their first trilateral exercise in 2023. In 2024, Japan and the Philippines held their first bilateral maritime exercise in the South China Sea.

The new agreements will make such cooperation easier and more frequent. They will also help Philippine forces operate more effectively with Japanese-supplied equipment.

Better-trained and better-equipped Philippine forces would make it harder for China to rely on harassment tactics. If Manila can resist these pressures more effectively, Beijing may face a choice: escalate further and risk triggering U.S. intervention, or pull back.

Still, the balance of power remains uneven. The China Coast Guard is far larger than its Philippine counterpart and operates some of the world’s biggest patrol vessels. Its massive Zhaotou-class cutters – 165 meters long and displacing about 12,000 tons – dwarf anything in the Philippine fleet.

The agreements could also allow Japan to maintain a more sustained presence in the region. By improving logistics and operational access, they will make it easier for the Japan Maritime Self-Defense Force and the Japan Coast Guard to operate alongside regional partners in the South China Sea.

But this should not be mistaken for a formal alliance. Unlike the Philippines-U.S. treaty, the new framework does not oblige Japan to defend the Philippines in the event of an armed attack. Nor does it dramatically increase the likelihood that Japan would be drawn into a conflict there.

Instead, Tokyo’s role is more indirect. By strengthening cooperation and increasing its presence, Japan is reinforcing its vision of a “Free and Open Indo-Pacific” – a regional order based on rules rather than coercion. The partnership also signals to Beijing that continued pressure on the Philippines risks further damaging China’s relations with Japan.

On their own, the agreements will not transform the strategic landscape. But they form part of a broader pattern. Across the Indo-Pacific, networks of security cooperation involving Japan, the United States, Australia, and other partners are steadily expanding.

Each new agreement adds another layer to this emerging architecture. Taken together, these partnerships may gradually raise the costs of coercion – and make Beijing think twice before escalating its actions.

Jun J. Nohara is associate professor of international relations at Kyoto Sangyo University's Faculty of International Relations, located in Japan. He specializes in bridging theories of international relations and practices of naval affairs. 

Japan in the South China Sea

Japan Official Security Assistance (OSA)

Japan Philippines defense cooperation

Japan-Philippines reciprocal access agreement

U.S.-Japan-Philippines defense cooperation

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