Maritime Activities of Eight Countries Participating in P-8A Demonstrate Japan-U.S. Maritime Security in 2026

On September 20 in Yokosuka, maritime forces from eight countries, including Japan and the U.S., began joint activities as part of the ADMM-Plus Maritime Security Experts Meeting. According to the U.S. Indo-Pacific Command, the participating countries are Australia, Brunei, India, Japan, New Zealand, the Philippines, Singapore, and the United States, contributing vessels, aircraft, and personnel. Notably, the U.S. Navy's P-8A Poseidon and the Independence-class littoral combat ship USS Canberra are included in the same framework.

Reality of the Exercise

This activity is not a combat-oriented anti-ship attack exercise. It consists of disaster response tabletop training, expert exchanges, and vessel exhibitions in Subic Bay, Philippines, followed by an at-sea phase to confirm information sharing and interoperability of maritime skills. According to the U.S. announcement, the focus is on maritime domain awareness (MDA) and disaster response capabilities. In other words, it is an activity to align communication procedures, liaison officer systems, and command structures of aircraft and vessels among the participating countries, starting from humanitarian assistance in peacetime.

Significance for Japan

For the Japan Maritime Self-Defense Force, this is not merely a multilateral goodwill exercise. Japan is not a party to the South China Sea, but it relies on the SLOC (Sea Lines of Communication) that extends from that area to the East China Sea and further to the Japanese archipelago. Co-hosting with the Philippines and placing the U.S., Australia, India, and New Zealand in the same practical framework serves to expand the foundation for sharing maritime information in times of crisis without naming specific countries.

However, it is premature to interpret the participation of the P-8A as a straightforward advancement in anti-submarine warfare cooperation. What can be confirmed from the announced content is limited to training for disaster response and maritime situational awareness. There are separate barriers to sharing practical anti-submarine intelligence, such as classification standards, data links, and the speed of political approval. The value of the exercise lies not in the number of flashy vessels but in whether the procedures for "informing" can be maintained in times of crisis as they are in peacetime.

Overlooked Risks

As the number of participating countries in joint activities increases, the likelihood of settling on a minimum common denominator agreement also rises. What Japan should next confirm is which communication standards and liaison procedures remain with each country's forces after the training. True deterrence lies not in a group photo of the eight countries but in the practical ability to update the same maritime chart at the same speed, regardless of typhoons or emergencies.

Main Sources

• U.S. Indo-Pacific Command, “U.S. naval forces join eight nations in exercise to strengthen maritime security,” 21 September 2026
• Japan Maritime Self-Defense Force, Press Release (Official Announcement List of ADMM-Plus Multinational Maritime Cooperation)