Japan Boosts Security Cooperation With ‘Quasi-Allies’ UK and Australia

This August saw several landmark developments in Japan’s security, further deepening its cooperation with its two key “quasi-allies,” the United Kingdom and Australia. These events will pave the way for Tokyo’s more active participation in AUKUS, the trilateral security framework in the Indo-Pacific comprising Australia, the United Kingdom, and the United States.

First, in a major milestone for the Japanese government and its defense industry, the Australian government announced on August 5 that it has selected Mitsubishi Heavy Industries’ upgraded Mogami-class frigate, also known as 06FFM and New FFM, for the Royal Australian Navy’s (RAN) new general purpose frigate program, called Project Sea 3000.

Under this decade-long project, Australia aims to acquire 11 upgraded Mogami-class vessels to replace the RAN’s aging Anzac-class frigate. The first three ships will be built in Japan, with the first to be delivered to the RAN in 2029 and enter service in 2030. The remaining eight will be built at Henderson shipyard in Western Australia.

This would be Japan’s biggest defense export in history with an estimated value of up to A$10 billion (US$6.4 billion). But this only covers costs for the first three frigates until 2034. The Australian government is widely expected to allocate additional funds beyond 2035 for acquiring the remaining eight ships, building facilities, and maintenance costs.

In July 2025, the Japan Maritime Self-Defense Force and the United States and Australian navies signed a trilateral naval logistics agreement, allowing them to cooperate in ship ammunition supplies, maritime refueling, repairs, and parts supply, and to share data on maintenance. The U.S., Tokyo’s only formal military ally, thus provided a timely boost to Japan, which was then competing with Germany to win Australia’s new general-purpose frigate project.

Australia and Japan seriously began to strengthen their security cooperation toward a quasi-alliance in the 2010s, in response to regional security challenges, including China’s maritime assertiveness.

In 2010, they signed the Acquisition and Cross-Servicing Agreement (ACSA), enabling mutual provision of supplies and services between the Japan Self-Defense Forces (JSDF) and the Australian Defense Force (ADF).

In 2012, they also signed the General Security of Military Information Agreement (GSOMIA), allowing Tokyo and Canberra to share classified defense and intelligence information more effectively and securely.

In 2014, the two nations also signed the Agreement on the Transfer of Defense Equipment and Technology, creating a framework for the export of Japanese defense equipment and the transfer of defense technologies to Australia.

These three agreements – the ACSA, the GSOMIA, and an agreement governing military technology transfers – serve as a benchmark for measuring which countries are closest to Japan in terms of security.

Between 2013 and 2017, Japan successively put into effect these three bilateral agreements with the United Kingdom as well. Tokyo regards London as a “quasi-ally” just like Canberra. Based on the three agreements, the defense authorities of Japan and the U.K. can share goods, technology, and information with each other.

Japan and the United Kingdom made a further groundbreaking move this August. For the first time, a F-35B stealth fighter jet carried by Royal Navy aircraft carrier HMS Prince of Wales conducted “cross-deck drills” – in which fighters land and take off – from the deck of JS Kaga (DDH 184), the second Izumo-class helicopter carrier (locally classified as a “helicopter destroyer”) of the JMSDF.

It was the first time that a British F-35B had ever landed or taken off from one of Japan’s helicopter carriers.

The U.K. F-35B landing on the Kaga was conducted during joint training as part of the United Kingdom’s Carrier Strike Group 25 (CSG25), which made its first port call in Japan from August 12 to September 2. 

“Cross-deck trials saw two 809 Naval Air Squadron jets land on JS Kaga” in the Philippine Sea, the British Navy Lookout news outlet reported on August 12.

The successful landing and takeoff of two British F-35B fighter jets on the Kaga sent a message to the world, especially China, that Japan’s relationship with the U.K. is being strengthened with increased interoperability.

In addition, Japan and the United Kingdom both sent their navies to a multilateral exercise also involving Australia, Norway, Spain, and the U.S. in the Western Pacific from August 4 to 12. On August 13, the Japanese Defense Ministry announced that the JMSDF’s destroyers Kaga and Teruzuki had provided protection to the Prince of Wales and other vessels based on Article 95-2 of the Self-Defense Forces Law during the exercise. This is the first time such protection has been provided to British troops.

“Providing security for British troops demonstrates the improved interoperability between the forces of Japan and the U.K., our closest security partners in Europe, enabling even closer collaboration. This is an extremely important development for Japan-U.K. defense cooperation to ensure peace and stability in Japan and the region,” the Japanese Defense Ministry said. “Through such efforts, we will continue to elevate our defense cooperation with the U.K. to a new level.”

In a speech in Canberra in November 2023, former Japanese Prime Minister Aso Taro – then serving as the vice president of Japan’s ruling party – emphasized the need for Japan, the United States, and Australia to unite and demonstrate a stance that will not tolerate China's attempts to change the status quo through force.

At the time, he proposed expanding cooperation via AUKUS, which also includes the United Kingdom: “Let's add Japan to AUKUS to make it ‘JAUKUS.’”

AUKUS consists of Pillar I, which will provide Australia with nuclear-powered submarines, and Pillar II, which will jointly develop advanced capabilities such as on undersea, quantum, artificial intelligence (AI), cyber, hypersonic, and electronic warfare weapons.

While the nuclear submarine portion is limited to the three namesake AUKUS members, Pillar II has greater potential for expansion. In September 2024, AUKUS partners released a joint statement that mentioned exploring cooperation on Pillar II with Japan, as well as Canada, New Zealand, and South Korea.

Although Japan is not a formal member of AUKUS yet, the latest events position Tokyo as a quasi-member, facilitating greater interoperability among AUKUS navies.

In the 2024 joint statement, Australia, the U.K., and the U.S. “welcomed progress on consultations with Japan on improving interoperability with Japan’s maritime autonomous systems as an initial area of cooperation” linked to AUKUS Pillar II.

On July 23, the Japanese Defense Ministry announced it had participated inTalisman Saber 25, held in Australia, with the U.S., Australia, and the U.K. During the exercise, the four partners conducted an experimental exercise involving maritime autonomous systems and underwater acoustic communications as part of efforts under AUKUS Pillar II.

Strategically, all of these events represent Japan’s de facto alignment with AUKUS objectives, particularly the maintenance of a free and open Indo-Pacific and balancing against China’s naval expansion.