The announcements last week of an upgrade in the US-Japan alliance and the formation of a new trilateral partnership between the US, Japan and the Philippines are a significant addition to the defence networks that have marked the region since the end of World War II. They also signal a shift from the existing pattern.

Up until now, the region’s defence networks have been dominated by the US-led formal “hub and spokes” alliances with Australia, Japan, the Philippines, South Korea and Thailand. Other countries in the region – for example, Singapore, Indonesia and Malaysia – were linked in less formal defence partnerships with the US. For years, countries outside the US-led formal alliances saw no point in moving into more formal arrangements with the US and its allies, given concerns that this would rile China, the rising power in the region.

Last week’s announcements represent a step change: Japan as a US ally would link up countries in South-east Asia, starting with the Philippines, as part of what Japanese Premier Fumio Kishida calls a “multi-layered defence network”. This development is a major advance towards the broader strategic goals envisioned by the late Japanese prime minister Shinzo Abe: Japan taking on a bigger regional leadership role and the building of a “free and open Indo-Pacific”.

Biggest Upgrade Since 1951

Arguably, the upgrade to the US-Japan alliance is the biggest since its formation in 1951.

The two militaries would be put under a joint operations command led by a US four-star general, thus enabling “seamless integration of operations and capabilities for greater interoperability and planning”. This means the alliance set-up in Japan would more closely resemble US forces in South Korea, which serve with the Korean military as part of a joint command under a “fight tonight” ready-to-deploy posture. Currently, coordination is hampered because Japanese forces have to coordinate with US Indo-Pacific Command in Hawaii, several time zones away.

Japan would also be melded into the second pillar of Aukus, the trilateral security agreement between Australia, the UK and the US. This pillar involves the development of advanced capabilities such as cyber assets, artificial intelligence and quantum capabilities.

To give the US-Japan alliance a broader remit, Japan would also participate in US-Japan-UK trilateral exercises starting in 2025. Tokyo would also participate in an annual multi-domain exercise involving the US and South Korea. It would also acquire Tomahawk land-attack missiles to strike land targets at ranges of up to 1,600km (think China and North Korea). The statement was couched in language which would be familiar to observers of the Quad (Quadrilateral Security Dialogue), a grouping involving Australia, Japan, India and the US. The two allies pledged to uphold the “free and open international order based on the rule of law that has allowed so many nations to develop and prosper”.

Two things also underscore the gravity of the US-Japan alliance upgrade: putting aside convention, Mr Kishida singled out China by name, saying the two allies would “respond to challenges concerning China”. The statement also alluded to the alliance’s global footprint.

To get a sense of how far the alliance has come, one must examine where it has been. In 1997, the two allies announced that Japan could provide logistical and non-combat support for the US military in “areas surrounding Japan”. In April 2021, this was made more specific, with the two allies stressing the importance of “peace and stability across the Taiwan Strait”.

Integrating Manila into the alliance

The inclusion of the Philippines in a trilateral arrangement with Japan and the US does not augur a three-way alliance, given that Japan and the Philippines are not formal allies. But institutional linkages will grow, and serve as a deterrent to assertive Chinese actions, particularly in the South China Sea.

More importantly, the trilateral arrangement would integrate the Philippines, a key Asean member, deeper into the activities of the Quad. This is significant given that Asean countries have long fretted that the Quad would undermine the grouping’s centrality and provoke Beijing, which sees the Quad as a multilateral coalition of resistance against it.

Among other things, the three countries said they would advance trilateral defence linkages, through combined naval exercises and “additional partners” such as Australia and South Korea.

The trio also set out a slew of activities and exercises. Later in 2024, crews from the Japanese and Filipino coast guards will embark on a US Coast Guard vessel on patrol in the Indo-Pacific (the three countries held a first-ever coast guard exercise in 2023). The three will also conduct a maritime training exercise around Japan in 2025. On April 7, the three countries conducted a naval exercise in the South China Sea. Manila and Tokyo are also reported to be in the final stages of talks for a reciprocal access agreement that would make it easier for the two countries’ militaries to conduct joint exercises. The US and the Philippines have a similar set-up under their Visiting Forces Agreement.

Japan as the link between the Quad and SE Asia

Taken together, the two developments – the US-Japan alliance upgrade and the US-Japan-Philippines trilateral – serve to put Japan in a strategic position of linking the Quad and Asean. While Quad membership is out of the question, South-east Asian countries have become increasingly receptive to working with Quad countries to strengthen regional stability.

In August 2023, navies from Australia, Japan, the Philippines, and the US conducted a multilateral exercise in the South China Sea. Also in August 2023, the Talisman Sabre exercises involved not just Australian and American soldiers, but also soldiers, marines and aviators from Japan, South Korea and the UK. Military personnel from the Philippines, Singapore and Thailand attended as observers. In September 2023, Singapore and Japan were part of a large-scale Super Garuda Shield exercise by Indonesia and the US. Brunei, Malaysia and the Philippines participated as observers.

Taken together, these working arrangements do not constitute an anti-China defence network; rather, it means South-east Asian countries can pursue flexible arrangements with the US and its allies in the pursuit of their national interests and regional stability.

The common denominator in these exercises was Japan. The fact it has been a longstanding contributor to the security of South-east Asian countries would also make it more palatable for South-east Asian countries to work with Quad countries.

In 2013, Japan delivered 10 multi-role patrol vessels to the Philippine Coast Guard, and dispatched another two patrol boats in 2022. Tokyo has delivered six second-hand fishery patrol ships to Vietnam, with an additional six to be sent by 2025. Japan has also agreed to sell Indonesia eight Mogami-class stealth frigates.

More importantly, the fact that Japan – like many South-east Asian countries – takes a more nuanced approach to China helps it gain traction in the region. As opposed to the US, which at times has used megaphone diplomacy to harangue China, Japan tends to take a more low-key approach, calling for the upholding of the rule of law in the face of Chinese assertiveness. Tokyo recognises the need to confront yet engage China. In 2018, for example, Mr Abe forged 52 memorandums of understanding with China to facilitate bilateral cooperation in third-country markets.

Japan has generally been highly regarded by South-east Asian countries. In the 2024 State of South-east Asia Survey published by the ISEAS – Yusof Ishak Institute, Japan was ranked as the third most strategically relevant dialogue partner of Asean (after China and the US). It was ahead of other middle powers, including South Korea, the UK, Australia and India.

Not a home run yet

That said, Japan’s heightened security role in the Quad and Asean does not mean that it could coordinate a strong South-east Asian response against China in the event of a conflict.

In the ISEAS survey, Beijing enjoyed a surge in influence across South-east Asia. This will make it hard for Japan (and the US) to rally support from Asean members in the event of a conflict in the region’s four hot spots: the South China Sea, the Senkaku/Diaoyu islands, the Korean peninsula and the Taiwan Strait.

Beijing will also not take the developments in the US-Japan alliance and the trilateral arrangement lying down. China already has the largest navy in the world and its naval modernisation efforts continue apace, including in areas such as anti-ship ballistic missiles, submarines and unmanned vehicles. In a crisis over, say, Taiwan, the question remains if China would be deterred by the US and its allies and partners.

Most importantly, there are the questions over Trump 2.0 and the likely impact of a return to the White House of the notoriously transactional Donald Trump, who is not particularly vested in alliance-building. In short, recent developments lend momentum to the US and Japan’s efforts to put the brakes on China’s assertive regional ambitions, but as the saying goes, it ain’t over till the fat lady sings.

• William Choong is a senior fellow at the ISEAS – Yusof Ishak Institute and the managing editor of Fulcrum, the institute’s commentary and analysis website.