Is the UK already in a Military Alliance with Japan?

In March 2025, a ministerial delegation travelled to Japan for the 'Economic 2+2', a meeting of the trade and foreign ministers for the UK and Japan, heralded by this Government as an opportunity to propel growth and resilience. In a rather au-milieu way, UK policy identifies the Indo-Pacific as critical to the economy and security, with the UK-Japan relationship described as an enhanced global strategic partnership. To what extent is this fully reflective of the Japanese experience of their security relationship with the UK?

Signalling their perceived severity of the security environment, previous PM Kishida warned that 'Ukraine today may be East Asia tomorrow'. Japan now dynamically balances its peace-loving approach with a hard realism that bolsters deterrence with hard power and alliances. The Hiroshima Accord is evidence of, at least, Japan’s clear intention to increase the quality and depth of its deterrence partnerships. This article considers whether the state of the UK’s entanglement with Japan is actually a vivid collection of co-dependencies that, with some small imagination of the Japanese and US policymaker, sufficiently meet the conditions of being in an Alliance.

Stocktaking the UK-Japan Relationship

A shared UK-Japan world view towards securitisation of economies and geopolitics drove the 2+2 to focus on AI, defence investment and joint research. Joining the collection of joint programmes is GCAP, the UK-Japan-Italy 6th-generation fighter programme, which from a political perspective has cemented the convergence of national-strategic goals with a programme that supports national prosperity and defence-industrial sovereignty.

These fundamentals did not happen overnight. The race to partner with the Indo-Pacific centres of economic and technological gravity is a sport for many western nations, yet the UK has comparatively outstripped its competitors in this regard.

It is possible that an enemy or ally could create an impression of a UK-Japan entanglement from this collection of agreements and quasi-alliances. Both the full-spectrum cyber-partnership and GCAP contain obligations arising from co-dependencies. The commitment to maintain a high technology partnership needs to account for the interactive nature of the threat, where a responsive defence-industrial feedback-loop maintains the asymmetric advantage. GCAP's Convention requires reciprocal access to information and industrial-led spiral development. An aggregate impression could be that these agreements pass the test for the definition of an alliance.

The Meaning of an Alliance

Japan has seen its relationship with the US generate defensive securitisation. As part of their shield-role, Japan is obligated to anchor the island chains, with an emphasis on survivability. The UK’s span of collaboration is already providing Japan with its survivability edge and to assist with holding China at range, albeit that scenarios for Chinese precursor operations to a Taiwan invasion include the writing down of Japanese military installations and US bases. This means that the UK must be clear on the defence obligations with Japan, based upon a revised understanding of what it means to be in an alliance.

In its review of the relevant security agreements, this article finds that mutual security clauses are not common. Where they do exist, the agreement text contains profound ambiguity on what is actually obligated in the event of a security violation. NATO's Article V has a habituated understanding of an automatic clause to mobilise military force, yet there are not any details on the assistance that is perceived as necessary:

'…if such an armed attack occurs, each of them . . . will assist the Party . . . attacked by taking forthwith, individually and in concert with the other Parties, such action as it deems necessary'.

More explicitly, Article 3 of the Russia-DPRK Alliance states the need 'for consultations in order to coordinate positions’, whilst within the US-Japan Treaty of Alliance each party 'declares that it would act to meet the common danger'.

Such broad interpretations of security guarantees may burden an ally with concern about the appetite of the guarantor to take risk on its interests, yet the ally does enjoy the signalling of an increased security intention. Moreover, the vagueness within treaty obligations keeps adversaries in the dark, creating ambiguity over the circumstances in which a treaty would be enacted.

The 1902 Anglo-Japanese Alliance is an example, well-recalled by the Japanese, where a mutual security clause was not a foundational element. The UK used de-escalatory interventions, based upon the isolation of Russia and the deterrence of France, that obviated the need to deploy forces. Similarly, a new Treaty of Alliance elect for aterritorial deterrence. Rather than deploy hard power, the focus should be on resilience to withstand and endure attack. Domains such as cyber and space exist in the aterritorial global common. National resilience may start in these realms and remain as interactive as physical conflict. This approach to an Alliance - domain hardening - is clearly within the broad yet ambiguous explications of Treaties and Alliances.

This article further find finds that formal alliances invariably include the US, particularly in Asia-Pacific. This means that a UK-Japan Treaty of Alliance would need careful perception management; the US agreement with Japan does not mean its unilateral protection by the US, and Japan’s many agreements show that the US tolerates Japanese bilateralism. However, neither the UK nor Japan would wish the US to take exception to a Treaty of Alliance that builds further deterrence, increases Japanese security sovereignty and infers overreach of the UK and Japan into a US prerogative for techno-military dominance.

Weighing the Costs and Benefits

The US-Japan Alliance will continue to be the superior hard-power relationship, providing essential extended deterrence and security cooperation. However, the UK is on a pathway that pushes the Japan relationship into a territory of security co-dependency that was once the preserve of the US. Moreover, both the UK and Japan are thinking the unthinkable about the reliability of US hard power security guarantees, with Japan perhaps seeing in the Afghanistan withdrawal a measure of peril in the US’ abandonment of an ally. Russia's illegal war in Ukraine shows that deterrence has new dimensions beyond hard-power guarantees, where a complex and interactive system prioritises the spiral development of high-end, widely dispersed, low-cost technology.

Such factors point to possible objectives for a UK-Japan Treaty. Japan's global leadership in super-computing, AI and machine-learning would secure asymmetric survivability in cross-domain capabilities such as electronic warfare. Collaboration on advanced autonomous systems would compensate for numerical gaps in hard-power, backed-up by enhanced, bi-lateral intelligence-sharing. Not least, Japan is a credible space power, with independent vertical launch capabilities, whilst both the UK and Japan have mature Space Domain Operations, all of which are vital to respond to emerging national resilience threats.

This focus on the digital frontline binds the UK and Japan as deterrence-twins with an aim for asymmetric advantage over not just foes but also allies. This could present a challenge to fragile US-primacy within the Indo-Pacific and key domains. China would also see a threat to its interests in such a Treaty, although arguably this ground has already been broken by AUKUS and a newly assertive Japan, openly squaring-up to China with increased defence spending.

A Way Ahead

In order to guard against US misperception, specifically of UK overreach, the US must be involved in the drafting of a UK-Japan Treaty. This would set the boundaries for twin-deterrence and enhance collective messaging towards China. The aggregate impact of UK-Japan agreements and programmes is arguably already an alliance, just not the kind that the UK has experienced in the Indo-Pacific for over a century. Importantly, the UK is – for once – leading this race, where a collection of partnerships has moved beyond what is currently agreed by Japan with any other state, perhaps including the US. The cost of losing US confidence in either the UK’s intentions or Japan’s primary security relationship is untenable to both, which is why a UK-Japan Treaty of Alliance would be a bold, but much needed, step.

Group Captain Stuart Gregory RCDS MA RAF. The author is a Royal Air Force Strategist and Logistician. The article consists of his personal views, and in no way represents the views of His Majesty’s Government, or the policy of the Ministry of Defence.

© Stuart Gregory, 2025, published by RUSI with permission of the author.

The views expressed in this Commentary are the author's, and do not represent those of RUSI or any other institution.

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