The pathway to Australia acquiring nuclear-powered submarines, under Pillar 1 of the AUKUS partnership, assigns a prominent role to the UK, as the designer and co-builder of the future SSN AUKUS for the British and Australian navies. Britain’s commitment to forward-deploy a nuclear attack submarine (SSN) to Western Australia, from around 2027, has received less attention, but also carries potentially weighty implications for the UK, including its overall deterrence posture.

The decision to send one of the Royal Navy’s seven SSNs to the far side of the world was no small commitment for the British government to make, while under fiscal strain and dealing with the war in Ukraine. US Chief of Naval Operations Admiral Mike Gilday has said the US Navy will also commit up to four of its SSNs to the new Submarine Rotational Force—West (SRF-West), which will operate out of HMAS Stirling, near Perth. The addition of an Astute-class SSN to that total will give the Royal Navy a resident undersea presence in the region, plus an opportunity to train Australian submariners in situ and trial joint operational concepts with its AUKUS partners. But Britain will inevitably be the junior partner in SRF-West, which is likely to be a US-led venture in whatever form it takes, especially once Australia’s own Virginia-class boats arrive in the early 2030s.

Sceptics might therefore be tempted to dismiss a solitary UK boat as tokenism, a faint operational footprint in the AUKUS submarine endeavour, where UK equities remain stacked towards a technical-industrial role in delivering SSN AUKUS. A lone Astute-class submarine is likely to be expensive to sustain in Australia, given that it will require its own local maintenance and supply chain from the UK.

On closer inspection, the implications for the UK run deep. The SRF-West commitment is conceptually distinct from other UK defence initiatives in the Indo-Pacific, including expeditionary deployments. The purpose of basing foreign submarines in Australia, in the long run-up to SSN AUKUS, is to forestall the emergence of an Australian ‘capability gap’ by maintaining a sufficient regional balance, concentrated in a key category of military capability, and thereby hopefully dissuading China from resorting to force in the South China Sea, Taiwan or other maritime contingencies. Compared to the Royal Navy’s other experiment in forward deployment to the region, involving a pair of roving patrol vessels, the decision to dedicate an SSN full-time to the Indo-Pacific is more consequential. Submarines can perform other tasks, like intelligence-gathering, but primarily they are platforms for high-intensity maritime conflict. SRF-West is about deterrence, not engagement, and the UK will be an integral participant in that effort.

Much will depend on the concept of operations to be worked out for SRF-West, as well as for Australia’s future SSNs. For example, will they be integrated into US naval task groups, or tasked with solo missions, such as tracking China’s nuclear-armed ballistic missile submarines (SSBNs) in the South China Sea? Will their area of operations extend Indo-Pacific-wide, taking full advantage of SSNs’ speed and endurance? Will the British submarine operate as an ‘interchangeable’ asset alongside the US and Australian navies, per recent alliance rhetoric, or will Whitehall place caveats on what it can do, where and with whom? As a function of the UK’s Indo-Pacific ‘tilt’, British-led amphibious and carrier task groups will be passing through the region on a more regular basis. It’s fair to assume that the British submarine in SRF-West will be pulled into some of these requirements. Canberra and Washington are unlikely to object. And when Type 31 frigates eventually replace the Royal Navy’s forward-based patrol vessels, there will be more capable UK surface assets available in the region.

But what if deterrence fails and there’s an outbreak of conflict over Taiwan, or in the South China Sea, and US submarines in SRF-West are ordered to respond; would the British SSN also be committed in support? While that is a sovereign decision for the UK’s future political leadership, the fact of the vessel’s forward deployment in theatre alongside US and Australian submarines could make it more difficult to say ‘no’ than to an expeditionary deployment from the UK. Forward basing has a different dynamic, especially of a high-value asset.

Further potential implications flow from the fact that the UK and China are both nuclear powers. The 2021 UK defence command paper highlights numerous defence concerns around China’s military modernisation and naval build-up, but has nothing to say about its nuclear posture or capabilities. It remains to be seen if the impending command paper refresh will go further. Currently, the UK publicly articulates its nuclear deterrent largely in relation to Russia, as well as passing mentions of North Korea and (prospectively) Iran. It is hard to find any reference within UK nuclear thinking that entertains even a notional deterrence relationship with China. The matter is considered either too sensitive for public consumption or not serious enough.

And yet China now regularly tops intelligence-informed threat assessments in Britain. That hasn’t carried over to the Ministry of Defence, presumably because distance influences calculations of military threat to a greater extent than espionage or economic coercion. However, China is steadily its expanding nuclear arsenal (as, significantly, the UK itself is also poised to do). Meanwhile, Britain is drawing militarily closer to the US and Australia through AUKUS.

AUKUS members have been at pains to stress that Australia’s nuclear-powered submarines will be conventionally armed. However, deterring a nuclear power by purely conventional means is not a straightforward proposition, especially when SSNs operating from Australia could easily come into contact with China’s SSBNs, accidentally or intentionally. Unlike Australia, which depends on the US umbrella, the UK has an independent nuclear deterrent, so must make its own calculations about how to respond to a developing deterrence relationship with China. (If that sounds overdone, Russia’s recent resort to nuclear threats against NATO countries should give pause for thought.)

It is not clear that the UK has the capability to deter Russia and China concurrently. If a decision is made that the nuclear deterrent should also cover China, Britain may—for various reasons—need to consider extending SSBN patrols into the Indian or Pacific ocean, while it gradually expands its nuclear warhead inventory, from 225 to the new cap of 260. This may come across as a somewhat alarmist extrapolation from the deployment of a single, conventionally armed British submarine to Australia. However, ahead of that deployment it would seem prudent for officials in Whitehall to be weighing possible adjustments to Britain’s nuclear doctrine and posture against the entirely plausible future of a full-spectrum UK deterrence relationship with China.