The Liminal Army: Proposing a Term for Cross-Domain Operational Effect

I want to propose a term. Not as doctrine, not as a formal concept, but as a way of naming something I can see forming at the edges of UK defence thinking that does not yet have a clean label. I am going to call it the Liminal Army, with ‘Liminal Force’ as the broader principle it sits within. I am aware that naming things carries risk. Names can be wrong, or premature, or they can flatten nuance. So take this as early thinking, offered in that spirit, and push back if it does not fit.

What the Royal Navy Already Has

The Royal Navy’s ‘hybrid fleet’ concept is well established and increasingly official. The Strategic Defence Review (‘SDR’) 2025 uses the language of ‘uncrewed wherever possible, crewed only where necessary,’ and the Navy has been trialling this model actively. In October 2025, a flotilla of USVs (‘Unmanned Surface Vehicles’, remotely operated vessels without crew aboard) shadowed Royal Navy warships in a milestone capability demonstration, with sailors, Royal Marines, and Army personnel all operating the systems together.

The hybrid fleet concept is primarily a platform description. The question it answers is: what is on the vessel? Crewed platforms, USVs, and UUVs (‘Unmanned Underwater Vehicles’, remotely operated submersibles) working as a layered system. It is a meaningful and important idea, and the Royal Navy is further along in operationalising it than most NATO (‘North Atlantic Treaty Organisation’, the 32-nation military alliance that underpins European security) allies. But it is a different idea to what I want to talk about.

Effect Over Platform: The Distinction That Matters

The Liminal Army is not about platform description. It is about operational effect.

The question at its heart is not ‘is the vehicle crewed?’ It is ‘which domain is the effect landing in, and does it match the domain the operating service traditionally owns?’ When an Army unit operates a USV in the near-littoral to generate a maritime targeting effect, something interesting is happening that ‘hybrid’ does not quite capture. The boundary between land force and maritime force has become permeable. The Army is operating liminally, at and across the threshold of its traditional domain. The platform is just the mechanism. The effect is the point.

‘Liminal’ is borrowed from anthropology and architecture, where it describes the threshold state, the in-between space, neither one thing nor another. An Army unit generating maritime effects from a land base is neither a traditional land force nor a traditional naval force. It is operating in the space between. That, in short, is the concept.

Ukraine Showed Us This Before Anyone Named It

The most compelling evidence for the Liminal Army is not theoretical. It has already happened, and it happened at scale.

In April 2022, Ukrainian forces sank the Russian Black Sea Fleet flagship Moskva using Neptune anti-ship cruise missiles launched from land positions. No carrier, no fleet, no admiral. A land force generating decisive maritime effect using weapons fired from the coast. Subsequently, Ukrainian operators deployed USVs in the Black Sea to extend that reach further, conducting persistent maritime strike and harassment operations against Russian naval assets from land-adjacent positions. The cumulative effect was the effective withdrawal of the Russian Black Sea Fleet from its western positions.

That is the Liminal Army operating in practice, before anyone gave it a name. The institutional and doctrinal structures that would normally govern such operations did not exist. The technology moved faster than the framework. What emerged was improvised, effective, and historically significant.

What the Liminal Army Looks Like in Practice

Closer to home, the concept finds expression in the near-littoral space, the coastal and near-shore environment where Army operations and maritime operations have historically met awkwardly.

The Royal Marines have been the traditional bridge here. Their transformation programme, ongoing since 2021, has explicitly embraced USVs and autonomous systems as part of a future littoral strike capability. But the Royal Marines are a relatively small force, and the near-littoral is a large and operationally important space. The argument for the Liminal Army is not that it replaces the Royal Marines. It is that the Army, equipped with the right uncrewed systems and operating under the right doctrine, can generate complementary effects in that same space.

A British Army unit equipped with USVs could conduct ISR (‘Intelligence, Surveillance and Reconnaissance’, meaning finding, observing, and tracking targets) in near-shore environments. It could provide maritime targeting data feeding into a joint fires network. It could support logistics operations in contested littoral environments. UUVs extend this further, into sub-surface ISR and potentially mine countermeasures in the near-littoral zone.

The same logic, pushed a little further, points at other things. Ground-launched anti-ship missiles give an Army unit maritime strike reach without any vessel at all. Loitering munitions, slow-flying autonomous strike systems that orbit a target area before engaging, can be tasked against naval targets from a land position. Long-range precision fires can reach into the maritime domain from a coastal deployment. The Liminal Army is not only a near-shore concept. It is a principle that extends wherever an Army unit can generate effects that were previously the exclusive preserve of another service.

The Structures Have Not Caught Up

The technology that makes the Liminal Army possible is developing faster than the institutions around it, and the gap is worth being specific about.

Training is the most immediate problem. The British Army trains soldiers to operate in the land domain. The Royal Navy trains sailors to operate in the maritime domain. The idea that an Army unit might routinely generate maritime effects using uncrewed systems does not sit cleanly in either pipeline. The skills required, operating USVs in a contested near-littoral environment, fusing maritime ISR into a joint fires picture, understanding sub-surface sensor behaviour, belong to a space between the two. That space does not yet have a training home.

Procurement amplifies the problem. The incentive structure in UK defence acquisition tends to produce service-specific capability rather than cross-domain effect. The SDR’s push from ‘joint to integrated’ is the right direction, but the institutional machinery that turns strategy into contracts is still largely organised around single-service requirements. Nobody is currently writing a requirement for an Army unit that generates maritime strike effects. The Liminal Army does not have a budget line because it does not yet have a name.

The command and control question is perhaps the sharpest edge. If an Army unit is generating maritime effects in the near-littoral, under whose doctrine does it operate? Who commands it, and through which chain? These are not procedural details. In a contested environment, the answer determines whether the capability can actually be used or whether it sits idle at the seam between two command structures that were not designed to share it.

Ukraine improvised its way through all of this because it had no choice. The British Army would need to design its way through it deliberately. That is harder, slower, and more political. But the alternative is watching the technology mature in a vacuum while the institution waits for someone else to write the doctrine first.

This Needs a Conversation, Not Just a Concept

I am offering Liminal Army as a working term for something I can see forming, and Liminal Force as the broader principle underneath it. The technology exists. The operational logic has been demonstrated, brutally and effectively, in the Black Sea. The near-littoral opportunity for the British Army is real and it is sitting in the gap between what the Royal Marines can cover and what uncrewed systems now make possible.

What does not yet exist is the institutional will to design around it. The training, the procurement logic, the command arrangements, the doctrine. These take time and they take someone being willing to name the problem clearly enough that it becomes impossible to keep deferring.

That is what I am trying to do here. Whether ‘Liminal Army’ is the right name is genuinely secondary. What matters is that the thing the name is pointing at gets taken seriously. If you are working in this space and you think the frame is wrong, or incomplete, or that it already has a name I have not found yet, I want to hear it. That conversation is more useful than any term I can coin in a blog post.