Agile Combat Employment

As NATO peer and near-peer adversaries have continued to advance their intelligence-gathering, targeting, and long-range strike capabilities. Consequently, airbases, which have typically been geographically separated from conflict areas and represented relative safe havens, are increasingly vulnerable. As such, NATO Allies and partner nations are streamlining to become more ‘agile’, to counter a range of threats and complicate adversary planning. Agile Combat Employment (ACE) represents a direct outcome of this, comprising a doctrinal evolution for NATO’s air component. 

Agile Combat Employment (ACE) is the latest air force doctrine employed by members of the North Atlantic Treaty Organisation (NATO), representing a paradigm shift for air arms across the alliance. Defined in United States Air Force Doctrine Note 1-21 (DN 1-21), ACE is “a proactive and reactive operational scheme of manoeuvre executed within threat timelines to increase survivability while generating combat power”. Built on the principles of flexibility and adaptability, ACE seeks to enhance the resiliency, survivability and responsiveness of NATO’s air component, and to enable the alliance to rapidly generate its full range of combat air power at any time and from any place across a theatre.

At its core is the operational practice of moving away from the model of few large, centralised airbases, and instead positioning a larger number of smaller and more dispersed packages of aerial assets throughout a theatre. This serves to complicate adversary targeting, and limits the potential damage which could be caused by any one adversary attack succeeding, thus improving the survivability of the air component as a whole.

ACE acknowledges NATO’s depleting quantity of permanent bases and, responds to commanders’ expectations that these will be targeted in the early phases of a confrontation. In being “proactive”, ACE serves to communicate with actors, providing support and reassurance to allied nations, and demonstrating intent to adversaries by improving force posture, to deter aggression and more effectively respond to potential attacks. Simultaneously, ACE is “reactive” in posturing forces in response to anticipated or executed adversary manoeuvres, meaning a trigger initiates dispersal away from enduring locations.

To a well-trained eye, nothing about ACE appears profoundly new. Indeed, dispersal and reinforcement away from main operating bases was a fundamental component of NATO’s Cold War strategy. For example, beyond ten main operating bases, the Royal Air Force (RAF) had close to thirty dispersal airfields to which the ‘V-Force’ of strategic nuclear bombers could be dispersed. So, what is special about ACE and why is it new?

ACE today arises from a security context wherein Western nations are emerging from two decades dominated by low-intensity operations focussed on counterterrorism and counterinsurgency. In this time, significant force and basing reductions have taken place, and the threat focus has shifted to peer and near-peer opponents. ACE represents NATO’s response to the contemporary military-political environment, re-learning and evolving to reinstate previously-neglected competencies which remain essential to fighting peer conflicts, in a political space dominated by domestic pressures, budgetary and logistical constraints that prohibit large-scale restructuring.

In the UK today, the RAF’s fixed wing assets occupy just five main operating bases. Meanwhile, the USAF has seen a 65% reduction in the number of its air bases overseas since the end of the Second World War, falling from 93 to 33. Similarly, the International Institute for Strategic Studies (IISS) reports that air combat fleets have reduced significantly across NATO since the cold war, with the UK alone decreasing from approximately 500 aircraft in 1973 to just 169 in 2023; France, Germany and Italy have also seen similar reductions. Air Marshall Harv Smyth, the senior officer in charge of ACE delivery for the RAF, described the current arrangement as having “eggs in single baskets”.

With Alliance resources stretched thin, ACE seeks to maximise friendly forces’ advantage against adversaries by being better at doing more with less. In essence, ACE aims to make forces “ready to execute missions quickly in unpredictable ways – or rapidly respond to adversary moves” according to a spokesperson from the 100th Air Refuelling Wing (100th ARW) at RAF Mildenhall, UK. The spokesperson told ESD that in Europe, ACE addresses the “tyranny of proximity” – referring to the short distance and timelines of potential threats originating from NATO’s Eastern Flank – adding that “the ACE playbook directs approaches and capabilities which must enable dispersed forces to adapt and prevail despite uncertainty, using the best information available to local commanders.”

In sum, through its dispersed operations model, ACE aims to achieve a number of goals: improving survivability, boosting responsiveness, sustaining operations to the required level in a high-intensity conflict, and enabling the continuity of those operations.

Mechanics

Generating and sustaining lethal combat air power remains a complex task, particularly when further complicated by dynamic basing that sees assets spread across a theatre. Beyond basing, ACE utilises ‘Multi-Capable Airmen’ (MCA) – personnel who are trained to and able to execute tasks beyond their primary trained specialism or trade – and creating cross-functional teams. This helps streamline the service, ensuring a majority of personnel are able to directly support expeditionary combat operations and that the entire force is functional.

The US’ ACE doctrine employs five core elements to balance the cumbersome nature of large-scale force mobilisation, which draws heavily on command and control (C2), logistics and supply chain infrastructure. According to the USAF’s Joint DN 1-21, ACE is built on a framework of five core elements:

DN 1-21’s ‘Five Core Elements’ summarised

• Posture: refers to the “starting position” of all forces in the ACE manoeuvre. This involves the pre-positioning of equipment and assets, optimisation for resupply, pre-determined forward enduring and contingency operational locations across a theatre, as well as resilient communications.
• C2: This encompasses the successful employment of Joint All Domain Command and Control (JADC2) to ensure the ‘unification of efforts’ across players in the battlespace. It seeks to enable fast, large-scale, rapidly-digestible C2 across a ramified theatre. ACE JADC2 must be mobile, survivable, secure and sustainable with redundancy and resiliency built in. DN 1-21 asserts that commanders should expect force elements to lose connectivity with commanders at some point during ACE, necessitating flexible plans and exhaustive training so that components are able to operate “limited direction” from above.
• Movement and manoeuvre: Assets begin at home stations or permanent deployed locations, known as Main Operating Bases (MOB) and Forward Operating Sites (FOS) respectively. These sites have high levels of existing, dedicated and enduring infrastructure and are referred to in ACE doctrine as “enduring locations”. Examples of a MOB might be RAF Coningsby or RAF Lossiemouth, the RAF’s permanent Typhoon stations in the UK. Meanwhile a FOS could be RAF Akrotiri, an enduring location with no permanently assigned assets in Cyprus that houses a longstanding rotational Typhoon deployment.Complementing the ‘enduring locations’ are “Contingency Locations” (CL), which serve as non-permanent sites across a theatre or nearer to the front-line, to which assets disperse upon a trigger during ACE’s reactive phase. CLs have varying degrees of permanency, which include:

• “Initial”, this refers to an austere site that has minimal external support beyond what is positioned there by the force conducting ACE itself, such as a disused airstrip or municipal airport.
• “Temporary”, these are sites that can draw on a small amount of external support on a short-term basis.
• “Semi-Permanent”, where consistent support is available to a level appropriate for sustaining operations for an extended duration.

MOBs and FOSs are associated with at least one CL to ensure streamlined logistics to and from each location, reducing information flow and creating C2 efficiency. CLs that are in close proximity are grouped in “base clusters”, which are assigned a single enduring location from which they receive C2, maximising C2 redundancy. These locations might look different for each nation conducting ACE, or the wider context in which ACE is being used. For example, the UK might execute the dispersal of Typhoons to civilian airports as acceptable CLs to preserve its Quick Reaction Alert capabilities domestically. Meanwhile, this might be considered an inappropriate approach for US strategic nuclear bomber dispersal overseas. These sites are pre-identified and agreed with host-nations. This aspect is the backbone of the concept and best described by Air Marshall Smyth who suggested that the “utopia” is assets “always being on the move”.

• Protection: Unlike the expeditionary operations of the past decades, where full operational packages are moved to remote locations on the periphery of battlespaces, ACE dictates that no bases should be considered safe havens for assets. ACE sees to it that all sites should be considered warfighting locations, necessitating both active and passive force protection including defensive counter air. “Base clusters” are therefore intended to share protection resources. Protection necessitates a whole-force, inter-service approach, to enable all sites to be protected against threats arising across all domains, while being able to continually generate effect. It also necessitates the effective application of intelligence units as well as the immediate integration of pre-existing and host-nation forces at dispersed locations to maximise responsivity and mitigate risk.
• Sustainment: As ACE is so dynamic, it stretches air forces’ logistics infrastructures, particularly because dispersed sites can span vast geographical scopes. Sustainment includes aircraft and airfield repair, prioritising aircraft sortie generation as ACE’s primary sustainment task. Alongside this, it is desired that existing “pull” material supply chains, which distribute resources as required, are replaced by “push” logistics systems. In this model, logistical depth is achieved through analysis of the operational environment, determining the necessary resources for sustainment. War reserve materials and operational equipment are then forward positioned before they are required to avoid shortfalls and distribution across multiple operating locations.

ACE so far

ACE’s origin story is convincing, and its mechanics look promising; but to what extent has ACE manifested itself so far and is it making an impact? Is this just an attempt to reinvent the Cold War wheel with a brand-new glossary? ACE first cut its teeth in Europe meaningfully in 2021, with a series of exercises held by the USAF across the continent. Notably, the 48th Fighter Wing simulated a FOS on a remote part of their home base at RAF Lakenheath in January 2021, marking US Air Forces in Europe’s (USAFE’s) first major trial of the concept. The exercise incorporated the refuelling of two F-15E Strike Eagles at the Leuchars Diversion Airfield in Scotland.

Since then, it has steadily matured and scaled up, with larger-scale demonstrations taking place across the content, both in dedicated environments and as part of broader exercise programmes, such as Steadfast Defender and Arctic Challenge. As part of this process, the RAF and USAF signed a ‘Combined Vision Statement’ for ACE on 12 September 2023. This involves aspects including the adoption of common terminology, concepts, tactics, procedures and materials to ensure interoperability. The process reinforces several factors, including the US’ commitment to European security, the US-UK special relationship and cements these nations as the forerunners of the doctrine’s development.

Exercises have now seen forward deployments to CLs across Europe, both proving the concept and reinforcing NATO’s readiness response to the ongoing war in Ukraine. In June 2023, Exercises Tempest Strike and Tower Guardian saw RAF Typhoons from No.1 (Fighter) Squadron, which were already deployed to Ämari Airbase, Estonia disperse into Norway, proving their ability to reach deep within the Arctic Circle and High North – one of Europe’s most highly-contested regions. These aircraft underwent refuelling at a forward arming and refuelling point (FARP) using rapidly-deployed infrastructure at Ørland Main Air Station, before proceeding to a CL at Bardufoss Air Station, where they carried out air-to-ground combat sorties. Security in the high North is increasingly important to European security, as the region is both resource rich and increasingly contested.

Following Finland’s recent accession into NATO, the Alliance’s collective security umbrella now has to account for Europe’s largest single land border with Russia. Therefore, not only the ability to rapidly deploy, but also the capacity to effectively operate in the adverse conditions presented by the High North are essential to the development of ACE. Though a small-scale iteration, scenarios such as the above help to visualise the real-world impact the concept might have. Additionally, although NATO might not know the thoughts of its adversaries, successful execution of ACE effectively communicates intent and ability, reinforcing ACE’s deterrent nature. Moreover, the aircraft involved in the aforementioned exercise were already engaged with NATO Air Policing tasks in Estonia – thus, the exercise demonstrated successful employment of already-active assets, as opposed to generating additional assets from home bases, illustrating ACE’s flexibility and responsiveness.

More recently, alongside testing of coalition Integrated Air and Missile Defences, Exercise Astral Knight 2024 involved the extensive application ACE profiles of the UK and the US, alongside four other NATO Member States, Estonia, Latvia, Lithuania Poland. Within this, a contingent of RAF Typhoons once again relocated from an Air Policing tasking, this time from Romania to Poland. Using an A400M Atlas to transport fifty personnel and the necessary equipment, Polish personnel then integrated with the RAF upon their arrival to generate defensive counter-air sorties alongside their F-16s. Meanwhile, the USAF’s 555th Fighter Squadron relocated a number of F-16s from Aviano AB, Italy to Šiauliai AB, Lithuania, while the 100th ARW dispersed KC-135R Stratotankers across the continent to support various operations.

These exercises demonstrate the diverse range of what ACE involves beyond combat aircraft and how ACE is a whole-force effort. It is not simply about the movement of combat aircraft from point to point, but also requires ensuring the survivability and function of critical force enablers such as air mobility and air-to-air refuelling, without which ACE’s potential is inhibited significantly. Astral Knight was a clear demonstration of the US-UK cooperation on ACE’s development, while representing progress in ensuring the whole Alliance is becoming competent in the concept.

Recently, the 100th ARW at RAF Mildenhall conducted an innovative tabletop ACE wargame, ‘Exercise Kingfish ACE’. A 100th ARW spokesperson told ESD that such exercises help “introduce ACE concepts to airmen in a structured way”. Explaining the scenario, the spokesperson stated that “teams are challenged with accomplishing tasks which drive them to implement creative solutions to logistical challenges”. The spokesperson added that this approach aims to “allow [the USAF] to leverage [its] young Airmen’s creativity in a low/no cost environment to find solutions that could translate to real-world applications. If the idea shows promise, it can then be introduced to subject matter experts to refine the idea and help determine its feasibility in the real world.”

Challenges

Though progressing quickly, ACE has not yet reached its fully-evolved form, with fundamental challenges remaining. A holistic one-size-fits-all approach is clearly not appropriate for ACE, nor is it desirable given the vast operational scope which could range from Special Operations Forces to Air Combat. However, across all dimensions, effective adaptive basing remains an underlying issue for ACE practitioners, owing to the diverse nature of aerial assets.

A good example is the RC-135W Rivet Joint strategic signals intelligence (SIGINT) aircraft. American and British examples can be found operating across Europe’s Easternmost reaches almost daily, monitoring adversary activities from international airspace in the Baltic, Barents and Black Seas. These platforms provide real-time intelligence to combatant commanders, launching and recovering from their home stations at RAF Mildenhall and Waddington the majority of the time. Being both high-priority and extremely vulnerable, their sensitivity and role means that they necessitate particular security requirements, airfield requirements, and specific infrastructure that might call for different basing procedures. Not only this, but intelligence, surveillance and reconnaissance (ISR) assets are critical force enablers in both a pre-conflict environment and during conflicts, meaning ensuring their persistent availability for to tasks is imperative. These assets epitomise the purpose of ACE and thus, through the US-UK Rivet Joint cooperation program, ‘hot-refuelling’ for the RC-135 has been trialled, where an aircraft refuels upon landing, but without shutting down its engines. Similar trials have also taken place with KC-135 Stratotankers, which fulfil the crucial aerial refuelling role. For the KC-135, hot-refuelling meant that ground time was reduced from 6 hours to 1 hour, demonstrating the increased output and flexibility the procedure enables.

Of course, this is familiar territory for tactical assets, such as fighters and helicopters, but it is certainly a non-standard tactic for SIGINT and tanker aircraft. However, by employing unconventional approaches in this way, in theory this means that some of the challenges presented by adaptive basing and combat scenarios can be mitigated. For instance, some prerequisites necessary for dispersal can be mitigated, as assets can remain mobile without spending too long in one place. Meanwhile, expanding the assets capable of hot refuelling drastically reduces the burden on the tanker supply chain, which is already extremely stretched. Flight Lieutenant Dan Wilkes also noted that, for the RC-135 in particular, hot refuelling aids the “quick reaction capability” and enables the RAF to “push the aircraft towards its peak performance”. Similarly, the 100th ARW has also begun trialling low-level flight operations in another unconventional approach toward increasing potential.

A spokesperson from RAF Mildenhall told ESD that these sorties help “develop airmanship and advanced mission planning”, and while ACE was not mentioned explicitly, it was asserted that “these skills will lend themselves to any ACE-type operation” as the concept seeks to leverage novel approaches to expand the miliary options available to combatant commanders. Nevertheless, for the 100th ARW, ESD was told that large-scale restructuring to enable ACE has been unnecessary as their mission “has involved deploying small teams forward to meet refuelling requirements for decades.” However, MCA training has been implemented within the Wing, as with many others, to ensure that their same tasks can be completed with a much smaller operational footprint.

A further challenge is constructing a mutual understanding between allies and organisations. Air Marshall Smyth recently alluded to difficulties surrounding the use of civilian airports to conduct military operations in the UK. In an environment where suitable facilities are not plentiful, it is integral to build these relationships in the next phases of ACE’s development so as to ensure that sufficient options are available both with which to rehearse ACE but also fulfil it should the situation arise. One potential solution for the UK could be the use of the many former military sites around the country that still hold some basic airfield infrastructure, such as at the former RAF Sculthorpe and RAF Dishforth, where military training still regularly takes place. This is something which the USAF have done by reactivating its airfield on the island of Saipan.

The above exposes how ACE might manifest differently for different nations or in different contexts. For example, Air Marshall Smyth highlighted that the UK lacks an advanced integrated missile defence system. Therefore, at home, British ACE might prioritise the continuous movement of assets to new locations to ensure their survivability. Nevertheless, other nations may face different challenges, such as a lack of refuelling tanker capabilities. This might demand either external support or increased hot-refuelling competency. This demonstrates how a one-size-fits-all approach to ACE is unviable, while highlighting that a mutual understanding not only of the concept, but each ally’s requirements must be achieved, as mitigation can be ensured through cooperation to enable the successful execution of ACE in the years to come.

Senior officials, including Air Marshall Smyth have asserted that ACE remains a developing concept. There are cases in which practitioners do not possess all the necessary resources to fulfil ACE from skills to specialist equipment. The current period is therefore certainly one of identifying gaps and filling them where resources allow, or working around them to mitigate risk.

Projections

A key component of the doctrine’s development is found in the intent to include an ‘ACE element’ in the majority of large-scale exercises that take place. In 2023, Air Marshall Smyth explained this in terms of a no-notice, rapid relocation to a specific CL at some point during the wider exercise, while continuing to fulfil the exercise’s training objectives as they would from the exercise’s MOB. An example of this was demonstrated during the trilateral exercise Atlantic Trident (30 October to 10 November 2023), involving French, UK, and US forces.

During the exercise, Armée de l’Air et de l’Espace Rafales assumed RAF Waddington as their MOB, while RAF Typhoons and USAF F-35As operated from their home stations at RAF Coningsby and Lakenheath. The RAF announced that these aircraft were airborne conducting a routine exercise sortie when they were notified that their MOBs had been incapacitated, necessitating their dispersal to RAF Leeming, North Yorkshire. Despite being an active RAF airfield, Leeming lacks specialist infrastructure for these aircraft types – the French contingent’s commander, Lieutenant Colonel Veuille remarked how the exercise participants “hadn’t planned” where they would be going, how to get there or what the logistical support would be. Also participating were RAF F-35Bs operating from the HMS Queen Elizabeth alongside the HMS Diamond Type 45 destroyer. During the dispersal phase, the RAF stated that secure communications were established between the vessels and in-land C2; this demonstrated cross-domain integration and the centrality of collaborative C2 in enabling dynamic ACE operations. The F-35Bs then arrived at Leeming and executed refuelling, increasing their endurance by four hours.

Going further, in 2023, Air Marshall Smyth alluded to ongoing progress towards the execution of a first ‘ACE Evaluation’ exercise in 2024. Describing this as similar to Cold War NATO ‘Tactical Evaluations’ (TACEVALs), which tested force’s ability to conduct no-notice responses. TACEVALs served to “assess against prescribed criteria the operational potential of NATO Command and assigned units” and to use a standardised grading criteria to “indicate deficiencies and make recommendations where necessary”. Therefore, a similar ACE EVAL would likely see a whole-force effort to test the principles and competencies around ACE, massing forces to fulfil all of the concept’s objectives and tasks. Details of when this might take place, what this would look like and who might participate are so far unavailable – however, the four main grading criteria (Alert Posture and Reaction; Mission Effectiveness; Support Functions; Ability to Survive) for a TACEVAL certainly appear relevant to what an ACE EVAL-type exercise might look like, aligning neatly with ACE’s core principles.

Smyth asserted that at this stage, such an exercise would not be to prove the concept, but to develop it. According to Smyth, as it stands, developing and implementing ace is “as much by failing” and learning how to mitigate the risk and maximise operational potential.

Luca Chadwick is an international relations graduate and Terrorism & Insurgency Master’s student at the University of Leeds. Luca’s primary interests centre around air power studies and coercion.