Reserves Without Purpose: The Hidden Weakness in Modern Militaries

One of the key lessons from the Russian invasion of Ukraine is that military mass has an inherent property that cannot be replaced with other capabilities.[1] The obvious problem is that mass is expensive to maintain. In war, military mass has great utility; in peacetime that same mass has limited utility at best yet must be maintained in case of, or to deter, war. This gets to the heart of the idea of strategy as ‘the use of military force to achieve political outcomes at a tolerable cost’.[2] How can military force be maximised, while minimising the costs in peace as well as war? At its simplest, the answer is reserve forces.

Reserve forces trade time for mass. A professional army is deployable in a relatively short timeframe, but the cost of maintaining a commensurate standard of training and readiness is high. A reserve army, comprised of part-time volunteers, maintains a lesser standard of training and readiness at greatly reduced cost, but in return can only be deployed within a moderate timeframe. Building any reserve force requires a series of decisions from policymakers and military planners. What ratio of reserves to regulars should comprise the whole force? What level of training should reserve forces maintain? How can you recruit and motivate them? However, all decisions are subordinate to one key decision: what function should that reserve force fulfil in your national security strategy?

This article will use the example of the British Army to examine the three possible strategic functions of any reserve force – augmenting the professional army, reinforcing the professional army, or defending the homeland – before arguing that without a defined strategic function a reserve cannot be resourced, trained or employed coherently. While this article focusses on Britain, the same strategic dilemma confronts all advanced militaries when balancing readiness, cost and mass.

Augmenting the Professional Army

The first principal use of a reserve force is augmentation, in which reservists are attached to an existing regular force either in small detachments or individually. This can be to backfill the professional army and bring it up to strength, or to insert civilian expertise into technical areas of the regular force. In either case, individual augmentees facilitate a transfer of knowledge between the professional army, the reserve army and industry. By completing a rotation with the professional army, volunteer officers and soldiers gain valuable operational experience that they can bring back into the reserve force. The professional army, in turn, uses specialist augmentees to bring cutting-edge industry knowledge into defence operations. For example, where the regular army cannot realistically expect to operate at the forefront of many medical, engineering or information security disciplines, it can rely on civilian experts to bring in specialist knowledge at a fraction of the consulting price. Examples include formed detachments within the Army Medical Service or Royal Engineers, or individual expertise through groups such as the Specialist Group Information Services. Additionally, ex-regular soldiers retain a call-up liability and can be mobilised in a national emergency. Outside land forces, augmentees may be a preferable model for reinforcing air and naval forces, where individuals operate platforms rather than generate mass. Where it takes longer to build the platform than it does to train the manpower, individual augmentation may be preferable.

In capability terms, augmentation enables the first echelon. It reduces the cost of maintaining an expensive first echelon force, allowing it to be brought up to strength quickly with additional combat power and specialist knowledge from industry. This first echelon provides an initial deterrent capability, denying an adversary the ability to win quickly or cheaply, but lacks strategic depth. Without a second echelon to support it, an army cannot sustain prolonged operations.

Figure 1

In practice, post-1945 mobilisations of British reserve forces have been to augment the professional army in times of war. Even in Korea, over 7,000 reservists were mobilised not as formed bodies of men, but as individual augmentees.[3] Then, as now, augmentees have been a tried-and-tested way of backfilling the professional army to bring it up to strength.

Reinforcing the Professional Army

The second possible role for any reserve force is to reinforce the professional army – not individually, but as cohesive, deployable formations. Where the professional army provides the first echelon of fighting force, it cannot do so indefinitely. In effect, the role of the professional army is to buy time; the professional army must survive long enough to allow the reserve force to mobilise, train and deploy as a second echelon, relieving those troops already on the front line. The reserve army, therefore, enables sustained land warfare.[4]

In capability terms, reinforcement expands the force by creating a second echelon at relatively low cost. This second echelon is created either by mobilising the existing reserve force, or by using the existing force structures to rapidly integrate and train civilians into a significantly expanded reserve. The presence of a second echelon creates a powerful deterrent effect, indicating a capacity for sustained resistance if the first echelon fails to deter or defeat an initial attack. The cost is less an economic issue, but rather the time it takes to mobilise.

In WWI, the second echelon was created through two mechanisms. The Territorial Force was formed in 1908 to create an army of 300,000 volunteers for homeland defence in the case of German invasion, while the bulk of the regular army was deployed overseas as the British Expeditionary Force. When war was declared it was immediately clear that the Territorial Force was insufficient, leading Kitchener to recruit heavily not for the Territorials, but to create a ‘New Army’ of 1,000,000 volunteers for active overseas service. Of course, the creation of the New Army was not without challenge. While there was no shortage of volunteers, there were huge shortages of equipment, clothing, accommodation, and, crucially, officers and NCOs to command and train the men. The lack of officers and NCOs proved so critical that they were plundered from the Territorial Force and Indian Army officers at home on leave.

The challenges identified in creating the New Army were central to mobilisation strategy in WW2. In 1939, rather than creating another ‘New Army’, the Territorial Army was doubled in size, with each regiment and sub-unit splitting in half. This allowed a massive, controlled expansion of the reserve force, with enough officers and NCOs to facilitate command and training of volunteers. The lesson from British mobilisation in the 20th century is that the reserve army is not itself intended to fight. Instead, the reserve army maintains the infrastructure that allows faster mobilisation of the general population. It is this rapidly expanded reserve that must then deploy in support of the professional army.

While the Territorial Army was designed around this principle, British reserve formations have not been mobilised beyond individual sub-units for any conflict since WWII, whether for short-notice wars such as the Falklands War, or low-intensity, long duration campaigns such as the Troubles or Afghanistan. Indeed, the sole exceptions are a small number of Territorial Army sub-unit deployments to Iraq in the early years of the campaign.[5] This stands in contrast with many peer reserve armies, which are principally deployed on operations as formed bodies.[6]

Homeland Security and Defence

The final possible function for any reserve force is that of homeland defence or security. Fear of invasion was the driving force behind the creation of militias, the yeomanry, the Territorial Force and the Home Force, and remains the prime motivation for Total Defence economies such as Israel and the Scandinavian states. Britain, on the other hand, has not faced a credible threat of invasion since 1940. Instead, current UK doctrine recognises a potential role for the Reserves in homeland security and resilience. Operation Rescript is often cited as an example of the reserve contribution to resilience, where 2,300 reservists mobilised in response to COVID-19. In practice, however, resilience taskings are the preserve of the professional army. Outside ceremonial events, most scenarios envisaged under the Military Assistance to Civil Authorities framework are crisis response, resourced through taskings to the high readiness ‘standby battalions’ from the Regular Army.

As Britain faces no credible or threatened prospect of a conventional invasion today, the requirement for a homeland defence force might seem remote. However, it is clear that our adversaries have the capability and intent to conduct hostile operations on the UK mainland. Such a statement might be considered to be a continuation of the discussion around concepts such as ‘Unrestricted Warfare’ or the ‘Gerasimov Doctrine’, but even this year Operation Spider’s Web and Operation Rising Lion demonstrated the ability of Ukraine and Israel to use smuggled drones to strike critical military and national infrastructure from within an adversary’s territory. If our allies fight in this manner, we can be sure that our adversaries will. More practically, the point is that the next war will be a novel experience for Britain – it will not be purely expeditionary, and the ‘front line’ will extend onto the UK mainland. The potential manpower requirements for homeland security should not be underestimated – Palestine Action has recently demonstrated the challenge of defending defence infrastructure even in peace, but in a war scenario there may be significant demand for military forces to reinforce the police and other civil authorities. Establishing a credible homeland defence force would allow the reallocation of regular forces to expeditionary or deterrent tasks.

Lessons for Other Militaries

These strategic functions can be observed in peer militaries across the world. In the United States, the Army Total Force Policy articulates how the Army fights as a single unit comprised of the US Army, the Army Reserve and the National Guard, with the Active Component being nearly double the size of the Reserve Component. The Army Reserve has few combat capabilities but augments the Active Component with specialist capabilities on demand. The National Guard, unusually, has two functions. It is capable of deploying operational units overseas to reinforce the Active Component. The Army National Guard deployed 11 Brigade Combat Team deployments over the course of the Iraq campaign, for example. More commonly, the National Guard provides homeland defence and security, typically providing disaster relief and emergency response in support of civilian authorities. These two roles are enabled by a system of dual control: reinforcement of the professional army is always under federal control, whereas the defence function is broadly under state control.

As with Britain, US forces are designed to fight expeditionary warfare, with little threat of invasion. For those forces designed for expeditionary warfare, the delineation between augmentation, reinforcement and homeland defence works well. The Netherlands, which has operated a broadly expeditionary force structure, maintains two distinct groups of reservists. The National Reserve Corps is the territorial component, responsible for homeland security and defence. Within the regular army are ‘Reservists with Specific Expertise’, responsible for augmenting the force with specialist capabilities. However, there is no organisation capable of reinforcing the professional army, and the National Defence Corps is realistically too small to mobilise the population.[7] With no second echelon capability, the Netherlands is incapable of sustaining land warfare in the event of Russian aggression.[8] The same criticism might be levelled at France, which maintains an ability to augment the professional army through the Réserve Opérationnelle, but lacks a reserve organisation capable of reinforcing the professional army.

For those countries that face a realistic prospect of invasion, the model changes slightly to one in which the primary strategic function of the reserve force is defence of the homeland. Countries such as Norway, Finland, and Israel maintain reserve forces more than double that of their professional armies. This is sustained by conscription, which equips a large portion of the population with the base knowledge required for mobilisation. Their reserve armies provide scalable combat power without the requirement to maintain large standing formations, allowing the population to focus on productive economic activity.

Evolution of UK Reserve Forces

For the 230 years preceding 2011, British reserve organisations had clearly defined and well-understood functions. The yeomanry and militias that sprung up in the 18th Century were intended for homeland defence in the event of a French invasion. The Territorial Force was raised in 1908 in response to fears of a German invasion. Tasked specifically with homeland defence, the Territorials could, in general, not be sent to France, so in 1914 Kitchener raised the New Army to reinforce the British Expeditionary Force. By 1940, Britain once again faced a threat of invasion, and the Home Guard was formed for homeland defence, allowing the Regular Army to concentrate on preparing for and conducting offensive operations abroad. The Home Guard was disbanded at the end of WWII, and only briefly reconstituted in the 1950s at the height of the Cold War.

Augmentation has been a cornerstone of reserve contributions to fighting power almost as long as home defence. Over the course of the Boer War, 23,000 men of the Militia Reserve were attached to Regular battalions.[9] The Militia Reserve was replaced by the Special Reserve (1908-1924), the Supplementary Reserve (1924-1951) and the Army Emergency Reserve (1951-1967), with the latter organisations dedicated to augmenting the Regular Army with specialist troops.[10]

WWI showed the limitations of the Territorial Force in sustaining a continental land war, necessitating the New Army to be raised from scratch. In 1921 the Territorial Force was disbanded and reconstituted as the Territorial Army, intended to reinforce the Regular Army overseas as an expeditionary second echelon, a role which it held until 1967.

In 1967 all existing reserve organisations were amalgamated into the Territorial and Army Volunteer Reserve (1967-2014). The three principal categories of this new Territorial Army being Class I (the ‘Ever-Readies’) to provide technical augmentees, Class II (the Volunteers) to reinforce the Regular Army and Class III (Home Defence) to provide home defence. In 2001, the Territorial Army experimented with maintaining an explicit resilience force called the Civil Contingency Reaction Force, but this was stood down in 2009. Despite Britain fighting in a number of wars, the Territorial Army was not called up in any meaningful sense until Iraq 2003. Wars were either fought too quickly to mobilise reservists (such as the Falklands), or where political and legal constraints prohibited mobilisation (as in Northern Ireland). Where reserves did deploy in large numbers, such as the Gulf War, it was as technical augmentees.

In 2011, in response to criticism that the Territorial Army had atrophied since the end of the Cold War, the Territorial Army was replaced with the Army Reserve. In doing so the Army Reserve lost any capacity to reinforce the Regular Army, solely focussing on providing augmentees for Afghanistan and, more recently, missions such as NATO’s Enhanced Forward Presence in the Baltic states. The latest recommendations, issued in 2021, propose three new groupings within the Army Reserve: ‘Reinforcement Reserves’, ‘Operational Reserves’ and ‘Strategic Reserves’.[11] The problems are manifold. Firstly, there are no clear definitions of Reinforcement or Operational reserves, while the Strategic Reserve simply restates the existing call-up liability that ex-Regulars already hold. Each is a slightly different mechanism of augmenting the Regular Army. Secondly, homeland defence and security are envisaged as a tasking for all reservists and regulars, rather than being a specialist capability in its own right. Any deployment of the Army in an expeditionary setting will be to the detriment of its ability to resource resilience operations at home, and vice versa. Finally, there is no stated aspiration to field a reinforcement capability, without which the UK will be incapable of sustaining land operations. These are serious flaws in an environment where fear of a major land war has become mainstream opinion.[12]

Political Constraints

Several political realities shape the UK’s reserve structure, including alliance expectations, recruitment and retention, and integration with civilian agencies for resilience.

A common argument against investing reserve forces is that land conflict will be sustained through allied forces, principally through a NATO First approach.[13] This strategy outsources force sustainment to allied countries, reducing costs and avoiding domestic backlash against mobilisation. But NATO demonstrates how national under-investment risks becoming collective failure. As an example, the UK has let its ground-based air defence capabilities atrophy, arguing that layered air defence will be provided by allied militaries while ignoring the fact that air defence has atrophied across the whole alliance.[14] In the same manner, when NATO members collectively struggle to generate military mass, the UK cannot rely on allies to backfill mobilisation depth. Additionally, dependence on allies impedes the British government’s stated aim of launching independent operations in support of sovereign interests.[15]

As with other Western economies struggling to invest in their militaries, several economic and demographic factors affect regular and reserve recruitment alike, including aging populations, changing employment habits and a widening rural-urban divide. However, British Reserve recruitment is directly challenged by the lack of strategic coherence across the force. Historically, reinforcement has been politically unpalatable in the UK, even when campaigns such as Northern Ireland and Afghanistan required surge forces – but why would reservists join if not to be used? Augmentation is the preferred reserve model for the British Army, but the number of reservists who can take career breaks to voluntarily mobilise is limited. Finally, potential recruits motivated by the opportunity to support their communities through domestic resilience taskings may well object to the accompanying call-out liability for active overseas service.

Homeland security is a strategic function to which reserves should be well suited. Civil emergencies require surge capacity from local troops with local knowledge. The UK preference for maintaining ‘standby battalions’ at readiness runs counter to the benefits of reserve forces, being expensive to maintain, rarely located where they are required, while reducing the Regular resource available for other operational commitments. Where reservists are criticised for being slow to deploy using mobilisation pipelines designed for augmentation or reinforcement, a reserve organisation dedicated to homeland security could resolve this with a rapid mobilisation pipeline. This is not theoretical, but actively practiced by the US National Guard, which maintain Quick Reaction Forces, designed for short-notice domestic response. While civil-military integration has, historically, been routinely practiced across the US during disaster relief and emergencies, the integration is deepening, with the National Guard recently being ordered to prepare not just for intervening in civil contingencies, but civil unrest. Even France, which has historically been as averse to homeland military deployments as the UK, has been increasingly integrating the military with the police force since 2015 as part of Opération Sentinelle.

Conclusion: Does having a Reserve Matter?

While no army must have a reserve, the cost of a professional standing army with the same opportunities for breadth, depth and resilience would be too high for any modern state to bear. Instead, a well-designed reserve can offer policy options to governments. A government with only an augmentative reserve can fight quickly, but unsustainably. A government with only a reinforcing reserve will mobilise slowly, but cheaply. A government with only a homeland defence force can fight domestically but not abroad.

The central difficulty is that no single reserve organisation can fulfil more than one strategic function. Augmentation, reinforcement and homeland defence each require different structures, training models, legal frameworks and mobilisation timelines. Yet a coherent national security strategy requires that all three functions are fulfilled. The essential strategic dilemma lies in balancing these strategic functions against the threats and resources facing the nation.

In Britain, the reserve capability has atrophied and requires radical overhaul. Most importantly, military planners must be explicit about how they expect Britain to fight in any coming war. Augmentation, long the predominant reserve model in the UK, can create a powerful first echelon, but one that is incapable of sustained land operations. Reinforcement can sustain a land war by generating a second echelon but requires a reserve structure several times larger than that currently envisaged. Homeland defence has not been relevant since 1940 but will be an inevitable component of any war the British Army is preparing to fight in the future. Currently, the Reserves aim to fulfil only two of the three strategic functions – augment and defend – within one organisation, and yet basic organisation theory tells us that without a clear, singular mission, that organisation will fail. Within the current force structure, the existence of Reserves is seen as a problem that needs a solution. Military planners must be clearer about the problems we face before defining any solutions required.