Facing a threat from an aggressively hostile state that is destabilising Europe, the British government decides on an act that has enormous symbolic effect, but also practical outcomes. Britain announces that is going to double its Army reserve forces. This act, it hopes, sends a clear message that Britain takes the threat seriously. The potential enemy is already gobbling up a smaller neighbour and is lining up future victims. Britain’s proclamation is aimed at four audiences. The aggressor (the British government hopes) will be left in no doubt about how seriously London regards the threat, and this will reinforce deterrence. Similarly, the message targets Britain’s allies, who will be assured about its commitment, as will neutral states. Finally, the massive expansion of part-time land forces sends a stark message to the British population about the nature of the threat, the prospect of major war, and the need for mobilisation of the state and society on a scale that has not been seen for many years. 

There is a price to pay. Doubling the reserves means splitting some existing units in half and building new units on the cadres. This disrupts the cohesion, training and battle-readiness of the original units. It also involves some de-skilling, as there is only a limited number of trained officers, Non-Commissioned Officers (NCOs) and ordinary soldiers, and these will now be spread thinly between parent units and their offspring. During this process the reserve units of the Army will undoubtedly be less effective than they were at the beginning. But at least this massive upheaval will take place in what remains of peacetime. Far better that the Army endures the pain now, rather than suffering significant disruption, with all that entails, while a war is being fought. That happened in 1914 with the raising of Kitchener’s Army, and the precedents are not good. Hopefully the worst of the chaos will have been overcome when the balloon goes up, and the Army is plunged into major combat operations.

This is a possible scenario for a British government in the near future, but it also happened, exactly like this, many years ago. The phrase ‘when the balloon goes up’ is the giveaway. This expression, with its origins in the trenches of the First World War, was much in use in the late 1930s. On 29 March 1939, Leslie Hore-Belisha, the flamboyant Secretary of State for War, who as Minister of Transport had given the nation the Belisha Beacon, announced the massive expansion of the Territorial Army (TA). This was the part-time force of civilians in uniform, which was renamed ‘Army Reserve’ in 2012. Hore-Belisha’s announcement was essentially a political rather than a military move. It was intended to send a message about the UK’s determination to stand up to Nazi aggression.

In late 1938, such a course of action seemed unthinkable. When, on 30 September 1938, Prime Minister Neville Chamberlain had arrived back from the Munich conference with Hitler, he announced that it was ‘peace for our time’. Chamberlain had agreed to the dismemberment of Czechoslovakia, believing that Hitler’s hunger for territorial expansion was now sated. Then, in March 1939, Hitler’s army occupied the rump of Czechoslovakia. The scales dropped from Chamberlain’s eyes. Neville Chamberlain has a popular reputation as a political innocent, but actually he was a heavyweight politician with a formidable record on the Home Front. Hitler, however, had made him look a fool. Now Chamberlain was angry, and the doubling of the TA was a clever response. It fired a shot across Hitler’s bows, and sent messages of resolution to allies, neutrals and the British people.

In the event, the expansion of the Territorials failed to make the right impression on Hitler. Neither did the introduction of peacetime conscription only a month later, in April 1939. Hitler’s belief that Britain would not fight was a factor in his decision to attack Poland in September 1939. That says more about Hitler taking the wrong decision than about Britain taking the right one. For the doubling of the TA and the move to a conscript army were sensible and ultimately successful moves, both in sending clear messages to the British people, and taking the (albeit faltering) steps towards creating the mass army needed for a major war.

What this underlines is that the British Army, like all armies, is an instrument of political power. It can have an impact without firing a shot. Britain’s Nato allies, such as Germany, Sweden and Finland, have recently announced significant rearmament measures, to reinforce the existing deterrence of Russia and, if necessary, to defend themselves. Building up armed forces does not necessarily lead to war. The idea that arms races inexorably lead to conflict is simply not true. Two of the most important – the Anglo-French naval competition in the late 19th century, and the US-Soviet nuclear arms race in the Cold War – ended in détente, not war. For weapons and armed forces can deter an adversary, at the very least causing them to think twice before carrying out provocative moves. The ancient axiom ‘Si vis pacem, para bellum’ (if you desire peace, prepare for war) remains as relevant as ever.

Prime Minister Keir Starmer and the current UK government have shown that they are alive to the threat posed by Putin’s Russia, and to the implications of the current instability in Nato. Whatever Starmer’s domestic difficulties, he has proved capable on the world stage. What has attracted far less attention than the PM’s dealings with world leaders is the Labour government’s moves to reinforce the nation’s defences. A hefty increase in defence spending and increased support for Ukraine are the headlines. The 2025 Strategic Defence Review (SDR) included two ‘ambitions’: to ‘move to war-fighting readiness – establishing a more lethal “integrated force” equipped for the future, and strengthened homeland defence’; and a ‘whole-of-society approach – widening participation in national resilience and renewing the Nation’s contract with those who serve’.

In July 2025, the Starmer ministry also released the UK Government Resilience Action Plan. This is not solely concerned with external threats, but also with pandemics and other sources of insecurity. In addition, this summer saw the publication of the cross-party Defence Committee’s report ‘Defence in the Grey Zone’. Former Conservative Defence Secretary Ben Wallace has defined the grey zone as ‘that limbo land between peace [white]  and war [black]’. This is a realm of sabotage, cyberattacks, destabilisation operations, propaganda and the like, including the sort of thing associated in the Second World War with Fifth Column activity. Chiming in with the SDR, the Defence Committee urged a ‘whole of society’ stance for defending the UK’s Home Front.

Taken together, while there is no room for complacency, these are all steps in the right direction. And Britain’s Reserve Forces have a vital part to play in the new reality. If we look at the first half of the last century, it is clear that the UK’s Reserve Forces had three critical roles: as a deterrent; as reinforcements for regular forces; and as a means of mobilising society for war.

Deterrence, as Karl Mueller states, is, at its simplest, ‘causing someone not to do something because they expect or fear that they will be worse off if they do it than if they do not’. Deterrence revolves around the ‘three Cs’: capability, credibility and communication. Capable armed forces enhance the credibility of a state’s defences, which can be communicated to potential adversaries in various ways. Obviously, the UK’s Armed Forces, of which the Reserves formed a part, did not deter Germany either in 1914 or in 1939. But that is not to argue that conventional deterrence never works. As Mueller has noted: ‘successful deterrence is usually invisible – an adversary may back down conspicuously during a confrontation in response to a deterrent threat, but the causes of crisis outcomes are often ambiguous’. The UK’s primary deterrent is the strategic nuclear forces operated by the Royal Navy, but exclusive reliance on nuclear weapons as a deterrent narrows the options of a government to a dangerous degree. When faced with an adversary with robust conventional forces, at the very least a potential aggressor’s decision-making is made more complex and challenging.

Russia’s re-emergence as a threat to NATO and the UK has moved these debates out of the university seminar room and into the political arena. Investing in the reserve forces, which consists of superb, dedicated women and men, worthy successors of the citizen-troops of the past, enhances the credibility of Britain’s deterrent. The current situation in the UK’s reserves is laid out in the annual statutory report for 2024 produced by the Council of Reserve Forces’ and Cadets’ Associations Scrutiny Team (full disclosure: I was a member of that team for three years several years ago, and this article draws upon material I contributed to the 2022 and 2023 reports). I am not advocating a massive expansion back to Cold War levels, let alone to that of 1914 or 1939. Financial and political reasons rule that out. However a greater concentration on reserve forces sends a clear message to friend and foe alike that the UK is taking conventional defence and deterrence seriously, in line with SDR and Government Resilience Action Plan.

At the beginning of both world wars, reserve forces provided vital reinforcement for the regulars. In October 1914, battalions of Territorials fought in the front line in Flanders, where they proved to be vital reinforcements for the hard-pressed regular Army. The defensive action of the London Scottish on Messines Ridge on Halloween 1914 became justly famous. Likewise, the defence of Cassell in the Dunkirk campaign in 1940 by the 4th Oxfordshire and Buckinghamshire Light Infantry, the Oxford City Territorials, showed that reserve forces could fight as hard as their regular counterparts.  Indeed, some of Britain’s best divisions in both world wars were originally Territorials, the 51st (Highland) and 56th (London) among them. It was not just the Army that benefitted from reinforcement by reserves. Twenty squadrons of the Royal Auxiliary Air Force (RAuxAF) were deployed alongside the regular RAF in 1939. During the Battle of Britain, 14 out of 62 Fighter Command squadrons were from the RAuxAF.

In both wars, reserve forces and regulars together held the line, metaphorically and literally, buying time for Britain to mobilise. In more recent wars of choice, in Iraq and Afghanistan, reservists have been an integral and vital role in the UK’s military deployments. For example, in 2004, some 1,200 TA soldiers (about 10 per cent of which were women) were involved in Operation Telic 4 in Iraq. The TA contingent comprised 14 per cent of the UK force.

Lord Kitchener, Britain’s principal warlord at the beginning of the Great War, remarked at a particularly stressful moment: ‘Did they remember, when they went headlong into a war like this, that they were without an army, and without any preparation to equip one?’ This was an exaggeration, but one with a strong element of truth. Before 1914, there were not even paper schemes to create a mass army, which was seen as being politically inexpedient, unnecessary and unthinkable – right up to the moment when it was created. The methods used, calling for volunteers, took no account of the penalty paid by the British war economy of removing key workers from industry. There was some planning based on the experience of 1914-18 during the run-up to war in the 1930s, and so many of these problems were avoided. The Territorial Army was the primary vehicle for the expansion of land forces.

If history is any guide, in any future major conflict, reserve forces will have to carry out the dual role of reinforcing the regulars and acting as a cadre for the expansion of the UK’s armed forces. I am not suggesting that this is likely, but a clear message from the past is that failure to plan for this eventuality in peacetime led to an ad hoc response in time of war. As a consequence, under-trained, inexperienced and poorly equipped troops were sent into battle, with sometimes disastrous results. This was the fate of some Territorial units in the Norway campaign of 1940, and in France a few weeks later. Investing in reserve forces in peacetime is best seen as an insurance policy. Like any type of insurance, you might gripe at the premiums, but when the worst happens you are mightily glad that you kept up the payments.

Expansion of the TA in 1939 helped bring home to the British population the gravity of the situation, and that need for huge efforts in the event of war. This ‘cultural mobilisation’ ran alongside physical mobilisation. In 1939 a journalist remarked on the number of young men on their way to work in the City of London who were wearing the ties of Territorial regiments – vastly more than had been the case 12 months previously. Although it came late in the day, shining the spotlight on reserve forces did much to address the criticisms made in 1932 by the Chiefs of Staff that the public had not been sufficiently alerted to the need to rearm to preserve peace. Today, not the least benefit of an increased emphasis on the reserves would be to draw attention to the elephant (or bear) in the room. Russian aggression against Ukraine means the return of interstate war to the continent of Europe, and this poses a threat to the UK; this is still not well understood. As the SDR makes clear, a build-up of conventional defences is necessary to enhance the UK’s deterrence.

Britain has moved a long way from the militarised, National Service society of the early Cold War period. Then, military service was generally uncontroversial. Modern society is less deferential, more questioning, than six or seven decades ago. In recent years, the Iraq War did much in the eyes of a section of public opinion to discredit the use of war to achieve political aims. Building up reserve forces would help bridge the gap between the population at large and the UK’s armed forces, especially by reforging links at the local level. Concentration on the defensive and, above all, deterrent value of reserve forces in the face of an emerging threat helps to counter accusations of militarism: the UK has a long tradition of part-time citizen-troops defending the country in time of crisis.

The world is different in so many ways from 1938, and the challenge presented today by Putin’s Russia is not the same as that posed in the past by Hitler’s Germany. Nonetheless, there are uncomfortable parallels. Prudence demands increased investment in defence. Thankfully, the UK government has demonstrated, at least, that it understands this simple truth. It is sometimes said that to prepare to fight makes war more likely. On the contrary: in the circumstances of 2025, just as in the 1930s, to show weakness is to court danger. I am sceptical about the existence of straightforward ‘lessons’ from history, ut the past does, as the naval historian Andrew Gordon has argued, provide important approximate precedents applicable to the present. That being so, it is clear that reserve forces have a vital role in defending the UK’s interests, in war and peace, and in the space between the two extremes.