Robert Lyman outlines why Britain in general and the British army in particular were so materially and intellectually unready for the war that broke out in 1939:

… the British Army was catastrophically unprepared for war in 1939. But it wasn’t just the Army that was unprepared. Despite a last-minute rush to re-arm, so too was the whole country. In Britain a deep-seated passivity had set in following the end the Great War. This belied the reality that in Europe the ending of the war in fact opened the door to unheralded political chaos and instability that was in time to overcome the forces of stability and would lead directly to yet another devastating war. In the years immediately following the arrival of peace in 1918 Britain hoped it could close the door on any future European or continent commitment and return to the halcyon days when its only security commitments were the defence of its widely flung Empire.

The weakness at the heart of British planning for war was a direct reflection of Britain’s strategic, political, societal and economic situation during the inter-war period. Britain – both the British public and the country’s various governments – simply wasn’t mentally prepared to go to war again so soon after the trauma of the Great War. As a result, it made no proper preparation for another full-on industrial war against a peer opponent on the continent. This was fundamentally a failure of political and military imagination; the inability to think through what a potential war might look like and to prepare for this possibility accordingly.

We have identified five primary causes of the decline of British military effectiveness in 1939. In the first place there was no clear strategic plan for the Army. Strategies are determined by having a clear understanding of who a future enemy might be. Following the end of the Great War, until the late 1930s no one seemed bothered to define this essential point of direction. There was a remarkably inadequate grand strategic conversation (i.e., at a national, governmental level) about the purpose, structure, and nature of the Army. There was plenty of talking, but very little of it focused on realistic determination as to who it might have to fight, and how. This was a problem, because it meant that Britain was unable to determine the precise structure its armed forces needed to be, and its cost. Was the focus of the army to be the continent, or the Empire, or both? No one knew. As a result, the last known plan reasserted itself – Imperial defence, à la 1914. This meant that the army wasn’t structured or equipped to fight a specified enemy in a defined set of circumstances. Instead, the British Army and its cousin, the Indian Army, was expected to be a generic jack-of-all-trades, without the structure, doctrine, training, or equipment to fight the type of war it had become the master of in 1918. While there was some doctrine, and considerable doctrinal debate, little was anchored in a clear definition of what future war was expected to look like. There was no operational design for the British Army derived directly from an analysis of the threat it faced. If it had done, the BEF would have been thoroughly prepared for the German Blitzkrieg in France and the Low Countries in 1940 or the similar Japanese Kirimoni Sakusen in 1941 and 1942. The British Army wasn’t prepared to fight a first-class European Army in 1939 for the simple reason that Britain hadn’t prepared itself to do so. Likewise, when it came to fighting the Japanese in 1941 and 1942 in Malaya and Burma, the British found that not only had it failed to prepare adequately for a potential Japanese invasion of its vulnerable Far Eastern colonies, but that it had no idea as to how to fight the Imperial Japanese Army. There were two connected failures here. The first was one of strategic preparedness, the blame for which was both governmental and strategic. The second was of training, doctrine and military preparedness by the British Army in Europe and Asia to fight. When they emerged out of their assault boats at Kota Bahru on the morning of 8 December 1942 the Japanese could as well have come from Mars, given how little the British knew about them and their warfighting methods.

Second, as a country, Britain was unprepared both politically and culturally for another war so soon after the last. In 1919 the country seemed to want to look backward to embrace the days of peace that had preceded the cataclysm of war, to drape itself with Edwardian comfort. It was tired and disillusioned, and felt no victor’s triumph. The country looked to itself, and to its Empire, eschewing the complications of commitments on continental Europe that had recently resulted in the loss of so much blood. The losses sustained in the Great War resulted in the overwhelming national sentiment that war must never again be undertaken as a form of politics. Clausewitz was dead. Part of this sentiment evidenced itself in the rise of pacifism. In the army, a pervasive belief existed that the Great War was an aberration, and nothing like it would again afflict western civilisation. Any lessons from the war were therefore irrelevant to the future structures or doctrine of the British Army, for whom the defence of the Empire was the crucial issue. But whether it liked it or not, the world was changing fast, in ways that Britain struggled to comprehend and from which it could not ultimately escape. The Russian Revolution, the rise of fascist dictators in Europe, isolationism in the USA (except for a new American assertiveness in Asia) and the increasing militancy of Japan, began changing the global landscape in ways that were hard to understand for a country seemingly once in total charge of the certainties of statecraft. Now it struggled to find its way in a new world of tension, turmoil and rapid change.

Third, no one in the British Army thought to capture the reasons for operational success in 1918. The dramatic reduction in troops numbers at the end of the Great War meant that those best able to convert the learning from 1918 into doctrine left for civilian life, taking their knowledge and experience with them. It was never recovered. There was therefore no template in the years afterward on which to build a successful military doctrine based on the successful warfighting experience that had culminated in the victories of 1918.

Fourth, political naivety led to a dramatic economic stringency being applied, including the underlying Treasury assumption in the early 1920’s of the ‘Ten Year Rule’, an assumption that kept rolling over, year after year. This meant that there wasn’t enough money to do what was necessary to protect British interests from impending harm. The Army butter was thinly spread on the imperial bread, with the result that insufficient investment was made in the core of the army’s warfighting capability. This stringency was exacerbated by the impact of the Great Depression at the end of the 1920s into the early years of the next decade.