Dr. Robert Lyman writes about the shocking contrasts between the British Army (including the Canadian and Australian Corps) during the Hundred Days campaign of 1918 and the British Expeditionary Force that was driven from the continent at Dunkirk:

There was a fleeting moment during the One Hundred Days battles that ended the First World War in France in which successful all-arms manoeuvre by the British and Commonwealth armies, able to overturn the deadlock of previous years of trench stalemate, was glimpsed. But the moment, for the British Army at least, was not understood for what it was. With hindsight we can see that it was the birth of modern warfare, in which armour, infantry, artillery and air power are welded together able successfully to fight and win a campaign against a similarly-equipped enemy. Unfortunately in the intervening two decades the British Army simply forgot how to fight a peer adversary in intensive combat. It did not recognise 1918 for what it was; a defining moment in the development of warfare that needed capturing and translating into a doctrine on which the future of the British Army could be built. The tragedy of the inter-war years therefore was that much of what had been learned at such high cost in blood and treasure between 1914 and 1918 was simply forgotten. It provides a warning for our modern Army that once it goes, the ability to fight intensively at campaign level is incredibly hard to recover. The book that General Lord Dannatt and I have written traces the catastrophic loss of fighting knowledge after the end of the war, and explains the reasons for it. Knowledge so expensively learned vanished very quickly as the Army quickly adjusted back to its pre-war raison d’etre: imperial policing. Unsurprisingly, it was what many military men wanted: a return to the certainties of 1914. It was certainly what the government wanted: no more wartime extravagance of taxpayer’s scarce resources. The Great War was seen by nearly everyone to be a never-to-be-repeated aberration.

The British and Commonwealth armies were dramatically successful in 1918 and defeated the German Armies on the battlefield. Far from the “stab in the back” myth assiduously by the Nazis and others, the Allies fatally stabbed the German Army in the chest in 1918. The memoirs of those who experienced action are helpful in demonstrating just how far the British and Commonwealth armies had moved since the black days of 1 July 1916. The 27-year old Second Lieutenant Duff Cooper, of the 3rd Battalion The Grenadier Guards, waited with the men of 10 platoon at Saulty on the Somme for the opening phase of the advance to the much-vaunted Hindenburg Line. His diaries show that his experience was as far distant from those of the Somme in 1916 as night is from day. There is no sense in Cooper’s diaries that either he or his men felt anything but equal to the task. They were expecting a hard fight, but not a slaughter. Why? Because they had confidence in the training, their tactics of forward infiltration, their platoon weapons and a palpable sense that the army was operating as one. They were confident that their enemy could be beaten.

[…]

It would take the next war for dynamic warfare to be fully developed. It would be mastered in the first place by the losers in 1918 – the German Army. The moment the war ended the ideas and approaches that had been developed at great expense were discarded as irrelevant to the peace. They weren’t written down to be used as the basis for training the post-war army. Flanders was seen as a horrific aberration in the history of warfare, which no-right thinking individual would ever attempt to repeat. Combined with a sudden raft of new operational commitments – in the remnants of the Ottoman Empire, Russia and Ireland – the British Army quickly reverted to its pre-1914 role as imperial policemen. No attempt was made to capture the lessons of the First World War until 1932 and where warfighting was considered it tended to be about the role of the tank on the future battlefield. This debate took place in the public arena by advocates writing newspaper articles to advance their arguments. These ideas were half-heartedly taken up by the Army in the later half of the 1920s but quietly dropped in the early 1930s. The debates about the tank and the nature of future war were bizarrely not regarded as existential to the Army and they were left to die away on the periphery of military life.

The 1920s and 1903s were a low point in national considerations about the purpose of the British Army. The British Army quickly forgot what it had so painfully learnt and it was this, more than anything else, that led to a failure to appreciate what the Wehrmacht was doing in France in 1940 and North Africa in 1941-42.