late May and early June 1940, the British Expeditionary Force withdrew from France after being defeated at Dunkirk by the Wehrmacht, leading to the fall of France under Nazi occupation. Dunkirk is one of the greatest battles on the Western Front in World War II and the greatest defeat of British imperial forces. Thus, England was threatened with invasion, being subjected from September 1940 to June 1941, the Blitzkrieg Luftwaffe.
Ten-year rule
At the end of World War I, in 1919, the British Government announced The Ten-Year Rule, respectively. It is assumed that the British Empire would not be engaged in any major war in the next ten years and that no expeditionary force was needed for this purpose. The main functions of the military and air force were to support the garrisons in India, Egypt, the new mandated territories, and all other (non-self-governing) territories under British control, as well as to support civilian power at home. Therefore, the immediate demobilization of the troops brought the Entente victory in the Great War, leaving only the RAF (Royal Air Force) forces that will control the desert, in the words of the historian Marks Sally. Respectively, British planes will ensure the security of the Empire in the Middle East.

Thus, in 1919, the English Government did not give an important place to the reorganization of the army and, in particular, to the ground troops, in the national defense strategy. Moreover, in the interwar period, there was no well-defined plan in which to design the defense of the Empire. So, in 1928, The Ten Year Rule was reconfirmed, only to be abandoned in 1932.
Implicitly and perhaps unconsciously, it is confirmed that Britain achieved in the interwar period what Paul Kennedy calls the “imperial overstretch”, respectively imperial overstretch. Only in December 1937, at a meeting of the Government led by Neville Chamberlain, the defense priorities of the British Empire were established, as follows: in the first place was the security of the British Isles, especially in the face of an air attack, secondly, the protection of routes of communication of the Empire, and in third and fourth place were, in order, the defense of imperial possessions and “cooperation to defend the territories of any Allies England would have at war.”
Undersized Army
For 20 years, London reduced the costs allocated to the military, which would make it impossible for it to carry out its battles at the optimum level in terms of armaments and training in 1939 when war broke out. The British army had been undersized between 1919–1939, which directly led to the failure of Dunkirk. Until 1936, but also after this date, it had a small number of tanks, which can be seen from the fact that it spent four times more on horse feed than on oil needed for the panzer. Therefore, in May 1940, the British Expeditionary Force was in an acute tank crisis, and those used erroneously against the Wehrmacht were not the right ones.

In addition, the main enemy, at the military level, was considered Japan, not Germany, so in 1935, the Naval Treaties were concluded, which allowed Berlin to build an equal number of submarines with the British and a proportion of 35% of ships.
Focusing on internal affairs
Also, after 1919, England constantly refused to grant, at the political level, security guarantees to its former ally in the Great War, namely France. The Treaty of Versailles contained such legal assurances. However, it had a clause that made London accept that provision only after it was sanctioned in the US Congress. As the latter rejected the Peace Treaty in its entirety, the Anglo-American guarantees could not be granted to Paris either. The United Kingdom did not want to commit itself unilaterally to France because it would have meant that it had to defend its security on the Rhine, that is, to take action on the continent whenever necessary.
English society, which was essentially peaceful, and the foreign policy decision in the Government, moreover, opposed an active, extended role of England in Europe. Therefore, in the twenty years between 1919 and 1939, London often refused to assist Paris in this regard, respectively, to make statements that would discourage Berlin.

Until now, the United Kingdom has erroneously believed that Germany was much more affected by the First World War than France, so it was trying to tip the scales in favor of Berlin than Paris. London set out to move away from European affairs in order to focus on imperial affairs, given that after 1931 the Commonwealth had created the Westminster Status and that its foreign policy could no longer be decided unilaterally.
In the late 1930s, England offered the guarantees that France had sought since the First World War, but it was far too late. Thus, it was impossible to effectively plan the combined attack of Anglo-French troops in the face of possible German aggression. As such, the lack of political and military cooperation between Paris and London did not have a deterrent effect on Berlin.

Moreover, General Guderian, the leader of the German armies, entered French territory at the Sedan, a space where a Nazi attack was not expected; as such, he was poorly defended. Therefore, the Allied strategy, almost non-existent, and the fact that the French army was in a situation that did not recommend it for a battle of such magnitude caused the failure of the Entente in May 1940. Also, US neutrality, the Ribbentrop Pact, and Molotov, as well as Japan’s accession to the Axis, led to the isolation of Great Britain, which was unable to find new allies.
French Prime Minister Paul Reynaud had sensed this since the morning of May 15, when in a phone call to Winston Churchill, he told him that “we are beaten. I lost the fight. “ However, although the British forces were hastily withdrawn from France, the failure at Dunkirk is turned into a victory in the mentality of British society. The Dunkirk Spirit, as it is called, translates into the unity of the British in the face of the war effort, which contains the premises for the final victory.

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