Ever since the expansion plan for the Royal Air Force (RAF) was adopted in 1934 under the name Plan A, the British had been working systematically to build a modern air force. The most important steps in this process were the expansion of a network of air bases, the development of a base of trained ground personnel and a cadre of pilots, and later - roughly in the two years between the Munich Agreement and the Battle of Britain - the equipping of the RAF with modern bombers and fighters.
On September 1, 1939, the German Reich attacked Poland, France and Great Britain declared war on Germany on September 3, and World War II had begun.
In several blitzkriegs, Germany was able to inflict massive losses on the Allies in 1940 and occupy the Benelux countries and large parts of France during the Western campaign. British troops on the mainland were encircled at the northern French port of Dunkirk, but were just saved from annihilation in Operation Dynamo. The rescue of the Expeditionary Force of about 240,000 troops and of 100,000 more soldiers of allied countries greatly boosted British morale. Since all heavy weapons had to be left behind during the evacuation, the successful defense of the British Isles against a German invasion was not yet certain. However, the overwhelming defensive success of a few days formed the decisive basis for Churchill's categorical no to enter into peace negotiations with the German Reich and was the early beginning of the end of the invasion plan against England. Hitler decided to attack the Soviet Union, the main ideological opponent, without first having defeated the enemy in the West or having reached a modus vivendi with him.
On 22 June 1940, an armistice was signed between the French army and the German Wehrmacht, much to Britain's disappointment, in the same month the German Wehrmacht managed to occupy the Channel Islands (Guernsey, Jersey, Alderney, Sark and Herm) as the only British territory until the end of the war in 1945.
Justified by the rapid victory over all enemies of the war except Great Britain, signals were expected from Hitler expressing a desire to end the fighting on the part of the British. In fact, there were political and popular currents that were ready for this.
But Arthur Neville Chamberlain, who until then had advocated the appeasement policy, had resigned as prime minister on 10 May 1940, and the energetic Winston Churchill took his place. He made it clear on May 13 that the "war against a monstrous tyranny, such as has never been surpassed, in the sinister catalogue of the crimes of mankind" must end only with "victory at any price." British RAF attacks on German cities began with the raid on Mönchengladbach on 11 May 1940 with 35 bombers.
On July 16, 1940, Hitler gave the order to prepare for Unternehmen Seelöwe. In order to carry out this plan, the German General Staff was sure, one must first win air supremacy over England.
Hitler's appeal to England's common sense that further bloodshed could be avoided, uttered in a speech to the Reichstag on July 19, led to no reaction.
From today's perspective, the plan to land in Great Britain is considered unrealistic. Neither the equipment of the navy nor the army was suitable for this project. There was a lack of transport facilities for an invading army. A war of conquest against Great Britain had not been thought of in the rearmament phase until 1939.