Adlertag ("Eagle Day") was the first day of Unternehmen Adlerangriff ("Operation Eagle Attack"), which was the codename of a German military operation by the Luftwaffe (German Air Force) to destroy the British Royal Air Force (RAF). By June 1940, the Allies had been defeated in Western Europe and Scandinavia. Rather than come to terms with Germany, Britain rejected all overtures for a negotiated peace.
Hitler gave the German armed forces (Wehrmacht) a directive (Directive No. 16) which ordered the preparation for the aerial attack and pacification of Britain. The military invasion of Britain was codenamed Operation Sea Lion (Unternehmen Seelöwe). Before this could be carried out, air superiority or air supremacy were required before any sea or land operations could take place. The Luftwaffe was to destroy the RAF in order to prevent it from attacking the invasion fleet or providing protection for the Royal Navy's Home Fleet which might attempt to prevent a landing by sea. Hitler ordered the Luftwaffe's commander-in-chief, Reichsmarschall (Marshal of the Realm) Hermann Göring and the Oberkommando der Luftwaffe (High Command of the Air Force) to prepare for the assault.
The essential target was RAF Fighter Command. The services' destruction would deny the British their air superiority asset. Throughout July and early August, the Germans made preparations for Adlertag. The date of the assault was postponed several times because of bad weather. Eventually, it was carried out on 13 August 1940. The German attacks on 13 August inflicted significant damage and casualties on the ground but, marred by poor intelligence and communication, they did not make a significant impression on Fighter Command's ability to defend British air space.
Göring had promised Hitler that Adlertag and Adlerangriff would achieve the results required within days, or at worst weeks. It had meant to be the beginning of the end of RAF Fighter Command, but Adlertag and the following operations failed to destroy the RAF. The engagements resulting from Adlertag became known as the Battle of Britain. During the course of that campaign, the Luftwaffe was prevented from gaining air superiority or air supremacy by the RAF. As a result, Sealion was cancelled.