bombing. It was ultimately this critical error that helped lead to Fighter Command's recovery. The airfields were repaired, and pilot numbers were stabilized and then gradually increased through the influx of new pilots. On 15 September 1940 Göring sent nearly 1,000 aircraft against London suffering losses of 175 aircraft destroyed or damaged in the day's fighting. On 17 September 1940 Hitler postponed the invasion. The Luftwaffe switched to a terror bombing campaign against British cities that lasted until the spring of 1941, when most bomber units were redeployed for the imminent invasion of the USSR. The raids often caused spectacular damage but did little to harm the British war effort. Hitler again postponed the invasion on 13 October 1940 until the spring of 1941. But on 18 December 1940 Hitler issued Directive 21, beginning preparations for the attack on the USSR, effectively canceling the invasion. Although defeated the Luftwaffe remained formidable: as Winston Churchill concluded, "it was not the beginning of the end but the end of the beginning." The Battle of Britain cost the Luftwaffe 873 fighters and 1,014 bombers. The RAF lost 1,023 fighters. Never again did the Luftwaffe operate in such numbers over Britain. General Werner Kreipe described it as a "strategic (Luftwaffe) failure" and a "turning point in the Second World War". The German Air Force was described as "bled almost to death, and suffered losses that could never be made good throughout the course of the war". Quoting Dr (Karl) Klee "The invasion and subjugation of Britain was made to depend on that battle, and its outcome therefore materially influenced the further course and fate of the war as a whole". It is argued by historians that Britain's defeat of the Luftwaffe during the Battle of Britain was the most significant in causing the downfall of the German Air Force. North Africa and the Mediterranean 1941–1944 In North Africa and the Mediterranean, the Luftwaffe mainly saw action in support of the ground operations conducted by General Erwin Rommel's Afrika Korps. The Afrika Korps fought in North Africa from February 1941 to May 1943. Before Rommel's arrival, Mussolini's invasion of Greece, in October 1940 and the Italian offensive against the British in Egypt the previous month had been a complete disaster. The British had driven the Italian forces back into Libya, and now seemed poised to sweep them out of Africa altogether. The Greek Army had