The Defence of the Reich (German: Reichsverteidigung) was the strategic defensive aerial campaign fought by the Luftwaffe of Nazi Germany over German-occupied Europe and Germany during World War II against the Allied strategic bombing campaign. Its aim was to prevent the destruction of German civilians, military and civil industries by the Western Allies. The day and night air battles over Germany during the war involved thousands of aircraft, units and aerial engagements to counter the Allied bombing campaigns. The campaign was one of the longest in the history of aerial warfare and with the Battle of the Atlantic and the Allied naval blockade of Germany was the longest of the war. The Luftwaffe fighter force defended the airspace of German-occupied Europe against attack, first by the RAF Bomber Command and then against the RAF and United States Army Air Forces (USAAF) in the Combined Bomber Offensive.
In the early years, the Luftwaffe was able to inflict a string of defeats on Allied strategic air forces. In 1939, Bomber Command was forced to operate at night, due to the extent of losses of unescorted bombers flying in daylight. In 1943, the USAAF suffered several reverses in daylight and called off the offensive over Germany in October limiting their attacks to western Europe as they built up their force. During the war the British built up their bomber force, introducing better aircraft with navigational aids and tactics such as the bomber stream that enabled them to mount larger and larger attacks while remaining within an acceptable loss rate. In 1944 the USAAF introduced metal drop tanks for all American fighters including the newly arrived North American P-51D Mustang variant, which allowed fighter aircraft to escort USAAF bombers all the way to and from their targets. With a change of focus on destroying the German day fighter force, by the spring of 1944 the Eighth Air Force had achieved air supremacy over Western Europe, which was essential for the Allies so they could carry out the invasion of France. The strategic campaign against Germany eased as the Allies' Transport Plan focused their resources on isolating northern France in preparation for the invasion.