In 1940, the war in the west was as a whole a conventional war, complying with certain rules like saving the civil ones (cf conventions of Geneva).
The German actions against London or Coventry at the end of the summer 1940 (554 died at the time of an attack to the bomb flamer) convince finally Churchill to launch air raids of reprisals on Germany.
As of August 1941, leave England of the bombers charged to destroy the potential of production of Reich (it is the strategic bombardment) and to weaken moral German population.
The arrival of VIIIème floats American in 1942, then by XIIème (based in Italy) in 1943 increase the number of bombers in the German sky. It is a very hard battle and loophole which begins, with the strategic results than disputed more today, since the German potential of production will reach its maximum in 1944 all the same. (the factories are camouflaged, buried and the camps of work with their prisoners transformed into workshops for the industry of armament). In 1942, the taboos indeed broke down: the British recommend bombardments targeted on the civil ones in order to break their moral, the Americans refuse there. But their planes, to escape from Flak, release their bombs at very high altitude, the carpets of bombs crush as well the industrial districts as the dwellings...
If the effectiveness of the bombardments is thus more than debatable, it remains nevertheless that the German command, not more than before him the British command, could not let enemy airplanes fly over the own territory with impunity. It is important from now on to develop an effective system of defense.