The Germans conducted mass air attacks against industrial targets, towns and cities, beginning with raids on London towards the end of the Battle of Britain in 1940, a battle for daylight air superiority between the Luftwaffe and the Royal Air Force over the United Kingdom. By September 1940, the Luftwaffe had failed and the German air fleets (Luftflotten) were ordered to attack London, to draw RAF Fighter Command into a battle of annihilation. Adolf Hitler and Reichsmarschall Hermann Göring, commander-in-chief of the Luftwaffe, ordered the new policy on 6 September 1940. From 7 September 1940, London was systematically bombed by the Luftwaffe for 56 out of the following 57 days and nights. Most notable was a large daylight attack against London on 15 September.
The Luftwaffe gradually decreased daylight operations in favour of night attacks to evade attack by the RAF, and the Blitz became a night bombing campaign after October 1940. The Luftwaffe attacked the main Atlantic sea port of Liverpool in the Liverpool Blitz and the North Sea port of Hull, a convenient and easily found target or secondary target for bombers unable to locate their primary targets, suffered the Hull Blitz. Bristol, Cardiff, Portsmouth, Plymouth, Southampton and Swansea were also bombed, as were the industrial cities of Birmingham, Belfast, Coventry, Glasgow, Manchester and Sheffield. More than 40,000 civilians were killed by Luftwaffe bombing during the war, almost half of them in the capital, where more than a million houses were destroyed or damaged.
In early July 1940 the German High Command began planning Operation Barbarossa, the invasion of the Soviet Union. Bombing failed to demoralise the British into surrender or do much damage to the war economy; eight months of bombing never seriously hampered British war production which continued to increase. The greatest effect was to force the British to disperse the production of aircraft and spare parts. British wartime studies concluded that cities generally took 10 to 15 days to recover when hit severely but exceptions like Birmingham took three months.
The German air offensive failed because the Luftwaffe High Command (Oberkommando der Luftwaffe, OKL) did not develop a methodical strategy for destroying British war industry. Poor intelligence on British industry and economic efficiency led to OKL concentrating on tactics rather than strategy. The bombing effort was diluted by attacks against several sets of industries instead of constant pressure on the most vital.
In the 1920s and 1930s, air power theorists such as Giulio Douhet and Billy Mitchell claimed that air forces could win wars, obviating the need for land and sea fighting. It was thought that bombers would always get through and could not be resisted, particularly at night. Industry, seats of government, factories and communications could be destroyed, depriving an opponent of the means to make war. Bombing civilians would cause a collapse of morale and a loss of production in the remaining factories. Democracies, where public opinion was allowed, were thought particularly vulnerable. The RAF and the United States Army Air Corps (USAAC) adopted much of this apocalyptic thinking. The policy of RAF Bomber Command became an attempt to achieve victory through the destruction of civilian will, communications and industry.