days to recover when hit severely, but some, such as 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 about 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. Background Luftwaffe and strategic bombing In the 1920s and 1930s, airpower theorists such as Giulio Douhet and Billy Mitchell claimed that air forces could win wars, obviating the need for land and sea combat. It was thought that "the bomber will always get through" and could not be resisted, particularly at night. Industry, seats of government 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. The Luftwaffe took a cautious view of strategic bombing but the OKL did not oppose the strategic bombardment of industries or cities. It believed it could greatly affect the balance of power on the battlefield by disrupting production and damaging civilian morale. did not believe air power alone could be decisive and the Luftwaffe did not adopt an official policy of the deliberate bombing of civilians until 1942. The vital industries and transport centres that would be targeted for shutdown were valid military targets. It could be claimed civilians were not to be targeted directly, but the breakdown of production would affect their morale and will to fight. German legal scholars of the 1930s carefully worked out guidelines for what type of bombing was permissible under international law. While direct attacks against civilians were ruled out as "terror bombing", the concept of attacking vital war industries—and probable heavy civilian casualties and breakdown of civilian morale—was ruled as acceptable. From the beginning of the National Socialist regime until 1939, there was a debate in German military journals over the role of strategic bombardment, with some contributors arguing along the lines of