The Blitz
In the 1920s and 1930s, air power theorists Giulio Douhet and Billy Mitchell espoused the idea that air forces could win wars by themselves, without a need for land and sea fighting. It was thought there was no defence against air attack, particularly at night. Enemy industry, their seats of government, factories and communications could be destroyed, effectively taking away their means to resist. It was also thought the bombing of residential centres would cause a collapse of civilian will, which might have led to the collapse of production and civil life. Democracies, where the populace was allowed to show overt disapproval of the ruling government, were thought particularly vulnerable. This thinking was prevalent in both the RAF and what was then known as the United States Army Air Corps (USAAC) between the two world wars. RAF Bomber Command's policy in particular would attempt to achieve victory through the destruction of civilian will, communications and industry.

Q: What kind of populace was believed to be most vulnerable?
Democracies