The fact that Harris said that bombing alone could do it was sheer hyperbole. Did he really believe that
Have read it (forget book) that following a meeting with Churchill at the outbreak of war Harris was asked by someone the likely outcome of a bombing campaign and Harris replied to the effect that he had no idea as it hadn't been tried before, not withstanding the WWI Zeppelin raids on the UK. "First Blitz" by Neil Hanson is an interesting account of the WWI campaign.