 Complete the following phrase:  As the war progressed, the Battle of Britain developed from a fight for air supremacy into the strategic and aerial bombing of London, Coventry and other British cities. In retaliation for this, the British built up the RAF Bomber Command, which was capable of delivering many thousands of tons of bombs onto a single target, in spite of heavy initial bomber casualties in 1940. The bomber force was then wielded by Arthur Travers Harris in an effort to break German morale and obtain the surrender which Douhet had predicted 15 years earlier. The United States joined the war and the USAAF greatly reinforced the campaign, bringing in the Eighth Air Force into the European Theatre. Many cities, both large and small, were virtually destroyed by Allied bombing. 