 Complete the following phrase:  Strategic bombing during World War II began on 1 September 1939 when Germany invaded Poland and the Luftwaffe German Air Force began bombing cities and the civilian population in Poland in an aerial bombardment campaign. As the war continued to expand, bombing by both the Axis and the Allies increased significantly. The Royal Air Force began bombing military targets in Germany, such as docks and shipyards, in March 1940, and began targeting Berlin in August 1940. In September 1940, the Luftwaffe began targeting British cities in the Blitz. After the beginning of Operation Barbarossa in June 1941, the Luftwaffe attacked Soviet cities and infrastructure. From February 1942 onward, the British bombing campaign against Germany became even less restrictive and increasingly targeted industrial sites and civilian areas. When the United States began flying bombing missions against Germany, it reinforced these efforts and controversial firebombings were carried out against Hamburg 1943, 