World War II (1939–1945) involved sustained strategic bombing of railways, harbours, cities, workers' and civilian housing, and industrial districts in enemy territory. Strategic bombing as a military strategy is distinct both from close air support of ground forces and from tactical air power. During World War II, many military strategists of air power believed that air forces could win major victories by attacking industrial and political infrastructure, rather than purely military targets. Strategic bombing often involved bombing areas inhabited by civilians, and some campaigns were deliberately designed to target civilian populations in order to terrorize them or to weaken their morale. International law at the outset of World War II did not specifically forbid the aerial bombardment of cities – despite the prior occurrence of such bombing during World War I (1914–1918), the Spanish Civil War (1936–1939), and the Second Sino-Japanese War (1937–1945).
Strategic bombing during World War II in Europe began on 1 September 1939 when Germany invaded Poland and the Luftwaffe (German Air Force) began bombing Polish cities and the civilian population 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, in retaliation for Luftwaffe attacks on the UK which started on 16 October 1939, began bombing military targets in Germany, commencing with the Luftwaffe seaplane air base at Hörnum on the 19–20 March 1940. In September 1940 the Luftwaffe began targeting British civilians 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 restricted and increasingly targeted industrial sites and civilian areas. When the United States began flying bombing missions against Germany, it reinforced British efforts. The Allies attacked oil installations, and controversial firebombings took place against Hamburg (1943), Dresden (1945), and other German cities.
