The RAF underwent rapid expansion following the outbreak of war against Nazi Germany in 1939. This included the training of British aircrews in British Commonwealth countries under the British Commonwealth Air Training Plan, and the secondment of many whole squadrons, and tens of thousands of individual personnel, from Commonwealth air forces. For example, by the end of the war, Royal Canadian Air Force personnel had contributed more than 30 squadrons to service with RAF formations; almost a quarter of Bomber Command's personnel were Canadian.[20] Similarly, about nine percent of the personnel who served with the RAF in Europe and the Mediterranean were seconded from the Royal Australian Air Force.[21] To these and other British Commonwealth personnel were later added thousands of men from other countries, including many who had fled from German-occupied Europe.[22]
The main RAF effort during the war was the strategic bombing campaign against Germany. From 31 May 1942 RAF Bomber Command was able to mount large-scale night raids, sometimes involving up to 1,000 aircraft. From mid-1942 increasing numbers of these aircraft were heavy four-engined bombers such as the Handley-Page Halifax and the Avro Lancaster. Noteworthy raids include Operation Millennium against Cologne, the first 1000-bomber raid; Operation Chastise, the 'Dambusters' raids on targets in the Ruhr Valley; Operation Gomorrah, the destruction of Hamburg; and the 'Battle of Berlin'. The lighter, fast two-engine de Havilland Mosquito fighter-bomber was used for tactical raids like Operation Carthage, a raid on the Gestapo headquarters in Copenhagen, as well as a night-fighter.[25]
