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.