During the Battle of Britain in 1940, the RAF defended the skies over Britain against the numerically superior German . In what is perhaps the most prolonged and complicated air campaign in history, the Battle of Britain contributed significantly to the delay and subsequent indefinite postponement of Operation Sea Lion, Hitler's plans for an invasion of the UK. In the House of Commons on 20 August, prompted by the ongoing efforts of the RAF, Prime Minister Winston Churchill made a speech to the nation, where he said "Never in the field of human conflict was so much owed by so many to so few".The largest RAF effort during the war was the strategic bombing campaign against Germany by Bomber Command. While RAF bombing of Germany began almost immediately upon the outbreak of war at first it was ineffectual; it was only later, particularly under the leadership of Air Chief Marshal Harris, that these attacks became increasingly devastating, from early 1943 onward, as new technology and greater numbers of superior aircraft became available. The RAF adopted night-time area bombing on German cities such as Hamburg and Dresden. Night time area bombing constituted the great bulk of the RAF's bombing campaign, mainly due to Harris, but it also developed precision bombing techniques for specific operations, such as the "Dambusters" raid by No. 617 Squadron, or the Amiens prison raid known as Operation Jericho.