In the summer and autumn of 1940, Germany's Luftwaffe conducted thousands of bombing runs, attacking military and civilian targets across the United Kingdom. Hitler's forces, in an attempt to achieve air superiority, were preparing for an invasion of Britain code-named "Operation Sea Lion." At first, they targeted only military and industrial targets. But after the Royal Air Force hit Berlin with retaliatory strikes in September, the Germans began bombing British civilian centers. Some 23,000 British civilians were killed in the months between July and December 1940. Thousands of pilots and air crews engaged in battle in the skies above Britain, Germany, and the English Channel, each side losing more than 1,500 aircraft by the end of the year. Prime Minister Winston Churchill, speaking of the British pilots in an August speech, said, "Never in the field of human conflict was so much owed by so many to so few." The British defenses held, and Operation Sea Lion was quietly canceled in October, though bombing raids continued long after. Workmen fit a set of paraboloids in a sound detector for use by anti-aircraft batteries guarding England, in a factory somewhere in England, on July 30, 1940. A Nazi Heinkel He 111 bomber flies over London in the autumn of 1940. The Thames River runs through the image. A ninety minute exposure taken from a Fleet Street rooftop during an air raid in London, on September 2, 1940. The searchlight beams on the right had picked up an enemy raider. The horizontal marks across the image are from stars and the small wiggles in them were caused by the concussions of anti-aircraft fire vibrating the camera. The German pilot released a flare, which left a streak across the top left, behind the steeple of St. Bride's Church.