Hitler launched "Operation Sea Lion," his plan to invade Britain. To succeed, the Royal Air Force had to be defeated. In July 1940, the Luftwaffe started a massive bombing campaign, aimed at destroying harbours and shipping faciliies in southern England. On August 24, German planes bombed several areas of London. In retaliation, the RAF bombed Berlin. Hitler then oredered the Luftwaffe to bomb London and other Brithish cities, this became known as "the Blitz".