In September of 1940 the German air force, the Luftwaffe, received orders to wipe out British cities. The Blitz began on 7th September with bombers attacking the London docks. Throughout the Blitz it was often the poor of the East End near the docks who suffered most, with loss of life and homes.
The Luftwaffe continued to Blitz London throughout the rest of 1940. Major events included the Battle of Britain, in which RAF fighter pilots fought with the Luftwaffe in the air above London.
The night of December 29th saw a giant raid that became known as The Second Great Fire of London. During that night some 24,000 incendiary bombs destroyed large parts of London. Hundreds hit St Paul's Cathedral. When Churchill heard that the great landmark of London was in danger he told the fire brigade "At all costs save St Paul's!" Although St Paul's continued to be hit, the fire brigade managed to keep the damage down to a minimum.
Londoners did all they could to protect themselves from injury. Many children were evacuated to the countryside. Blackouts were enforced to make it difficult for the Luftwaffe to navigate. Gas masks were provided and Anderson air raid shelters were built (made from corrugated metal, they could be easily built in the back garden). Originally the authorities were opposed to the use of underground stations as shelters because they feared the outbreak of disease in the close confines, and they also didn't think that many of the stations were deep enough to offer protection. The public however, insisted on being allowed down into the stations and many people used them nightly. The authorities were sadly proved right on a number of occasions when bombs made direct hits on stations, and killed many of those sheltering below.
Churchill and his team planned the military strategy from a reinforced basement in the Cabinet War Rooms, which are now open to the public. The Blitz continued until May 1941, when the much-weakened Luftwaffe turned their attention to the invasion of Russia.
During the Blitz, over 20,000 Londoners died and 1.5 million homes were destroyed or damaged. The Blitz was not, of course, a one sided affair. Britain and the allies were responsible for a much worse Blitz on the citizens of Germany. Allied raids on Dresden, for instance, destroyed 80% of the city, and the death toll was far greater.
Many of London's great buildings were badly damaged during the Blitz including St Bride's, St Mary-le-Bow, The British Museum, The Tower of London and Westminster Abbey. More details and fascinating reconstructions of the war years can be found at the Britain at War Exhibition and the Museum of London.