A blackout during war, or in preparation for an expected war, is the practice of collectively minimizing outdoor light, including upwardly directed (or reflected) light. This was done in the 20th century to prevent crews of enemy aircraft from being able to identify their targets by sight, for example during the London Blitz of 1940. In coastal regions, a shoreside blackout of city lights also helped protect ships from being seen silhouetted against the artificial light by enemy submarines farther out at sea.
Plans to black out British coastal towns in the event of war were drawn up in 1913 by Winston Churchill in his role as First Lord of the Admiralty; these plans were implemented on 12 August 1914, eight days after the United Kingdom had entered the war. On 1 October 1914, the Commissioner of the Metropolitan Police ordered that bright exterior lights were to be extinguished or dimmed in the London area and street lamps be partially painted out with black paint. Elsewhere, the matter was left to local authorities. Following the start of the German strategic bombing campaign early in 1915, ordinary people in towns without blackouts sometimes took the law into their own hands, smashing street lamps which they thought might attract an air raid. Blackout restrictions were extended to the whole of England in February 1916.
In France, a blackout was implemented for Paris at the start of the Zeppelin campaign in the spring of 1915, but was later relaxed, only to be reintroduced in the spring of 1918 when the Germans began using heavy bombers against the city. In Germany, a blackout was only enforced in a zone 150 kilometres (93 miles) behind the Western Front.