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, such as 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.