Operation Sea Lion (German: Unternehmen Seelöwe) was Germany's plan to invade the United Kingdom during the Second World War, beginning in 1940. To have had any chance of success, however, the operation would have required air and naval supremacy over the English Channel. With the German defeat in the Battle of Britain, Sea Lion was postponed indefinitely on 17 September 1940 and never carried out.

By early November 1939, Adolf Hitler had decided on forcing a decision in the West by invading Belgium, the Netherlands and France. With the prospect of the Channel ports falling under Kriegsmarine (German Navy) control and attempting to anticipate the obvious next step that might entail, Grand Admiral (Großadmiral) Erich Raeder (head of the Kriegsmarine) instructed his operations officer, Kapitän Hans Jürgen Reinicke, to draw up a document examining "the possibility of troop landings in England should the future progress of the war make the problem arise." Reinicke spent five days on this study and set forth the following prerequisites: