Operation Sea Lion (German: Unternehmen Seelöwe) was the plan to invade the United Kingdom by the Nazis during World War II. The plan began in 1940. However, Germany first had to control the sky and sea of the English Channel before a land invasion. The Battle of Britain was a military campaign of the Second World War, in which the Royal Air Force and Fleet Air Arm of the Royal Navy defended the United Kingdom against large-scale attacks by Nazi Germany's air force, The Battle of the Atlantic, the longest continuous military campaign in World War II, ran from 1939 to the defeat of Nazi Germany in 1945, covering a major part of the Naval history of World War II. The purpose of this study is to describe the German campaigns in the Balkans and the seizure of Crete within the framework of Hitler's military policy during the second year of World War II. The study is the first of a series dealing with large-scale German military operations in Eastern Europe; other historical studies such as Germany and Finland--Allies and Enemies in World War II, The Axis Campaign in Russia, 1941-45: A Strategic Survey, and German Army Group Operations in Russia will follow.