Operation Sea Lion, also written as Operation Sealion, was Nazi Germany's code name for their planned invasion of the United Kingdom. It was to take place during the Battle of Britain, nine months after the start of the Second World War. Following the Battle of France and that country's capitulation, Adolf Hitler, the German Führer and Supreme Commander of the Armed Forces, hoped the British government would accept his offer to end the state of war between the two. He considered invasion to be a last resort, to be used only if all other options had failed.

Nazi Germany, officially known as the German Reich and later the Greater German Reich, is a term used to describe the German state between 1933 and 1945, when Adolf Hitler and the Nazi Party controlled the country, transforming it into a totalitarian dictatorship. The Third Reich, meaning "Third Realm" or "Third Empire", referred to the Nazi claim that Nazi Germany was the successor to the earlier Holy Roman Empire (800–1806) and German Empire (1871–1918). The Third Reich, which the Nazis referred to as the Thousand-Year Reich, ended in May 1945, after only 12 years, when the Allies defeated Germany and entered the capital, Berlin, ending World War II in Europe.