Operation Sea Lion (German: Unternehmen Seelöwe) in OTL was Nazi Germany's plan to invade the United Kingdom during World War II, beginning in 1940. To have had any chance of success, however, the operation would have required air 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.

Operation Sea Lion has been discussed several times on various alternate history internet forums. Generally it is concluded that the chance of Operation Sea Lion succeeding are slim to none. For Operation Sea Lion to be successful, Germany would have needed control of the English Channel, air superiority, and resources to ship men and ammunition to the beachhead in Britain. Meanwhile the British defense would be intense and the Germans would lack the chance to use the Blitzkrieg when launching an amphibious assault.

In some timelines located in the multiverse, the plan was carried out:

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