Operation Sealion (Unternehmen Seelöwe) was Nazi Germany’s code name for the plan for an invasion of England during the Battle of Britain. Historically, the Royal Air Force successfully defended England and precluded the invasion, but it was a close call. Events and military choices might have driven the RAF from southern English skies and permitted the invasion to be carried out. Sealion is a two-player wargame simulating a hypothetical German invasion of England in September of 1940. The British have lost the aerial Battle over Britain. However, the RAF has not been completely vanquished, and the Royal Navy’s Home Fleet can still present a credible threat to the German landings and supply lines in the English Channel.

The game incorporates an unabstracted level of detail, including air and naval operations as well as all facets of land operations (operational and strategic). On the ground, the Germans have operational superiority, but the British Army can rely on their interior lines of communication and defensible terrain, including a crude network of hastily constructed fortifications. The units of maneuver are primarily divisions and brigades/regiments. Air units are Flieger divisions and groups. Naval units are flotillas and named capital ships representing each named ship plus escorts. Each game turn represents one to three days depending on the tempo of battle.