SEALION was the operational name of the German Wehrmacht's plan to land an army in the United Kingdom during the Second World War. Due to difficulties in securing air superiority over the English Channel and the Straits of Dover, as well as over Kent and southeastern Britain, and the lack of landing craft and overall inadequacy of the German navy, Fuhrer and Reichschancellor Adolf Hitler called off the invasion in late 1940.
The invasion of Britain in the summer of 1940 posed one of the greatest and overused alternate history scenarios of modern times. Since it presents an obvious POD with some very large changes, it's usually one of the first AH ideas a noob entering the field would come up with. As a result, various threads on SEALION have been started on AH forums across the internet; this in addition to the many thousands of pages in books and on web sites devoted to the plans and the actual implementation of SEALION.
There have been serious attempts though to prove if SEALION could be successful or not. The Royal Military Academy Sandhurst conducted wargames in 1974, which assumed the Luftwaffe had not yet won air supremacy, the Germans were able to establish a beachhead in England by using a minefield screen in the English Channel to protect the initial assault. However, the German ground forces were delayed by a layered series of defensive positions that had been built, each a combination of British Home Guard troops and physical barriers. At the same time, the regular troops of the British Army were forming up. After only a few days, the Royal Navy was able to reach the Channel from Scapa Flow, cutting off supplies and blocking further reinforcement. Isolated and facing regular troops with armour and artillery, the invasion force was made to surrender.
The scenario done by Sandhurst, however, did not carry out any exercises on the assumption if the Luftwaffe had won air supremacy.