Grand Admiral (Großadmiral) Erich Raeder of the German Navy (Kriegsmarine) oversaw numerous studies for a German naval assault across the English Channel. The earliest of these, made around November 1939, identified the conditions for invasion:
The German Army High Command (Oberkommando des Heeres, or OKH) originally planned an invasion on a vast scale, extending along most of the English Channel, from Dorset to Kent. This was far in excess of what the Navy could supply transportation for and final plans were more modest, calling for nine divisions to land by sea with around 67,000 men in the first echelon and an airborne division to support them. The chosen invasion sites ran from Rottingdean in the west to Hythe in the east.
The battle plan called for German forces to be launched from Cherbourg to Lyme Regis, Le Havre to Ventnor and Brighton, Boulogne to Eastbourne, Calais to Folkestone, and Dunkirk and Ostend to Ramsgate. German paratroopers would land near Brighton and Dover. Once the coastline was secured, they would push north, taking Gloucester and encircling London. There is reason to believe that the Germans would not attempt to assault the city but besiege it, and bombard it. German forces would secure England up to the 52nd parallel (approximately as far north as Northampton), anticipating that the rest of the United Kingdom would then surrender.
Adolf Hitler's initial warning order on 16 July 1940, reflected the most current thinking and set out the revised minimum pre-conditions. He prefaced his order by stating: "I have decided to prepare a landing operation against England and, if necessary, to carry it out".
Hitler's conditions for invasion were:
This placed responsibility for Sealion's success on the shoulders of Naval High Command (Oberkommando der Marine, or OKM) Grand Admiral (Großadmiral) Erich Raeder and Air Force High Command (Oberkommando der Luftwaffe, or OKL) Imperial Marshal (Reichsmarschall) Herman Göring.
Italian dictator Benito Mussolini offered to send Italian troops to participate in the projected invasion, but Hitler declined his offer. However, the Italian Air Corps (Corpo Aereo Italiano, or CAI) did participate towards the end of the Battle of Britain.
The aerial battles which resulted from Unternehmen Adler (Operation Eagle) later became known as the Battle of Britain. Adler's objective was for the Luftwaffe to achieve air superiority over the Royal Air Force and allow the German invasion fleet to cross the English Channel. However, the change in emphasis of the bombing from RAF Bases to bombing London turned Adler into a strategic bombing operation. This switch afforded the RAF, reeling from Luftwaffe attacks on its bases, time to pull back and regroup.
British intelligence erroneously believed the Luftwaffe had a 4:1 advantage in aircraft. This led to the Royal Air Force mobilising the last of its reserves and accelerating the rate of Spitfire production.
The French fleet, one of the most powerful and modern in the world, might have tipped the balance against Britain if operated by the Kriegsmarine. The destruction of the French fleet by the British during the attack on Mers-el-Kébir, as well as the scuttling of the French fleet in Toulon two years later, ensured that this could never occur.
Even had the Royal Navy been neutralised, the chances of making a successful amphibious invasion across the channel were remote. The transport ships to be used by the Germans for landing were primarily river barges since the Germans had no specialised landing craft. This would have limited the quantity of artillery and tanks that could have been transported, and restricted operations to times of good weather. The barges were not designed for the open sea in any event, even with almost perfect conditions progress would have been slow and the craft would have been very vulnerable to attack. Nor were there enough barges to transport the first invasion wave and the following waves and their equipment. Without specialised landing craft, the Germans would have needed to immediately capture one of the ports, an unlikely situation considering the strength of the British coastal defences around the south-eastern harbours at that time. The British also had several contingency plans, including the planned use of poison gas, which would have made an invasion even more difficult.
It was only a postponement at that stage. Prototypes of two designs of pefabricated jetty, similar in function to Mulberry Harbours, were built and successfully overwintered in the North Sea in 1941/42. After cancellation, they were installed on the Island of Alderney, where they remained until being demolished in 1978.
Not until 13 February 1942, after the disaster on the Russian front, were forces earmarked for the operation released to other duties.
The postponement coincided with a rumour that there had been an attempt to land on British shores at Shingle Street, but it had been repulsed with large German casualties. This was reported in the American press but was officially denied. British papers, declassified in 1993, have suggested this was a successfully-engineered example of British black propaganda to improve morale in Britain, America and occupied Europe.
Military historians are divided on whether Operation Sealion might have succeeded; some such as Michael Burleigh, Andrew Mollo, and Kenneth Macksey believe success was possible, others such as Peter Fleming, Derek Robinson and Stephen Bungay believe the operation would have most likely resulted in a disaster for the Germans if attempted. Adolf Galland, commander of Luftwaffe fighters at the time, claimed invasion plans were not serious and that there was a palpable sense of relief in the Wehrmacht when it was finally called off.
There were a number of errors in German intelligence, and whilst some of these might not have caused problems there were others (such as the inclusion of bridges that no longer existed or mis-understanding the usefulness of minor British roads) that would have been detrimental to German operations, and would have only added to the confusion caused by the layout of Britain's cities and the removal of road signs.
In wargames conducted at the Royal Military Academy Sandhurst 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 at the "Stop Lines" (e.g., the GHQ Line), 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.
Alberto Cavalcanti's 1942 film Went the Day Well? is centred on a German reconnaissance mission for Sealion being eventually repulsed by the efforts of the civilian population of a remote village. A less contemporary dealing with the fear of German invasion concerned the German paratroopers central to the plot of the 1971 film Dad's Army who have crashed with photographs vital to the invasion.