What was Operation Sealion?

Operation Sealion was the code name for Nazi Germany’s planned invasion of Britain. It was supposed to take place in September 1940 and, had it been successful, would have completed Adolf Hitler’s domination of western Europe.


In the preceding months, the German Army had already swept across much of the continent. Western Poland had fallen early on, in the autumn of 1939. Denmark and Norway had been defeated six months later, in the spring of 1940. Then came Belgium, the Netherlands and northern France in May and June. British troops on the mainland had also been defeated: at Dunkirk they had been forced to abandon their equipment and retreat back across the Channel. On paper, therefore, the invasion of Britain was the logical final step.

British troops returning after the evacuations at Dunkirk
At Dunkirk, British forces had retreated across the Channel. An invasion of Britain was the logical final step, says Keith Lowe. (Photo by Puttnam and Malindine/ Imperial War Museums via Getty Images)

How did Germany plan to invade Britain?

There were three aspects to the German invasion plan: the battle in the air, the battle at sea, and the amphibious assault on the British beaches.

The first and most important step was to neutralise the Royal Air Force: a cross-Channel invasion would be far too dangerous to attempt without first achieving command of the air. The head of the Luftwaffe, Hermann Goering, famously boasted that his planes could smash the RAF within just a few weeks. In mid-July, therefore, he launched a massive attack on British aircraft and airfields with the intention of destroying Britain’s capacity to defend itself. This was the beginning of the air battle that came to be known as the Battle of Britain.

The second prerequisite for an invasion was command of the sea. For the Germans this seemed like a far more daunting challenge. Though the Kriegsmarine had dozens of U-boats at its disposal, most of its big surface ships had already been sunk, damaged or worn out in the Norway campaign earlier in the year. Britain, by contrast, still had the largest navy in the world, which would in all likelihood destroy any invasion force even before it had the chance to land. The head of the Kriegsmarine, Grand Admiral Erich Raeder, therefore drew up plans to distract the Royal Navy with a decoy attack in the North Sea. Then, by laying vast minefields in the Channel, he hoped to be able to protect German forces just long enough for the invasion to take place.

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A British Spitfire aircrew 'scrambles', c1940
A British aircrew 'scrambles', c1940. Hermann Goering famously boasted that his Luftwaffe planes could smash the RAF within just a few weeks. (Photo by Hulton Archive/Getty Images)

The final part of the plan was the invasion itself. The Germans had no specialised landing craft of the sort that the Allies would use in the Normandy landings four years later, so they assembled hundreds of river barges instead, which they planned to tow across the sea with tug boats. The first wave onto the beaches would consist of the infantry, whose job would be to secure the beach heads. Next would come the horses, tanks and other vehicles, along with vital supplies to help with the break-out from the beaches. In the meantime, airborne troops would also drop behind the British lines to attack the defenders from the rear.

There were plans to distract the Royal Navy with a decoy attack in the North Sea

From the very beginning, there were major differences between the various German commanders about how to go about this mammoth undertaking. The chief of the army general staff, General Franz Halder, wanted to treat the invasion as if it were a mere river crossing: he imagined landing 13 divisions along 190 miles of coastline between Lyme Regis and Ramsgate. Grand Admiral Raeder scoffed at such a notion, arguing that the only way to get across the Channel safely was to concentrate their forces on a much narrower front between Eastbourne and Folkestone. Halder argued back that concentrating all his men on just a few beaches, like Raeder wanted, would be “complete suicide”.

German forces assembled hundreds of river barges
German forces assembled hundreds of river barges instead, which they planned to tow across the sea with tug boats. (Photo by RAF Official Photographer/ Imperial War Museums via Getty Images)

Why did the plan fail?

In the end these arguments were neither here nor there, because Germany failed to achieve any of their prerequisites for invasion. Despite Goering’s boasts, the Luftwaffe never managed to achieve command of the air. Against the might of the Royal Navy, winning command of the sea even for a short time also began to seem like a pipe dream. On 17 September, with the weather in the Channel becoming much more unpredictable, Hitler finally decided to postpone the invasion – indefinitely.

A map detailing Operation Sealion
A map detailing Operation Sealion, the proposed invasion of Britain by the Nazis. On 17 September, Hitler decided to postpone the invasion. (Photo by MPI/Getty Images)

It is unclear whether Operation Sealion was ever a serious plan, or whether it was merely a ploy to put pressure on the British to capitulate. Hitler’s ultimate aim had always been to invade the Soviet Union. He much preferred to do so without having to worry about fighting Britain at the same time – but when it became clear that the British were not going to seek terms, he dropped his invasion plans and concentrated on his real objectives in the east.

What might have happened if Germany had invaded Britain?

People have been asking this question ever since the war. Dramatists have depicted every possible outcome of a German invasion, from the heroic resistance of the 1942 film Went the Day Well? to the murkier world of collaboration depicted in the 2017 BBC drama SS-GB.

In reality, however, it is extremely unlikely that any invasion would have got much further than the beaches and their immediate surroundings. When the Allies invaded Normandy in 1944, they had almost total control of both the air and the sea – and yet they still struggled to break out of their beachheads. After the war, German Grand Admiral Karl Doenitz admitted in his memoirs that, “I myself had no faith in the success of this invasion,” and claimed that Raeder agreed with him.

In 1974, the Royal Military Academy at Sandhurst war-gamed Operation Sealion, and came to the same conclusion: while German troops might well have gained a small foothold on the coast, before long they would probably have been forced to surrender.


Keith Lowe is the author of The Fear and the Freedom: Why the Second World War Still Matters(Penguin, 2018) and the international bestseller Savage Continent, which won the PEN/Hessell-Titlman Prize and Italy’s Cherasco History Prize. His latest book, Prisoners of History, is published on 9 July 2020. You can find him on Twitter @KeithLoweAuthor.