Operation Sea Lion (1940): Hitler's Planned Invasion of Britain

Operation Sea Lion (in German, Unternehmen Seelöwe) was Nazi Germany’s plan to invade Britain in the late summer of 1940. After the fall of France, Hitler ordered preparations for a landing on the south coast of England, but the plan depended on the Luftwaffe winning control of the skies. When that failed in the Battle of Britain, the invasion was postponed indefinitely and never carried out.

Background: a war Hitler expected to end

By late June 1940 Germany had defeated Poland, Norway, the Low Countries and France. Hitler hoped Britain would now accept peace terms. When Winston Churchill’s government refused, he turned reluctantly to the idea of invasion, an operation the German armed forces had barely studied before the war.

On 16 July 1940 Hitler issued Directive No. 16. It ordered preparations for a landing in England, to be ready by mid-August if necessary. The directive made clear that the British air force had to be beaten down first.

The plan

The army, under Field Marshal Gerd von Rundstedt’s Army Group A, first wanted a broad landing along much of the south coast, from Kent to Dorset. The navy, led by Grand Admiral Erich Raeder, said it could not protect such a wide front. After weeks of argument, the plan was narrowed to landings mainly between Folkestone and the area east of Brighton, with the Ninth and Sixteenth Armies.

The first wave was to include about nine divisions, with paratroops landing behind the coast. Because Germany had no proper landing craft, the navy gathered around 2,000 or more river barges, along with tugs, motorboats and transports. Many barges had to be towed in pairs across the Channel at walking pace, and some were fitted with ramps or concrete floors for tanks and guns.

Why the plan depended on the air

Germany’s surface fleet had been badly weakened in the Norwegian campaign, including the loss of ten destroyers at Narvik. The Royal Navy’s Home Fleet remained far stronger. Only if the Luftwaffe could drive the RAF from the skies and keep British warships away from the Channel could a crossing be attempted.

Hermann Göring’s air force therefore began its main offensive against the RAF in August. The Battle of Britain became, in practice, the decisive preliminary to Sea Lion.

British preparations

Britain prepared on the assumption that invasion was likely. Coastal defences, anti-tank obstacles and lines of pillboxes were built, and volunteers joined the British Home Guard. Bomber Command and the navy attacked the barge concentrations in the Channel ports, sinking or damaging a significant minority of them.

On 7 September 1940 the code word “Cromwell” was issued, warning that invasion conditions existed. In some areas it was misread as a signal that invasion had begun, and church bells were rung.

Postponement

The Luftwaffe did not gain air superiority. After heavy losses on 15 September, now marked as Battle of Britain Day, Hitler postponed Sea Lion indefinitely on 17 September. The barges began to disperse. In October the operation was put off until the spring of 1941, but by then Hitler’s attention had turned to the invasion of the Soviet Union, and Sea Lion was never revived in earnest.

Could it have worked?

Historians have debated Sea Lion’s chances ever since. Most conclude that even a temporary German success in the air would not have solved the naval problem. The Royal Navy had dozens of destroyers and cruisers within reach of the Channel, while the slow, unseaworthy barge convoys would have needed calm weather for days on end. Supplying troops once ashore would have been harder still.

In 1974 the Royal Military Academy Sandhurst staged a war game of the invasion, with senior German and British veterans acting as umpires. The landing force secured beachheads but was cut off by British naval forces and defeated within about a week. A game is not proof, but its outcome matched the judgement of most German naval officers in 1940.

Myths and facts

It is sometimes said that Sea Lion was never seriously meant and was only a bluff. In fact, preparations were real and costly, and German planners drew up detailed orders, maps and even a list of people in Britain to be arrested after an occupation. Most historians agree, however, that Hitler was never enthusiastic and that the operation’s chances of success were very low.

Timeline

• 1940, 16 July: Hitler issues Directive No. 16 ordering preparations for an invasion.
• 1940, August: Army and navy argue over the width of the landing front.
• 1940, 13 August: The Luftwaffe begins its main offensive against the RAF.
• 1940, September: Barges and transports gather in Channel ports; RAF bombers attack them.
• 1940, 7 September: The British issue the “Cromwell” invasion alert.
• 1940, 15 September: Major air battles over London and southern England.
• 1940, 17 September: Hitler postpones Sea Lion indefinitely.
• 1940, 12 October: The operation is deferred until spring 1941.

Why it matters

Operation Sea Lion marks the moment the German advance across Western Europe stopped at the Channel. Its abandonment meant that Britain stayed in the war as a base from which the Allies would later return to the continent.