World War II: Operation Sea Lion

Barges intended for Operation Sea Lion
Invasion barges assembled at the German port of Wilhelmshaven. Deutsches Bundesarchiv

Operation Sea Lion was the German plan for the invasion of Britain in World War II (1939-1945) and was planned for sometime in late 1940, after the Fall of France.


With the German victory over Poland in the opening campaigns of World War II, leaders in Berlin commenced planning for fighting in the west against France and Britain. These plans called for the capture of ports along the English Channel followed by efforts to force Britain's surrender. How this was to be accomplished quickly became a matter of debate among the German military's senior leadership. This saw Grand Admiral Erich Raeder, commander of the Kriegsmarine, and Reichsmarschall Hermann Göring of the Luftwaffe both argue against a seaborne invasion and lobby for various types of blockades aimed at crippling the British economy. Conversely, the army leadership advocated for landings in East Anglia, which would see 100,000 men put ashore.

Raeder countered this by arguing that it would take a year to assemble the shipping required and that the British Home Fleet would need to be neutralized. Göring continued to argue that such a cross-channel effort could only be made as "final act of an already victorious war against Britain." Despite these misgivings, in the summer of 1940, shortly after Germany's stunning conquest of France, Adolf Hitler turned his attention to the possibility of an invasion of Britain. Somewhat surprised that London had rebuffed peace overtures, he issued Directive No. 16 on July 16 which stated, "As England, in spite of the hopelessness of her military position, has so far shown herself unwilling to come to any compromise, I have decided to begin to prepare for, and if necessary to carry out, an invasion of England...and if necessary the island will be occupied."

For this to succeed, Hitler laid out four conditions that had to be met to ensure success. Similar to those identified by German military planners in late 1939, they included elimination of the Royal Air Force to ensure air superiority, clearing of the English Channel of mines and the laying of German mines, the emplacing of artillery along the English Channel, and preventing the Royal Navy from interfering with the landings. Though pushed by Hitler, neither Raeder or Göring actively supported the invasion plan. Having taken serious losses to the surface fleet during the invasion of Norway, Raeder came to actively oppose the effort as the Kriegsmarine lacked the warships to either defeat the Home Fleet or support a crossing of the Channel.

German Planning

Dubbed Operation Sea Lion, planning moved forward under the guidance of Chief of the General Staff General Fritz Halder. Though Hitler had originally desired to invade on August 16, it was soon realized that this date was unrealistic. Meeting with planners on July 31, Hitler was informed that most desired to postpone the operation until May 1941. As this would remove the political threat of the operation, Hitler refused this request but agreed to push Sea Lion back until September 16. In the early stages, the invasion plan for Sea Lion called for landings on a 200-mile front from Lyme Regis east to Ramsgate.

This would have seen Field Marshal Wilhelm Ritter von Leeb's Army Group C cross from Cherbourg and land at Lyme Regis while Field Marshal Gerd von Rundstedt's Army Group A sailed from Le Havre and the Calais area to land the southeast. Possessing a small and depleted surface fleet, Raeder opposed this broad front approach as he felt it could not be defended from the Royal Navy. As Göring began intense attacks against the RAF in August, which developed into the Battle of Britain, Halder vehemently attacked his naval counterpart, feeling that a narrow invasion front would lead to heavy casualties.

The Plan Changes

Bowing to Raeder's arguments, Hitler agreed to narrow the scope of the invasion on August 13 with the westernmost landings to be made at Worthing. As such, only Army Group A would take part in the initial landings. Composed of the 9th and 16th Armies, von Rundstedt's command would cross the Channel and establish a front from the Thames Estuary to Portsmouth. Pausing, they would build up their forces before conducting a pincer attack against London. This taken, German forces would advance north to around the 52nd parallel. Hitler assumed that Britain would surrender by the time his troops reached this line.

As the invasion plan continued to be in flux, Raeder was plagued by a lack of purpose-built landing craft. To remedy this situation, the Kriegsmarine gathered around 2,400 barges from around Europe. Though a large number, they were still insufficient for the invasion and could only be used in relatively calm seas. As these were gathered in the Channel ports, Raeder continued to be concerned that his naval forces would be insufficient to combat the Royal Navy's Home Fleet. To further support the invasion, a myriad of heavy guns were emplaced along the Straits of Dover.

British Preparations

Aware of German invasion preparations, the British began defensive planning. Though a large number of men were available, much of the British Army's heavy equipment had been lost during the Dunkirk Evacuation. Appointed Commander-in-Chief, Home Forces in late May, General Sir Edmund Ironside was tasked with overseeing the island's defense. Lacking sufficient mobile forces, he elected to construct a system of static defensive lines around southern Britain, which were backed by the heavier General Headquarters Anti-tank Line. These lines were to be supported by a small mobile reserve.

Delayed and Cancelled

On September 3, with British Spitfires and Hurricanes still controlling the skies over southern Britain, Sea Lion was again postponed, first to September 21 and then, eleven days later, to September 27. On September 15, Göring launched massive raids against Britain in an attempt to crush Air Chief Marshal Hugh Dowding's Fighter Command. Defeated, the Luftwaffe took heavy losses. Summoning Göring and von Rundstedt on September 17, Hitler indefinitely postponed Operation Sea Lion citing the Luftwaffe's failure to obtain air superiority and a general lack of coordination between the branches of the German military.

Turning his attention eastward to the Soviet Union and planning for Operation Barbarossa, Hitler never returned to the invasion of Britain and the invasion barges were ultimately dispersed. In the years after the war, many officers and historians have debated whether Operation Sea Lion could have succeeded. Most have concluded that it likely would have failed due to the strength of the Royal Navy and the Kriegsmarine's inability to prevent it from interfering with the landings and subsequent re-supply of those troops already ashore.


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Your Citation
Hickman, Kennedy. "World War II: Operation Sea Lion." ThoughtCo, Apr. 5, 2023, Hickman, Kennedy. (2023, April 5). World War II: Operation Sea Lion. Retrieved from Hickman, Kennedy. "World War II: Operation Sea Lion." ThoughtCo. (accessed June 6, 2023).