Operation Sea Lion

Operation Sea Lion was the code name for Nazi Germany’s planned invasion of the United Kingdom during the summer and autumn of 1940. Conceived in the aftermath of the German victory in the Battle of France, it was intended as a contingency measure should Britain refuse to negotiate a peace settlement with Adolf Hitler. The operation never progressed beyond the planning stage, primarily because Germany failed to achieve the air and naval superiority deemed essential for a cross-Channel invasion.

Background

Following the collapse of France in June 1940, Hitler hoped that the British government would sue for peace. Before the fall of France he had made no preparations for amphibious warfare, and Germany lacked forces with experience in large-scale landings—unlike Japan, which had conducted such operations during the 1938 Battle of Wuhan. When Britain elected to continue the war, Hitler instructed the Wehrmacht High Command (OKW) to examine the feasibility of an invasion.
In September 1939 Germany’s invasion of Poland triggered declarations of war by France and Britain. Directive No. 6, issued on 9 October 1939, planned offensives against the Western Allies and sought to secure coastal positions vital for any future war against Britain. Within the Kriegsmarine, Grand Admiral Erich Raeder commissioned preliminary invasion studies, which identified major prerequisites: neutralising the Royal Navy, eliminating the Royal Air Force (RAF), protecting the invasion fleet from submarine attack and securing the approaches to British landing sites.
German studies in late 1939 and early 1940, including the Army’s Nordwest paper, envisaged an assault on England’s eastern coast using infantry, airborne forces and naval transport. The Kriegsmarine responded with detailed objections, emphasising its inability to challenge the Royal Navy and estimating that preparations would take at least a year. Hermann Göring’s Luftwaffe dismissed the concept outright, insisting that invasion would only be possible once Britain was already largely defeated.

The Fall of France and Renewed Invasion Discussions

Following the rapid victory in France in May–June 1940, Germany gained control of the Channel coast, placing Britain in immediate danger. Raeder met Hitler on 21 May and recommended a campaign of blockade and aerial attack rather than invasion. Losses during the Norwegian campaign (Weserübung) had severely depleted the Kriegsmarine: by late May it possessed only one heavy cruiser, two light cruisers and four destroyers suitable for operations.
Despite political moves within Britain urging peace negotiations, the British War Cabinet resolved to continue the fight. Germany nonetheless sought diplomatic pressure during July, while internally the OKW reassessed options to coerce Britain into submission.

Invasion Planning

On 30 June 1940, OKW Chief of Staff Alfred Jodl provided Hitler with a report outlining options to break British resistance. The immediate priority was destruction of the RAF to secure air supremacy; in the long term, intensified bombing could damage industry, morale and food supply. A full invasion would constitute a final blow once Britain was already effectively defeated.
Throughout early July, the Wehrmacht investigated practical requirements for Sea Lion. Meetings between senior Army and Navy officials revealed differing assumptions, but both services agreed that air superiority was essential. On 2 July the OKW formally requested preliminary planning after Hitler concluded that invasion was technically feasible under the right conditions. The Luftwaffe estimated on 11 July that it would require two to four weeks to defeat the RAF.
Meanwhile, Raeder continued to advocate an alternative strategy combining air and submarine warfare rather than a hazardous landing. Jodl’s memorandum of 12 July described the invasion as akin to a broad river crossing, provoking friction with the Kriegsmarine. On 13 July Hitler reviewed plans prepared by the Army but showed little interest in operational detail, merely instructing OKW to begin preparations.

Directive No. 16: Operation Sea Lion

On 16 July 1940 Hitler issued Führer Directive No. 16, ordering preparations for an invasion of Britain. The directive stated that, since Britain showed “no signs of willingness to come to terms”, Germany must prepare for a landing intended to eliminate the country as a base for continued resistance. The plan envisaged landing forces between Dover and the Isle of Wight, supported by airborne troops and a fleet of converted river barges, tugs and transport craft assembled along the Channel coast.
Hitler declared two essential preconditions:

• Air superiority over southern England and the Channel, and
• Naval superiority sufficient to protect the invasion fleet.

Neither condition was achieved at any time. The RAF resisted sustained Luftwaffe assaults during the Battle of Britain, inflicting mounting losses on German aircraft. The Royal Navy maintained overwhelming dominance in surface forces, and German mines, submarines and limited warships could not exclude it from the Channel.

Abandonment of the Operation

By September 1940 it was clear that the Luftwaffe had failed to defeat the RAF. On 17 September Hitler indefinitely postponed Operation Sea Lion. All preparations ceased shortly thereafter, and the invasion was never attempted.
The German Army and Navy had invested significant effort in preparations—training assault troops, modifying barges, adapting equipment and concentrating transport vessels along the coast—but the fundamental strategic requirements were unattainable. Hitler and the OKW increasingly viewed the invasion as impractical, doubting its prospects even under favourable circumstances.