In the summer of 1940, the UK faced the imminent threat of invasion by Nazi Germany. After its rapid victories across Europe, including the dramatic evacuation of British forces from Dunkirk, Adolf Hitler sought to eliminate Britain’s resistance and expand German control.

Operation Sealion (Seelöwe) was the code name for Germany’s planned invasion of Britain. Although the operation was meticulously planned, it never came to fruition, leaving it one of the greatest “what if” scenarios of WWII.

The Plan for Invasion

In July 1940 Hitler issued Directive No. 16 initiating the planning for Operation Sealion, following Britain’s rejection of a final offer for a negotiated peace settlement.

“Since England, in spite of her hopeless military situation, shows no signs of being ready to come to a compromise, I have decided to prepare a landing operation against England, and, if necessary, to carry it out.” Adolf Hitler in July 1940

Hitler’s goal was clear: neutralise Britain before its military could recover from its defeat in France. He aimed to land German forces on British soil by mid-August 1940. The German navy (Kriegsmarine) quickly began assembling a vast invasion fleet composed of nearly 2,000 barges, tugs, trawlers, and merchant ships, primarily at ports like Calais and Rotterdam. The invasion was to bring four divisions across the Channel initially, with five more following shortly thereafter.

For the invasion to succeed, the Germans needed to dominate both the air and the sea, however, Hitler had given his forces little over a month to make their preparations.

Hermann Göring, head of the Luftwaffe, believed his forces could defeat the Royal Air Force (RAF) within weeks, paving the way for the invasion fleet. Meanwhile, Hitler’s army over-simplified the realities of a combined amphibious and airborne invasion.

The Battle of Britain and The Royal Navy

In August 1940, the Luftwaffe launched a massive air assault on Britain, in what would become known as the second phase of the Battle of Britain.

Fought from July to October 1940, the Battle of Britain saw the Luftwaffe attempt to gain air superiority over the RAF in preparation for Operation Sealion. Despite extensive attacks on airfields and radar installations, the RAF’s Fighter Command, aided by advanced radar technology, put up a fierce defence and prevented the Luftwaffe from achieving its goal.

In a letter directed to Hitler, sourced from the Federal Archives for Germany reporting on the air raids over the channel, it was stated that the cost of losses to soldiers was disproportionally high. Furthermore, the damage had been significantly aggravated several times by hits on German ammunition depots.

By the end of October, the Luftwaffe’s failure to achieve air superiority became a pivotal turning point in the war.

Without control of the skies, any cross-Channel invasion was too dangerous to proceed. Furthermore, the British Navy, the largest in the world at the time, posed an even greater threat to the German invasion fleet. Grand Admiral Erich Raeder knew that, without air dominance, any invasion attempt would likely end in disaster.

Operation Sealion was postponed several times throughout August and September on the grounds that the essential conditions did not exist. Due to the nature of their plans, there were specific windows the invasion had to take place to coincide with tide and moonlight levels. If the invasion was postponed into the autumn, it would risk being impacted by bad weather.

Eventually, Operation Sealion was postponed indefinitely. Though Hitler never officially cancelled the operation, it became increasingly clear that the logistics and conditions for a successful invasion simply didn’t exist. Instead, Hitler shifted his focus toward his next major target: the Soviet Union.

German Junkers 88 brought down near Mallwyd on a raid to Liverpool duing Operation Sealion
German Junkers 88 brought down near Mallwyd on a raid to Liverpool

The UK’s Defensive Preparations

While the Germans prepared for their invasion, Britain was fortifying its defences. After Dunkirk, fearing an imminent invasion, the British government implemented extensive anti-invasion measures. Coastal defences were bolstered with pillboxes, tank traps, and minefields. The beaches were fortified with barbed wire, and inland stop lines, defensive lines designed to slow the progress of enemy forces, were established.

Britain also formed the Home Guard, which was composed of volunteers who were either too young, too old, or otherwise exempt from regular military service. They were tasked with protecting the country from potential invasion, and by the end of June 1940, over 1 million people had signed up. Despite being poorly equipped with shotguns, old hunting rifles, bayonets and an array of improvised weapons at the start, the Home Guard became a vital part of Britain’s defence network.

Additionally, the British established minefields in strategic areas, including beaches and airfields, further complicating any invasion plans.

Despite extensive post-war clearance efforts, these mines and other ordnance left behind have created long-term UXO risks. The legacy of these defensive measures is still managed today, underscoring the importance of companies like Brimstone UXO in addressing such hazards.

Home Guard parade

The Aftermath and Lessons

Although Operation Sealion never materialised, its effects on both German and British strategies were significant. The Luftwaffe’s defeat in the Battle of Britain marked a major failure of Hitler’s war machine and bolstered British morale. For the Germans, the postponement of Operation Sealion reflected the challenges of planning and executing a complex amphibious invasion, especially without clear command over the air and sea.

Today, historians largely agree that Operation Sealion was unlikely to succeed, even if it had been attempted. The logistical challenges, coupled with Britain’s formidable defences and naval superiority, made the operation a gamble that Hitler was ultimately unwilling to take.

Addressing UXO Risks

The story of Operation Sealion offers a fascinating glimpse into a pivotal moment of WWII. It was a time when Britain, though beleaguered, stood firm against an imminent invasion, and through its preparations and determination, managed to hold off one of the world’s most powerful militaries.

For companies like Brimstone UXO, addressing the legacy of wartime ordnance remains critical. Items of UXO from WWII continue to be uncovered on construction and development sites across the country, and our expertise ensures that these hazards are safely identified and mitigated. Whether your project faces potential UXO risks, or you want to ensure a safe working environment, Brimstone UXO is here to help.

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Thank you to The National Library of Wales for the images used in this blog.