“The goal was to land nearly a quarter of a million German soldiers along the British coast within the first two weeks of the assault.”

By Robert Schreiner

MOST students of Second World War history know how perilously close Allied forces came to total defeat in just the first few months of the war in Europe. Less well known was Hitler’s ominous plan to invade the British Isles – to bring the war directly to his greatest adversary, conquer Britain on her home soil, and complete the Nazi conquest of Western Europe.

In early 1940, Allied forces in Europe were falling back in the face of Hitler’s seemingly unstoppable war machine. German blitzkrieg attacks – lightning-fast armored and mechanized infantry assaults supported by overwhelming air operations – had swiftly conquered almost all of Western Europe. The previous September and October, the Nazis had defeated Poland in just over a month. In April 1940, Germany invaded Denmark and Norway, then turned westward. German victories came quickly. Denmark fell in just six hours, and the Netherlands and Belgium were defeated in less than three weeks. By early June, Norway had fallen, and reeling Allied British and French forces were being steadily pushed back, as the Wehrmacht swiftly consumed mile after mile of French territory.

Only days after Britain’s desperate evacuation from the French port of Dunkirk, Paris fell to German forces. Two weeks later, France surrendered, which effectively left Great Britain as the only remaining member of the “Allies” in Europe. Hitler initially hoped that the British would agree to peace negotiations with Germany. When it became clear that Britain would not capitulate, Hitler’s focus turned to bringing the war across the English Channel, directly to this stubborn enemy. Pursuing and destroying any surviving remnants of a defeated army is one of the oldest principles of warfare. Nazi Germany had soundly defeated Britain and France on the continent, and what was left of the British army had retreated across the English Channel, only 21 miles (34 kilometers) away.

On July 16, 1940, Hitler issued an order to his commanders to begin preparations for Operation Sea Lion or Unternehmen Seelöwe, a full military invasion of Britain to take place by mid-August 1940. It was to be a massive, combined arms undertaking, involving elements of the German army, the Kriegsmarine, and the Luftwaffe. The initial versions of the plan called for 25 to 40 divisions of German infantry and armor to land on the southern coast of England and then move swiftly inland – a resumption of the blitzkrieg, but this time on British soil.

The first phase of the plan would require the Luftwaffe to neutralize the Royal Air Force to establish complete air supremacy over the Channel and southern Britain. Such a seaborne invasion could not be carried out successfully if German forces attempting to land were under constant attack from the air. At this point, Germany had every reason to believe that the Luftwaffe was invincible. In the preceding months, its warplanes had dominated the skies over European battlefields with near impunity. In fact, Reichsmarshall Hermann Göring boldly predicted that the Luftwaffe could utterly destroy Britain’s air force in only four weeks.

Germany’s secondary – but nearly equal – concern for the success of Sea Lion was to keep and maintain an open seaborne route between German assembly ports in France and Belgium and the landing zones along the British coastline. However, the Kreigsmarine was no match for the Royal Navy, which made a sustained German naval blockade an impossibility. Instead, the plan called for the Kreigsmarine to clear the English coast of any British coastal mines while laying defensive “curtains” of German minefields to prevent Royal Navy ships from entering the Channel to attack the invasion fleet.

The first wave of the invasion itself would comprise German infantry units delivered by barges and smaller boats to multiple points along the southern English coastline. Supported by airborne troops, these initial infantry units would take and hold beachheads in preparation for subsequent waves, which would include field artillery, armored vehicles, equipment, fuel, and supplies delivered on larger transport vessels. The goal was to land nearly a quarter of a million German soldiers along the British coast within the first two weeks of the assault. In the months to follow, another half million or more men would be ferried across, as Germany pressed their ground assault northward into the heart of Britain, under Luftwaffe air cover.

In August 1940, Germany began to assemble an immense invasion fleet at French and Belgian ports. More than 2,000 river barges of various sizes were gathered from multiple countries, and engineers began to convert them to haul soldiers and equipment. The Germans brought in hundreds of support vessels, including tugboats that would drag the converted barges to their landing zones. The Kriegsmarine designated multiple U-boats, destroyers and torpedo boats as escorts for the invasion flotilla. Concurrently, German intelligence began a complex misinformation campaign intended to convince the British military that the invasion was planned for Britain’s east coast and that German agents and saboteurs were already in place throughout Britain.

It’s worth noting that at this point in history, amphibious assaults were not yet a well-developed component of modern warfare. Dedicated landing craft, as were used four years later by Allied forces at Normandy, had yet to be developed – thereby necessitating the use of converted barges and tugboats. Additionally, the German military had previously conducted only one minor armed amphibious landing, when it captured several small, poorly-defended Baltic islands from Russia in the First World War. An armed invasion of Britain was a vastly larger undertaking – and the scale of logistical planning and operational coordination required to make Sea Lion a success had never even been attempted by any nation’s military.

More importantly, the two most critical prerequisites for Operation Sea Lion’s success – complete German control of the skies and the seas – were much more difficult to achieve than initial Nazi planners had predicted. German overestimation of the Luftwaffe’s invincibility was the first and most critical error. When German aerial attacks upon RAF bases and aircraft began in earnest in mid-August 1940, Britain was much better prepared than Hitler had assumed.

The Battle of Britain, as this desperate aerial conflict came to be called, showed that the British military was far from beaten. For the first time since the beginning of the war, the Luftwaffe began to suffer losses, and appalling ones at that. Their previous successes in support of fast-moving ground troops below who could dislodge front-line enemy positions and air defenses could not be replicated on the British battlefield. Instead of facing meager outmatched air forces, the Luftwaffe now faced a disciplined and determined RAF. British pilots were skillful and well-trained, RAF Spitfires and Hurricanes were more agile than German fighter aircraft, and British use of ground-based radar to communicate German aircraft positions to their pilots in near real-time gave the RAF an extraordinary advantage.

While the RAF was bloodying the Luftwaffe in the air, it became clear to German naval commanders that the idea of simply “clearing a path” across the Channel and then protecting it from the Royal Navy with minefields on either end was fanciful at best. The Kriegsmarine simply lacked the ability to seize or control any portion of the Channel, let alone protect a plodding flotilla of invasion barges from a swift and vastly superior Royal Navy.

As the Luftwaffe faltered against the RAF, Germany’s hope of establishing aerial supremacy evaporated, and the timeline for Operation Sea Lion was pushed back. Counterattacking RAF bombing raids on the German assembly ports repeatedly inflicted damage to the invasion vessels and equipment sitting idle during the delay. In mid-September, after being advised by Göring and Field Marshall von Rundstedt that neither air superiority nor effective naval coordination could be achieved, Hitler postponed the operation and ordered the disbursement of the invasion fleet to protect it from further British attacks.

Eventually, all remaining plans for Operation Sea Lion were completely abandoned, as Hitler turned his attention to invasion plans for the Soviet Union on Germany’s eastern front. Several years later, in June of 1944, the Allies would turn the tables on the Nazi regime. American, British, and Canadian forces stormed the beaches of Normandy during Operation Overlord, the largest amphibious invasion in the history of warfare. The Allied foothold in French territory gained by the audacious D-Day invasion would turn the tide of the war in Europe – ultimately ushering in the beginning of the end of the Nazi regime.

ROBERT SCHREINER is the author of The Wolves and the Greyhounds: A Novel of the Great War. He is a former CIA Intelligence Officer, a consultant in the global private security industry, and a military historian. He lives in the Blue Ridge Mountains of Tennessee.