Operation Sea Lion

• 1. Introduction

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  Operation Sea Lion was Nazi Germany's plan to invade the United Kingdom, code-named Unternehmen Seelöwe in German. It was conceived just nine months into the Second World War, after Germany had already swept through France and the Low Countries with stunning speed. The plan was never carried out. Hitler postponed it indefinitely on the 17th of September 1940, and it remained shelved for the rest of the war.

  The questions at the heart of Operation Sea Lion are not just military. They are questions of ambition, of miscalculation, and of the gap between what a conquering army believes it can do and what it is actually capable of. How did Germany, which had just pulled off one of the most decisive military victories in modern history, find itself unable to cross a body of water barely 34 km wide? Why did the men at the top of the German command resist the plan even while preparing for it? And what did they actually build before the whole enterprise was called off?

• 2. A Last Resort from the Start

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  Adolf Hitler never wanted to invade Britain. Following the armistice with France, he hoped the British government would simply accept a negotiated end to hostilities. He viewed an amphibious assault as a last resort, to be attempted only if every diplomatic avenue had failed.

  Grand Admiral Erich Raeder, head of the Kriegsmarine, shared that reluctance but for different reasons. When he met Hitler on the 21st of May 1940, he raised the possibility of invasion but immediately warned of the risks, preferring a blockade strategy using air and submarine power. Raeder's caution was well-founded: the Norwegian campaign had gutted his fleet. After Operation Weserübung, the Kriegsmarine had only one heavy cruiser, two light cruisers, and four destroyers fit for operations. More than half of Germany's surface fleet had been sunk or badly damaged.

  Reichsmarschall Hermann Göring, head of the Luftwaffe, was equally hostile to the idea. As far back as December 1939, when the German Army issued its first invasion study paper, Göring had responded with a single-page letter declaring that a combined landing operation "must be rejected" and could only be "the final act of an already victorious war against Britain." The men assigned to execute Sea Lion did not believe in it.

  Even so, on the 16th of July 1940, Hitler issued Führer Directive No. 16, formally setting preparations in motion. His prefacing statement captured the reluctant logic: "As England, despite her hopeless military situation, still shows no signs of willingness to come to terms, I have decided to prepare, and if necessary to carry out, a landing operation against her."

• 3. The Naval Problem

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  Crossing the English Channel required the Kriegsmarine to protect an invasion fleet against a Royal Navy that outnumbered it in almost every category. The British Home Fleet alone held a very large advantage, and Britain was prepared to commit its naval forces completely in the event of an actual landing attempt. Raeder wrote in his memoirs after the war: "Up until now the British had never thrown the full power of their fleet into action. However, a German invasion of England would be a matter of life and death for the British, and they would unhesitatingly commit their naval forces, to the last ship and the last man, into an all-out fight for survival."

  The Luftwaffe was supposed to compensate for this imbalance by dominating the skies above the Channel. But the air force's record against naval vessels was poor. In the Norwegian campaign, despite eight weeks of continuous air supremacy, the Luftwaffe sank only two British warships: a light cruiser and a destroyer. German aircrews were not trained to attack fast-moving naval targets. The Luftwaffe made 21 deliberate attacks on small torpedo boats during the Battle of Britain and sank none of them.

  On the 3rd of July 1940, British forces destroyed a large portion of the French fleet in the Attack on Mers-el-Kébir, removing any possibility that Germany could press those powerful modern ships into service. Admiral Karl Dönitz, reflecting after the war, stated bluntly: "We possessed neither control of the air or the sea; nor were we in any position to gain it."

  By the 31st of July conference at the Berghof, Raeder told Hitler that barge conversions would not be ready until the 15th of September, leaving only a narrow window of possible 1940 invasion dates between the 22nd and the 26th of September, when weather was likely to be unsuitable.

• 4. Improvised Flotilla

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  Germany had no purpose-built landing craft suitable for an operation the size of Sea Lion. The Kriegsmarine was, as its planners acknowledged, starting from scratch. The Pionierlandungsboot 39, a self-propelled shallow-draft vessel that could carry 45 infantrymen and unload via clamshell doors, existed only as two prototypes by late September 1940. The 220-ton Marinefährprahm, a more capable craft still under development, would not be commissioned until April 1941.

  With barely two months to assemble a fleet, Germany turned to inland river barges. Approximately 2,400 were collected from across occupied Europe: 860 from Germany, 1,200 from the Netherlands and Belgium, and 350 from France. Of those, only about 800 were powered, and none were strong enough to cross the Channel under their own steam. All would be towed by tugs, two barges to a tug, side by side.

  The barges came in two main types. The péniche was 38.5 meters long and carried 360 tons of cargo. The Kampine, named for the Belgian Campine region whose canals the design was built to navigate, was 50 meters long and carried 620 tons. To make them functional as landing craft, workers cut openings in the bow, added internal ramps and concrete floors for tank transport, and welded longitudinal braces to improve seaworthiness.

  Variants were developed for specific purposes. The Type AS barge lined its sides with concrete for crew protection and carried ten assault boats on wooden slides along the hull, each powered by a 30-horsepower outboard motor. The Type AF converted unpowered barges by installing a pair of surplus 600-horsepower BMW aircraft engines mounted on iron scaffolding at the stern. By the 1st of October, 128 barges had been converted this way; by month's end, the number had risen above 200.

• 5. Tanks Beneath the Waves

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  Getting armour onto a hostile beach was one of the most technically demanding challenges Sea Lion planners faced. The solution they arrived at was to make some tanks float and make others walk underwater.

  The Schwimmpanzer II was a standard Panzer II tank weighing 8.9 tons, light enough to float when fitted with long rectangular aluminium buoyancy boxes packed with kapok sacks. Its tracks drove propeller shafts running through the floats, giving the tank a water speed of 5.7 km per hour. An inflatable rubber hose sealed the turret ring against water. The tank's 2-centimetre gun remained operational while the vehicle was still moving toward shore. The Germans converted 52 of these before Sea Lion was cancelled.

  The Tauchpanzer, or deep-wading tank, was a heavier solution for the Panzer III and Panzer IV. Every hatch, sight port, and air intake was sealed with tape or caulk. A rubber hose 18 meters long connected the crew and engine to air at the surface, attached to a float. Navigation underwater used a directional gyrocompass or radio instructions relayed from the transport barge. Tests conducted at Schilling, near Wilhelmshaven, in late June and early July showed that the tanks had to keep moving; if stopped, they sank into the seabed and became stuck. By the end of August, 160 Panzer IIIs, 42 Panzer IVs, and 52 Panzer IIs had been converted, giving Germany a theoretical armoured strength of 254 machines divided into four detachments labelled Panzer-Abteilung A through D.

  Each batch of submersible tanks was designed to carry enough fuel and ammunition for a combat radius of 200 km from the landing beach.

• 6. The Landing Plan

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  The order of battle finalised on the 30th of August 1940 called for nine divisions from the 9th and 16th Armies to land across four designated beaches. Beach B ran between Folkestone and New Romney; beach C between Rye and Hastings; beach D between Bexhill and Eastbourne; and beach E between Beachy Head and Brighton. A single airborne division would land in Kent, north of Hythe, with orders to seize the aerodrome at Lympne and the bridge crossings over the Royal Military Canal.

  The logistics operated in tightly sequenced waves tied to the tides. Loading of heavy equipment in Antwerp would begin nine days before what planners called S-Tag, the day of the landings. Troops would board their barges on S-Tag minus two or S-Tag minus one. The first echelon would hit the beaches at daybreak, roughly two hours after high tide. If all went to plan, 138,000 men would be ashore within the first two days and 248,000 within the first fortnight.

  General Halder's private assessment of the navy's timetable was withering. After Raeder reported on the 28th of July that ten days would be needed simply to get a first wave across on a narrow front, Halder noted in his diary that if that was true, "all previous statements by the navy were so much rubbish and we can throw away the whole plan of invasion."

  The British, for their part, expected any landing to come on the East coast, which offered better access to ports. The accumulation of German barges in French ports from late August pointed south, and the British main mobile reserve was held near London, positioned to move into Kent or Essex as circumstances required.

• 7. Why Sea Lion Never Sailed

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  The Battle of Britain, which began in earnest with Operation Eagle Attack on the 13th of August 1940, was the operational precondition for Sea Lion. Hitler's Directive No. 16 required the RAF to be "beaten down in its morale and in fact" before any crossing could begin. That condition was never met.

  Historians disagree on how close the Luftwaffe came. Some argue that the shift in bombing from RAF bases to London cost Germany the chance to win air superiority. Others argue the Luftwaffe was never going to achieve it: if British losses had become severe enough, the RAF could simply have withdrawn northward and regrouped, then redeployed to oppose the crossing itself. Still others point to the closing weather window as the decisive factor, independent of anything the RAF did or did not do.

  OKW Chief of Operations Alfred Jodl captured the deeper strategic anxiety in a memorandum written on the 13th of August 1940. His first point was categorical: "The landing operation must under no circumstances fail. A failure could leave political consequences, which would go far beyond the military ones." He concluded that if the Kriegsmarine could not meet the army's requirements for landing two divisions within four days followed by three more, then Sea Lion would be "an act of desperation" that Germany had no reason to attempt.

  Hitler postponed Sea Lion indefinitely on the 17th of September 1940 in light of mounting Luftwaffe losses and no sign of RAF collapse. Some of the improvised equipment developed for the operation went on to serve elsewhere: the motorised barges were used for landings on Russian-held Baltic islands in 1941, and the prefabricated Krupp jetties, tested for Sea Lion and ultimately installed off Alderney in the Channel Islands, were not demolished until 1978-79.