Operation Sea Lion, also written as Operation Sealion (German: Unternehmen Seelöwe), was Nazi Germany's code name for their planned invasion of the United Kingdom. It was to have taken place during the Battle of Britain, nine months after the start of the Second World War. Following the armistice with France after the Battle of France, Adolf Hitler, the German Führer and Supreme Commander of the Armed Forces, hoped the British government would accept his offer to end the state of war between the two. He considered invasion to be a last resort, to be used only if all other options had failed.
As a precondition for the invasion of Britain, Hitler demanded both air and naval superiority over the English Channel and the proposed landing sites. The German forces achieved neither at any point of the war. Further, both the German High Command and Hitler himself held serious doubts about the prospects for success. Nevertheless, both the German Army and Navy undertook major preparations for an invasion. These included training troops, developing specialised weapons and equipment, modifying transport vessels and the collection of a large number of river barges and transport ships on the Channel coast. However, in light of mounting Luftwaffe losses in the Battle of Britain and the absence of any sign that the Royal Air Force had been defeated, Hitler postponed Sea Lion indefinitely on 17 September 1940. It was never put into action.
Background
Adolf Hitler hoped for a negotiated peace with the UK and made no preparations for amphibious assault on Britain until the Fall of France. At the time, the only forces with experience and modern equipment for such landings were the Japanese, which they used during the Battle of Wuhan in 1938.
Outbreak of war and fall of Poland
In September 1939, the successful German invasion of Poland infringed on both a French and a British alliance with Poland and both countries declared war on Germany. On 9 October, Hitler's "Directive No. 6 for the Conduct of the War" planned an offensive to defeat these allies and "win as much territory as possible in Holland, Belgium, and northern France to serve as a base for the successful prosecution of the air and sea war against England".
With the prospect of the Channel ports falling under Kriegsmarine (German Navy) control, Grand Admiral (Großadmiral) Erich Raeder (head of the Kriegsmarine) attempted to anticipate the obvious next step that might entail and instructed his operations officer, Kapitän Hansjürgen Reinicke, to draw up a document examining "the possibility of troop landings in England should the future progress of the war make the problem arise". Reinicke spent five days on this study and set forth the following prerequisites:
- Eliminating or sealing off Royal Navy forces from the landing and approach areas.
- Eliminating the Royal Air Force.
- Destroying all Royal Navy units in the coastal zone.
- Preventing British submarine action against the landing fleet.
On 22 November 1939, the Head of Luftwaffe (German Air Force) intelligence Joseph "Beppo" Schmid presented his "Proposal for the Conduct of Air Warfare", which argued for a counter to the British blockade and said "Key is to paralyse the British trade" by blocking imports to Britain and attacking seaports. The OKW (Oberkommando der Wehrmacht or "High Command of the Armed Forces") considered the options and Hitler's 29 November "Directive No. 9 – Instructions For Warfare Against The Economy of the Enemy" stated that once the coast had been secured, the Luftwaffe and Kriegsmarine were to blockade UK ports with sea mines, attack shipping and warships, and make air attacks on shore installations and industrial production. This directive remained in force in the first phase of the Battle of Britain.
In December 1939, the German Army issued its own study paper (designated Nordwest) and solicited opinions and input from both the Kriegsmarine and the Luftwaffe. The paper outlined an assault on England's eastern coast between The Wash and the River Thames by troops crossing the North Sea from ports in the Low Countries. It suggested airborne troops as well as seaborne landings of 100,000 infantry in East Anglia, transported by the Kriegsmarine, which was also to prevent Royal Navy ships from getting through the Channel, while the Luftwaffe had to control airspace over the landings. The Kriegsmarine response was focused on pointing out the many difficulties to be surmounted if invading England was to be a viable option. It could not envisage taking on the Royal Navy Home Fleet and said it would take a year to organise shipping for the troops. Reichsmarschall Hermann Göring, head of the Luftwaffe, responded with a single-page letter in which he stated, "[A] combined operation having the objective of landing in England must be rejected. It could only be the final act of an already victorious war against Britain as otherwise the preconditions for success of a combined operation would not be met".
The fall of France
Germany's swift and successful occupation of France and the Low Countries gained control of the Channel coast, facing what Schmid's 1939 report called their "most dangerous enemy". Raeder met Hitler on 21 May 1940 and raised the topic of invasion, but he warned of the risks and expressed a preference for blockade by air, submarines and raiders.
By the end of May, the Kriegsmarine had become even more opposed to invading Britain following its costly victory in Norway; after Operation Weserübung, the Kriegsmarine had only one heavy cruiser, two light cruisers, and four destroyers available for operations. Raeder was strongly opposed to Sea Lion, for over half of the Kriegsmarine surface fleet had been either sunk or badly damaged in Weserübung, and his service was hopelessly outnumbered by the ships of the Royal Navy. British parliamentarians still arguing for peace negotiations were defeated in the May 1940 War Cabinet Crisis, but throughout July the Germans continued with attempts to find a diplomatic solution.
Invasion planning
In a report presented on 30 June, OKW Chief of Staff Alfred Jodl reviewed options to increase pressure on Britain to agree to a negotiated peace, which included a direct invasion on the British Isles or a peripherial strategy against British strongholds in the Mediterranean such as Malta, Gibraltar and the Suez Canal in cooperation with Italy and Spain. The first priority was to eliminate the Royal Air Force and gain air supremacy. Intensified air attacks against shipping and the economy could affect food supplies and civilian morale in the long term. Reprisal attacks of terror bombing had the potential to cause quicker capitulation but the effect on morale was uncertain. Once the Luftwaffe had control of the air and the British economy had been weakened, an invasion would be a last resort or a final strike ("Todesstoss") after the UK had already been practically defeated, but could have a quick result. At a meeting that day, OKH Chief of General Staff Franz Halder heard from Secretary of State Ernst von Weizsäcker that Hitler had turned his attention to Russia. Halder met Admiral Otto Schniewind on 1 July, and they shared views without understanding each other's position. Both thought that air superiority was needed first, and could make the invasion unnecessary. They agreed that minefields and U-boats could limit the threat posed by the Royal Navy; Schniewind emphasised the significance of weather conditions.
On 2 July, the OKW asked the services to start preliminary planning for an invasion, as Hitler had concluded that invasion would be achievable in certain conditions, the first of which was command of the air, and specifically asked the Luftwaffe when this would be achieved. On 4 July, after asking General Erich Marcks to begin planning an attack on Russia, Halder heard from the Luftwaffe that they planned to eliminate the RAF, destroying its aircraft manufacturing and supply systems, with damage to naval forces as a secondary aim. A Luftwaffe report presented to the OKW at a meeting on 11 July said that it would take 14 to 28 days to achieve air superiority. The meeting also heard that Britain was discussing an agreement with Russia. On the same day, Grand Admiral Raeder visited Hitler at the Berghof to persuade him that the best way to pressure the British into a peace agreement would be a siege combining air and submarine attacks. Hitler agreed with him that invasion would be a last resort.
Jodl set out the OKW proposals for the proposed invasion in a memorandum issued on 12 July, which described operation Löwe (Lion) as "a river crossing on a broad front", irritating the Kriegsmarine. On 13 July, Hitler met Field Marshal von Brauchitsch and Halder at Berchtesgaden and they presented detailed plans prepared by the army on the assumption that the navy would provide safe transport. To the surprise of Von Brauchitsch and Halder, and completely at odds with his normal practice, Hitler did not ask any questions about specific operations, had no interest in details, and made no recommendations to improve the plans; instead, he simply told OKW to start preparations.
Directive No. 16: Operation Sea Lion
On 16 July 1940 Hitler issued Führer Directive No. 16, setting in motion preparations for a landing in Britain. He prefaced the order by stating: "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. This operation aimed to eliminate the English Motherland as a base from which the war against Germany can be continued, and, if necessary, to occupy the country completely." The code name for the invasion was Seelöwe, "Sea Lion".
Hitler's directive set four conditions for the invasion to occur:
- The RAF was to be "beaten down in its morale and in fact, that it can no longer display any appreciable aggressive force in opposition to the German crossing".
- The English Channel was to be swept of British mines at the crossing points, and the Strait of Dover must be blocked at both ends by German mines.
- The coastal zone between occupied France and England must be dominated by heavy artillery.
- The Royal Navy must be sufficiently engaged in the North Sea and the Mediterranean so that it could not intervene in the crossing. British home squadrons must be damaged or destroyed by air and torpedo attacks.
This ultimately placed responsibility for Sea Lion's success squarely on the shoulders of Raeder and Göring, neither of whom had the slightest enthusiasm for the venture and, in fact, did little to hide their opposition to it. Nor did Directive 16 provide for a combined operational headquarters, similar to the Allies' creation of the Supreme Headquarters Allied Expeditionary Force (SHAEF) for the later Normandy landings, under which all three service branches (Army, Navy, and Air Force) could work together to plan, co-ordinate, and execute such a complex undertaking.
The invasion was to be on a broad front, from around Ramsgate to beyond the Isle of Wight.
Preparations, including overcoming the RAF, were to be in place by mid August.
Discussion
Grand Admiral Raeder sent a memorandum to OKW on 19 July, complaining about the onus placed on the navy in relation to the army and air force, and stating that the navy would be unable to achieve its objectives.
The first joint services conference on the proposed invasion was held by Hitler in Berlin on 21 July, with Raeder, Brauchitsch, and Luftwaffe Chief of Staff Hans Jeschonnek. Hitler told them that the British had no hope of survival, and ought to negotiate, but were hoping to get Russia to intervene and halt German oil supplies. Invasion was very risky, and he asked them if direct attacks by air and submarine could take effect by mid September. Jeschonnek proposed large bombing attacks so that responding RAF fighters could be shot down. The idea that invasion could be a surprise "river crossing" was dismissed by Raeder, and the navy could not complete its preparations by mid August. Hitler wanted the air attack to commence early in August and, if it succeeded, the invasion was to start around 25 August before weather deteriorated. Hitler's main interest was the question of countering potential Russian intervention. Halder outlined his first thoughts on defeating Russian forces. Detailed plans were to be made to attack the Soviet Union.
Raeder met Hitler on 25 July to report on navy progress: they were not sure if preparations could be completed during August: he was to present plans at a conference on 31 July. On 28 July, he told OKW that ten days would be needed to get the first wave of troops across the Channel, even on a much narrower front. Planning was to resume. In his diary, Halder noted that if what Raeder had said was true, "all previous statements by the navy were so much rubbish and we can throw away the whole plan of invasion". On the next day, Halder dismissed the navy's claims and required a new plan.