Operation Sealion

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In the spring of 1940, the German war machine rolled over the nations of Western Europe so quickly that it surprised everyone, including the Germans. 

With France, Belgium, the Netherlands, Norway, and Denmark conquered, there was only one more country left to fall: Great Britain.

To topple this final domino, the German high command prepared for the invasion of the island. 

Learn more about Operation Sea Lion, and the planned invasion of Great Britain, on this episode of Everything Everywhere Daily.

From a military standpoint, 1940 was a good year for Germany. One could argue that it was the high point for the Third Reich. From April to June, they basically conquered most of Western Europe. 

When they began operations, they assumed that the time it would take to conquer France would be much longer and would cost far more lives than it actually did. What the Germans thought would take months, only took weeks. 

On May 10, the German entered Belgium and Luxembourg and on June 23, Adolf Hitler entered Paris. 

The victory was almost even more complete. The British Expeditionary Forces on the continent were trapped near the French city of Dunkirk. If it wasn’t for a herculean effort on the part of British civilians in small ships who rescued over a third of a million soldiers, the British Army might have been defeated then and there. 

Hitler’s initial plan for Britain wasn’t for an invasion. He assumed that with all of Western Europe conquered, the UK would seek a negotiated peace. The Kriegsmarine, aka the German Navy, did draw up some very basic plans as early as 1939 for a possible invasion of Britain, but there was nothing specific, only broad goals outlining what they would need to do to make an invasion successful.

The Germans had been very successful up until this point via their blitzkrieg strategy and use of mechanized warfare. However, all of their success in Eastern and Western Europe had been on land. 

They had done very little by way of the sea. The closest thing to what they would have to do to invade Britain was the invasion of Norway. Norway, however, had a population of only 3 million, whereas the UK had a population of 47 million. Moreover, the Norwegians inflicted heavy losses to German naval units, without an impressive navy, and the British had the biggest navy in the world at the time. 

When Churchill and the British refused to negotiate, Hitler ordered that plans be drawn up for an invasion.

No one in the German Wehrmacht was really keen on the idea of an invasion of Britain. 

Field Marshal Alfred Jodl’s first idea was to simply dominate the air and destroy the Royal Air Force. If the Germans could gain air supremacy, they could shut down ports and shipping thereby crippling the British economy.  By this route, they could eventually force the British to the negotiating table. 

Hitler initially agreed. However, on July 16 he issued Führer Directive No. 16, which called to begin preparations for the invasion of Britain. 

In the order, he said, “As England, in spite of 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. The aim of this operation is 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 plan was dubbed Operation Sea Lion. 

Hitler demanded four things occur before any invasion:

  1. The Germans had to achieve air supremacy to protect any force attempting to land on the coast around the Straight of Dover.
  2. The English channel had to be cleared of British mines, and a barricade of German mines had to block either end of the English Channel. 
  3. The coast of France near England had to be packed with heavy artillery. 
  4. The Royal Navy had to be pinned down in the Mediterranean and the North Seal to allow their ships to cross freely.

Once these things had been established, then there could be a wide scale landing of troops in southeastern England. 

The Army was the biggest cheerleader of the plan, but its success required the Kriegsmarine and the Luftwaffe. Grand Admiral Erich Raeder, head of the Kriegsmarine, and Field Marshall Hermann Göring, head of the Luftwaffe, were very hesitant about the plans. 

The British navy was simply superior, especially their surface fleet. The super battleship the Germans were working on, the Bismark, wasn’t quite ready yet. 

Hitler was wanting an invasion in September. This had no basis in reality. Organizing at least 100,000 men for an amphibious landing, plus finding all the ships and the logistics would take a year at least, not a month. Moreover, by the time September rolled around, the weather would become worse, seas would become rougher, and the odds of a failed landing would be higher. 

Beginning in July and August, the first part of the Fürher Directive, establishing air supremacy began. We know this as the Battle of Britain, the world’s first great aviation battle. 

The Germans assumed they could achieve air supremacy in 7 to 14 days. That didn’t happen. The Battle of Britain went on through October. The Germans were at a massive disadvantage as every British pilot who was shot down and survived could be put back into action. Every German pilot who went down and survived was taken out for the rest of the war.

In the end, it was the British who achieved air supremacy by downing more German aircraft and producing more planes. 

While all of this was going on, the British were throwing everything into a homeland defense. The third of a million troops rescued from Dunkirk were all still in England. Civilians were mobilized and the entire nation was put on high alert. A half a million civilians were recruited into the home guard. Thousands of pillboxes were installed along the coast. 

The Germans were assembling barges from all over Europe to carry troops and equipment. However, these boats weren’t military vessels. They had no armor, no guns, and most of them weren’t even designed to be used in the open ocean. They were for coastal or river use. 

Compare this to the D-Day landing craft several years later, which were armored and specially built, and a German invasion might have been a fiasco. 

In the end, Hitler indefinitely postponed Operation Sealion on September 17.  

There have been several reasons offered as to why the invasion never happened. 

One is that Hitler himself never really believed an invasion was possible and that the risk of failure was too high. The entire operation might just have been an attempt to get the British to negotiate. 

Second, is that in a rare moment of taking his general’s advice, Hitler listened to Raeder and Göring. Both said that the necessary prerequisite of air and naval supremacy wasn’t possible in the short run, and in fact, it never materialized. 

Many people think that Hitler’s attention was simply elsewhere. While an invasion of Britain would have been risky, Britain itself didn’t really offer any immediate threat to Germany. They were bottled up on an island, dependent on the United States for trade and resources. 

Hitler’s attention was on the Soviet Union, which he viewed as the much bigger strategic prize. A land invasion of the Soviet Union would play much more to the strengths of the Germans than an amphibious landing on the shore of England would. In June of 1941, they would do just that with the start of Operation Barbarossa.

If Operation Sealion had been implemented, regardless if it had been successful or a failure, it would have radically changed the outcome of the second world war.