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Western Europe 1939-1945: Invasion

How worried was Britain about invasion 1940-41?

Details of Operation Sealion, the planned German invasion of Britain, September 1940

Catalogue ref: CAB 101/347

Source a

Operation Sealion, Sept 1940; CAB 101/347 Operation Sealion, Sept 1940; CAB 101/347

Source b

Operation Sealion, Sept 1940; CAB 101/347

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What is this source?

This source is a summary of the plans of the German armed forces to invade Britain across the English Channel in the early autumn of 1940.


These instructions were taken from German documents that were discovered after the war.

What’s the background to this source?

In May 1940 German forces drove British armies out of France. France surrendered on June 21st 1940. Soon afterwards German forces took over the Channel Islands. Hitler and his generals had clear plans to invade Britain in 1940.


The problem for the Germans was the Royal Navy. British ships would have destroyed their invasion ships. As a result they needed to control the air. As a result the first stage of the invasion was an attack on the Royal Air Force, which became known as the Battle of Britain.

It’s worth knowing that...

Operation Sealion was a massive operation. British intelligence estimated that the Germans would use around 35 divisions (up to 600 000 men) to invade Britain. There were detailed plans to invade on the south coast in Sussex and possibly in Dorset as well.


Although the invasion did not happen in 1940 the invasion craft and many of the soldiers remained in France until well into 1941. It was not until Hitler invaded the USSR that the idea of an invasion was completely abandoned. Most of the troops in France were then transferred to fight the USSR.

What level of alert should Britain be on?

  1. What was the main purpose of the German plan?
  2. When was the invasion due to take place?
  3. What were the Luftwaffe and the German navy supposed to do?
  4. What were the main stages of the invasion plan?
  5. What level of alert would you put Britain on as a result of studying this evidence?

Record your answer in your threat level table you can always change your mind as you study other sources.