Codebreakers: The Story of Bletchley Park’s Growing Influence in World War Two

History Hit Podcast

4 mins

20 Sep 2018

This article is an edited transcript of Bletchley Park: The Home of Codebreakers on Dan Snow’s History Hit, first broadcast 24 January 2017. You can listen to the full episode below or to the full podcast for free on Acast.

It’s widely believed that Bletchley Park didn’t start to meaningful impact Allied fortunes until after the Battle of Britain. This isn’t entirely accurate.

While it’s true that Bletchley Park’s contribution grew as the war went on, the team was already reading German navigation beam information as early as the Spring of 1940.

Bletchley Park’s analysts were already contributing during the Battle of Britain, and that role continued to grow in influence campaign by campaign, theatre by theatre, throughout the rest of the war.

Some of this intelligence didn’t require any codebreaking. For example, when German aircraft flew back to their home base, the pilots would talk in plain language because getting home is pretty important and they didn’t think there was any strategic value in what they were saying at that point of their mission.

Heinkel He 111 bombers during the Battle of Britain. Bletchley Park was already reading German navigation beam information as early as the Spring of 1940.

In fact, by listening to the German Air Force talking in plain language as they flew back, analysts were able to establish where they were for next time, which was actually very important information.

Bletchley Park’s analysts were already contributing during the Battle of Britain, and that role continued to grow in influence campaign by campaign, theatre by theatre, throughout the rest of the war.

How was Bletchley’s intelligence used in the Mediterranean?

By 1942, Bernard Montgomery, the new commander of the British Eighth Army,

was able to take advantage of extensive intelligence courtesy of Bletchley Park, including information about the German order of battle and what Rommel’s intentions were.

All this information relating to the German and Italian military in North Africa became invaluable, enabling Britain to sink convoys bringing supplies across the Mediterranean.

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There was a problem with such good information however. By acting on a piece of intelligence that couldn’t have been found out any other way, the Allies were effectively in danger of signalling their codebreaking capabilities to the enemy. The Allies had to be very careful about how they acted on intelligence.

The British Army developed strategies to get around this problem. For instance, when they had intelligence on ships in the Mediterranean, they would send reconnaissance aircraft to the area they knew a convoy was going to be, the reconnaissance aircraft would fly around, making sure it had been spotted, and then fly away again. A submarine was then sent to sink the convoy.

The idea was to present a conceivable reason for the convoy being found that had nothing to do with codebreaking.

Why was Bletchley Park so vital to the D-Day deception?

Ahead of D-Day, the Allies wanted to persuade the Germans they’re going to attack at the Pas-de-Calais, rather than Normandy. All sorts of cunning deception was employed, from building a fake army in Kent to using inflatable wooden tanks. Bletchley’s Enigma codebreakers played a key role in the ruse, called Operation Fortitude.

The German spy network in Britain, by then completely controlled by British Intelligence, would report to the German intelligence service, the Abwehr, mostly in Spain and Portugal.

Ahead of D-Day, the Allies wanted to persuade the Germans they’re going to attack at the Pas-de-Calais, rather than Normandy.

This information was then reported back to Berlin by wireless and read by British codebreakers. So, MI5 was able to put something in a message that was then fed back to Berlin via a double agent, then read the reply from Berlin.

British intelligence was not only able to provide information, it could then see how the Germans reacted to it. Such complete control of the information feed allowed them to play the Germans like a violin, nuancing and tweaking the information they fed them according to the responses.

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Total information dominance

By late 1944 Britain had total information dominance. By patiently studying German communications Britain had built a complete picture of the German Army in France and the Low Countries by the time of D-Day.

They knew all the units, who commanded them, what their rank structures were, how many tanks they had, where their defences were… Essentially, they were close to knowing everything.

Dummy landing craft were used as decoys before D-Day. Bletchley Park played a key role in the deception.

In contrast, the Germans knew almost nothing about the army that was attacking them, and what they did know about was at least 50% false, thanks to a deception operation they’d been fed.

Consequently, the Germans had a very exaggerated view of how big the invading army was. So, even though Britain had almost made its full commitment to Normandy, Germany believed that there was another equally big army ready to go to Calais.

We’ve even seen a letter from Hitler himself telling the troops in Calais not to go to Normandy because there was another invasion coming.