Armed Forces Day 2017

Last Updated: 23 Jun 2017
Topics: Our history
To mark Armed Forces Day we look back at one of the most decisive battles of WWII and the role that signals intelligence played in supporting the military.

Signals intelligence doesn't win wars: they are won by service personnel who fight battles. Our job is to make sure that they go into battle with the best possible information about what they are going to meet, and that their adversaries know as little as possible about what they intend to do.


Military personnel inside a battleship during Op FORTITIDE
Op FORTITUDE was the plan to deceive the Germans about the Allied D-Day landings. ©Crown Copyright


By the time of D-Day the Supreme Commander of Allied Forces, General Eisenhower1, enjoyed a decisive information advantage. This was largely because of signals intelligence (though the importance of the Double Cross system in blinding the Abwehr2 should not be ignored). The strategic picture, which had been outlined in the decrypted messages of Eshima, the Japanese Military Attaché in Berlin, was being filled out by the TUNNY3 decrypts of traffic between Berlin and OKW4, the German Command of forces in Western Europe which gave an insight into von Rundstedt's5 capabilities and intentions.  


A SECRET Appendix of the notional structure of the fictitious First US Army Group
Notional structure of the fictitious First US Army Group. ©Crown Copyright


Work on Enigma illuminated lower level moves of German forces between fixed locations in France. A major strategic deception campaign based on an understanding of German signals intelligence capability led the Germans to infer the existence of an entire Army Group in the south east of England. This kept them from deploying their strategic reserve to combat the allied landings.

The securing of allied strategic and operation communications had been largely achieved by mid-1944, which blinded the Germans to allied intentions. And these capabilities gave Eisenhower one more advantage: as troops landed on the Normandy beaches he could follow their early progress not through their own communications - they were far too busy fighting to establish long-haul communications in the first hours of the invasion - but because of the near 'real time' interception and processing of German Army communications.


A decoy battleship - a field with a truck and an aerial!
A decoy battleship - a field with a truck and an aerial! ©Crown Copyright



1 General of the Army Dwight D. Eisenhower, the Supreme Allied Commander in Europe (SACEUR), and later the 34th President of the United States.

2 German military intelligence.

3 TUNNY was the British code name for the German Lorenz high-grade cipher machine.

4 Oberkommando der Wehrmacht, the 'Supreme Command of the Armed Forces' of Nazi Germany in the west.

5 Generalfeldmarschall Gerd von Rundstedt, Field Marshal in the Wehrmacht and commander-in-chief of German forces in the Western theatre.