Editing Signals intelligence (section)

===World War II===
[[File:Colossus.jpg|thumbnail|left|A Mark 2 [[Colossus computer]]. The ten Colossi were the world's first programmable electronic computers, and were built to break the German codes.]]
The use of SIGINT had even greater implications during [[World War II]]. The combined effort of intercepts and cryptanalysis for the whole of the British forces in World War II came under the code name "[[Ultra (cryptography)|Ultra]]", managed from [[Government Code and Cypher School]] at [[Bletchley Park]]. Properly used, the German [[Enigma machine|Enigma]] and [[Lorenz cipher]]s should have been virtually unbreakable, but flaws in German cryptographic procedures, and poor discipline among the personnel carrying them out, created vulnerabilities which made Bletchley's attacks feasible.

Bletchley's work was essential to defeating the [[U-boat]]s in the [[Battle of the Atlantic]], and to the British naval victories in the [[Battle of Cape Matapan]] and the [[Battle of North Cape]]. In 1941, Ultra exerted a powerful effect on the [[North African campaign|North African desert campaign]] against German forces under General [[Erwin Rommel]]. General Sir [[Claude Auchinleck]] wrote that were it not for Ultra, "Rommel would have certainly got through to Cairo". Ultra decrypts featured prominently in the story of [[Operation Salam|Operation SALAM]], [[László Almásy]]'s mission across [[Western Desert (Egypt)|the desert]] behind Allied lines in 1942.<ref>Gross, Kuno, Michael Rolke and András Zboray, [http://fjexpeditions.com/resources/salam/operation_salam.htm Operation SALAM] – László Almásy's most daring Mission in the Desert War, Belleville, München, 2013</ref> Prior to the [[Normandy landings]] on D-Day in June 1944, the Allies knew the locations of all but two of Germany's fifty-eight [[Western Front (World War II)|Western Front]] divisions.

[[Winston Churchill]] was reported to have told King [[George VI]]: "It is thanks to the secret weapon of General [[Stewart Menzies|Menzies]], put into use on all the fronts, that we won the war!" Supreme Allied Commander, [[Dwight D. Eisenhower]], at the end of the war, described Ultra as having been "decisive" to Allied victory.<ref>{{citation |last=Winterbotham |first=F. W. |author-link=F. W. Winterbotham |title=The Ultra Secret |location=New York |publisher=Harper & Row |year=1974 |isbn=0-06-014678-8|pages=154, 191 }}</ref> Official historian of British Intelligence in World War II [[Harry Hinsley|Sir Harry Hinsley]] argued that Ultra shortened the war "by not less than two years and probably by four years"; and that, in the absence of Ultra, it is uncertain how the war would have ended.<ref>{{citation |last=Hinsley |first=Sir Harry |author-link=Harry Hinsley |title=The Influence of ULTRA in the Second World War |orig-year=1993 |year=1996 |url=http://www.cdpa.co.uk/UoP/HoC/Lectures/HoC_08e.PDF |access-date=23 July 2012}}</ref>

At a lower level, German cryptanalysis, direction finding, and traffic analysis were vital to Rommel's early successes in the [[Western Desert campaign|Western Desert Campaign]] until British forces tightened their communications discipline and Australian raiders destroyed his principal SIGINT Company.<ref>{{Cite web |last=P9-J |date=2015-08-08 |title=German SIGINT in the Desert Campaign |url=https://friendsintelligencemuseum.org/2015/08/08/german-sigint-in-the-desert-campaign/ |access-date=2023-12-05 |website=Friends of the Intelligence Corps Museum |language=en-GB}}</ref>
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