Editing Signals intelligence (section)

===Development in World War I===
[[File:Ztel2.jpg|thumb|[[Zimmermann Telegram]], as decoded by [[Room 40]] in 1917]]
Over the course of the [[First World War]], a new method of signals intelligence reached maturity.<ref name="Wheeler">{{cite journal|author=Douglas L. Wheeler|title=A Guide to the History of Intelligence 1800–1918|journal=Journal of U.S. Intelligence Studies|url=http://www.afio.com/publications/Wheeler_Hist_of_Intel_1800-1918_in_AFIO_INTEL_WinterSprg2012.pdf}}</ref> Russia’s failure to properly protect its communications fatally compromised the [[Russian Army]]’s [[Russian invasion of East Prussia (1914)|advance early in World War I]] and led to their disastrous defeat by the Germans under [[Erich Ludendorff|Ludendorff]] and [[Paul von Hindenburg|Hindenburg]] at the [[Battle of Tannenberg]]. In 1918, French intercept personnel captured a message written in the new [[ADFGVX cipher]], which was cryptanalyzed by [[Georges Painvin]]. This gave the Allies advance warning of the German 1918 [[German spring offensive|Spring Offensive]].

The British in particular, built up great expertise in the newly emerging field of signals intelligence and codebreaking (synonymous with cryptanalysis). On the declaration of war, Britain cut all German undersea cables.<ref>{{cite journal |last=Winkler |first=Jonathan Reed |date=July 2009 |journal=The Journal of Military History |title=Information Warfare in World War I |volume=73 |issue=3 |pages=845–867 |doi=10.1353/jmh.0.0324|s2cid=201749182 }}</ref> This forced the Germans to communicate exclusively via either (A) a telegraph line that connected through the British network and thus could be tapped; or (B) through radio which the British could then intercept.<ref name="Beesly">{{cite book |title=Room 40: British Naval Intelligence, 1914–1918 |last=Beesly |first=Patrick |year=1982 |publisher=Hamish Hamilton Ltd |location=Long Acre, London |isbn=0-241-10864-0}}</ref> Rear Admiral [[Henry Oliver]] appointed [[James Alfred Ewing|Sir Alfred Ewing]] to establish an interception and decryption service at the [[Admiralty (United Kingdom)|Admiralty]]; [[Room 40]].<ref name="Beesly" /> An interception service known as [[Y-stations|'Y' service]], together with the [[General Post Office|post office]] and [[Marconi]] stations, grew rapidly to the point where the British could intercept almost all official German messages.<ref name="Beesly" />

The German fleet was in the habit each day of wirelessing the exact position of each ship and giving regular position reports when at sea. It was possible to build up a precise picture of the normal operation of the [[High Seas Fleet]], to infer from the routes they chose where defensive minefields had been placed and where it was safe for ships to operate. Whenever a change to the normal pattern was seen, it immediately signalled that some operation was about to take place, and a warning could be given. Detailed information about submarine movements was also available.<ref name="Beesly" />

The use of radio-receiving equipment to pinpoint the location of any single transmitter was also developed during the war. Captain [[H.J. Round]], working for [[Marconi]], began carrying out experiments with [[direction finding|direction-finding]] radio equipment for [[British Expeditionary Force (World War I)|the army in France]] in 1915. By May 1915, the Admiralty was able to track German submarines crossing the North Sea. Some of these stations also acted as 'Y' stations to collect German messages, but a new section was created within Room 40 to plot the positions of ships from the directional reports.<ref name="Beesly" />

Room 40 played an important role in several naval engagements during the war, notably in detecting major German sorties into the [[North Sea]]. The [[Battle of Dogger Bank (1915)|battle of Dogger Bank]] was won in no small part due to the intercepts that allowed the Navy to position its ships in the right place.<ref>Livesey, Anthony, Historical Atlas of World War One, Holt; New York, 1994 p. 64</ref> It played a vital role in subsequent naval clashes, including at the [[Battle of Jutland]] as the British fleet was sent out to intercept them. The direction-finding capability allowed for the tracking and location of German ships, submarines, and [[Zeppelin]]s. The system was so successful that by the end of the war, over 80 million words, comprising the totality of German wireless transmission over the course of the war, had been intercepted by the operators of the [[Y-stations]] and decrypted.<ref>{{cite web|url=http://marconiheritage.org/ww1-intel.html|title=Code Breaking and Wireless Intercepts}}</ref> However, its most astonishing success was in [[cryptanalysis|decrypting]] the [[Zimmermann Telegram]], a [[telegram]] from the German Foreign Office sent via Washington to its [[ambassador]] [[Heinrich von Eckardt]] in Mexico.