It became far more central to military (and to some extent diplomatic) intelligence generally with the mechanization of armies, development of blitzkreig tactics, use of submarine and commerce raiders warfare, and the development of practicable wireless communications. For example, failure to properly protect its communications fatally compromised the Russian Army in its advance early in WWI and led to the disastrous defeat by the Germans under Ludendorf and Hindenberg at the Battle of Tannenberg. Similarly, the interception and decryption of the Zimmerman telegram was an important factor in the US decision to enter the War. On the negative side, the inability of British commanders to take seriously traffic information from intercepts was instrumental in the failure to achieve more than they did at the Battle of Jutland, thus losing what might have been an major opportunity. Finally, both traffic analysis and decryption of some of the intercepted traffic gave Admiral Nimitz the information he used to plan the ambush that resulted in the Japanese Navy's defeat at the Battle of Midway.

As sensitive information is often encrypted, SIGINT often involves the use of cryptanalysis. However, traffic analysis can be used to gather information even when the messages themselves cannot be decrypted.

Past and Present SIGINT Activities

• Ultra
• Purple code
• Echelon