World War II has been described as a signal intelligence (SIGINT) war. According to historian Ronald Lewin the efforts to intercept, locate, and decrypt the radio communications of the enemy became a salient characteristic of the conflict. Never before in history had belligerent nations expended so much effort in intelligence gathering to such great effect. On the allied side, tens of thousands of people across the world, laboring with some of the most sophisticated technology of its day, resulted in an intelligence triumph, which staved off defeat, shortened the war, and saved lives. From discovering the Japanese plans to attack Midway, to eavesdropping on Admiral Donitz’s orders to his wolf packs, to decoding Admiral Yamamoto’s flight schedule, to confirming German belief that the invasion was yet to come at the Pas de Calais, SIGINT provided a decisive edge. The secret war of 1939 – 1945 was the original example of what is now known as ‘information warfare’.

            On the Axis side, similar (although not as effective) efforts were carried on throughout the war. These intelligence efforts by both sides were rooted in the history of radio technology. Almost as soon as navies began using wireless in the early years of the 20^th century, their opponents began listening in on them. In the first world war radio intelligence had important political and military effects. The casual eavesdropping on Russian radio communications prior to the battle of Tannenberg directly led to the German’s victory, nullifying the Tsar’s effort to invade Imperial Germany from the East. In Britain, the famous Room 40 of the Admiralty was able to track the movements of the German fleet in near real time, warning them in time of the sortie that became Jutland. A year later their cryptanalysts decoded the famous Zimmerman telegram, causing the entry of the United States into the war, and ultimately, victory for the Allies.

·         Interception – without the raw material of copied traffic there would be no intelligence. Each military service, Army, Navy, Air Force, on both sides and in every theater deployed both fixed and mobile radio interception units. Tens of thousands of troops every day and all day throughout the war carried out this tedious and demanding job. Long shifts crouched over hot radio receivers, straining to listen into the nonsense dots and dashes of encrypted messages and ensuring that they were copied accurately was the lot for these signal troops. The British called this effort the “Y” service.

·         Direction Finding – although invented in the first world war for Low and Medium Wave transmitters, the common High Frequency transmissions of the Second World War required refined techniques and new engineering, resulting in a practice known as Huff Duff (HFDF). Huff Duff, by triangulation of multiple bearings from the same transmitter, could located the physical location of the unit. This was critical in identifying and locate ships, aircraft, and Army headquarters. Especially in the Battle of the Atlantic, HFDF allowed British Naval intelligence to track U-Boats in near real time, allowing for the destroyers and corvettes assisted in allowing the attackers to pinpoint their enemy.

·         Traffic Analysis – the study of the external characteristics of a message, address, length, time of transmission, frequency and any indicators, along with the location information provided by Huff Duff allowed intelligence officers to derive a lot of critical information even without reading the contents of a message. Over time the command circuits of the enemy could be identified, allowing for the generation of “Order of Battle” information, i.e. the specific identities of opposing units and their command relationship with superior and subservient units. Analysis of routine messages could identify such things as weather reports, contact reports, unit returns. Deviation from the volume and length of messages could indicate planning for movement or action.

·         Cryptanalysis – the most complicated step, was the actual breaking and reading of the enemy’s encrypted communications. Often information from traffic analysis was used to provide the cryptanalyst with clues to assist in the breaking of the message. Various techniques, both manual and mechanical, were used to recover the key, which changed daily, requiring the effort to be repeated all over again the next day. Once the plaintext was recovered, translators had to accurately translate the message, which because of technical and military terms and jargon, was much more complicated that simple civilian style translation. Finally, the results were sent to intelligence officers whose job it was to put the message into the larger context and interpret the meaning in military terms. This intelligence was codenamed ULTRA, the highest security level requiring special handling. The code breaking centers were far behind the lines, usually near the senior decision makers. The British centralized their high level cryptologic processing at the Government Code and Cipher School at Bletchley Park, north of London in present day Milton Keynes, and serviced all their intelligence customers, both military and political from there. The Germans were more decentralized with each military service, the High Command (the OKW), the Foreign Service and Goëring’s “Research Bureau” each having their own separate code breaking center.

            This process was a massive industrial process; the advantage went to the side that could provide the most resources in both talent and technology. The British cryptologic center, Bletchley Park, employed almost 10,000 people before the end of the war. It could be described as the birthplace of the information age; it was the first large organization whose sole purpose was the acquisition, processing and dissemination of information, which was classified TOP SECRET ULTRA. The Germans took a more decentralized approach to code breaking; they had neither the organization nor leadership to match the allies' effort.

            In addition, during the war a new type of signals intelligence developed, electronics intelligence (ELINT), which focuses on non-communications electronic signals. Starting in 1940, when the British detected radio navigation beams being directed over England to guide German bombers during the Battle of Britain, an alternating battle of detection, countermeasures, and counter-countermeasures took place. By 1944, both the RAF and the USAAF were flying numerous missions over continental Europe to detect, map out and jam German radar sites. By the end of the war ELINT techniques were being used against ground and naval targets.

            The experience of the war showed that SIGINT was the premier intelligence source. Human agents were often unreliable, could be turned into double agents, or if reliable, could take days or weeks to report. Photo Reconnaissance required clear weather, and could only show what was there, not what was going to happen. Only signals intelligence could peer into the mind of the enemy, and its authenticity was impeccable.  Despite the challenging efforts involved, the value of SIGINT to decision makers was so great that the enterprise continued to grow in peacetime, dwarfing other forms of intelligence to this day.