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, labouring 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 20th 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.
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.
Quo Fata Vocant-Whither the Fates call