A nations ability to read its opponents communications is a sign of its intelligence communitys prowess. Signals intelligence transformed modern warfare by enabling combatants to know their enemies plans in advance. This proved decisive in many conflicts during the 20th and 21st centuries.Early Development of SIGINTHMS Diana, a British warship that intercepted Russian naval signals, 1904. Source: Global Defence TechnologySignals intelligence is a far more modern concept than human intelligence. Its origins date back to the first use of wireless communications. During the Boer War, both the British and Boer armies used wireless sets to communicate. However, interception was rare. The first known signals intercept was when the British warship HMS Diana intercepted a Russian communication ordering the mobilization of its fleet to fight Japan in 1904. The Japanese learned from the British and successfully intercepted some Russian messages during the Russo-Japanese War.WWI saw a massive expansion in the use of SIGINT. France established a station on the top of the Eiffel Tower to intercept German messages. This helped the Allies stop several German advances towards Paris. At sea, the Central Powers struggled to send its ships and submarines out without being tracked by Allied intelligence. This was thanks to the efforts of Room 40, a station based in Britains Admiralty in Whitehall that intercepted enemy naval transmissions. The United States began employing SIGINT in 1918 by setting up stations on the Atlantic Coast.The most famous SIGINT operation of WWI was when British intelligence intercepted a telegram from Germanys foreign minister, Arthur Zimmerman, to the Mexican government offering German support for Mexico if it went to war with the United States. London showed the message to Washington with the hopes of swaying American public opinion towards the Allied cause. While Mexicos government did not declare war, the intercept played a role in convincing the United States to enter the war in 1917.The Practice of Direction FindingHuff-Duff set onboard HMS Belfast (date unknown). Source: Scholarly Community EncyclopediaIn addition to intercepting transmissions and messages, radio direction finding became a major component of SIGINT. In the early 1900s, researchers were experimenting with different types of antennas to track the location of transmitters. For instance, Captain Henry Round began testing antennas for the British Army after their deployment to France in WWI. Allied naval officers during the war also hoped to pinpoint the location of German U-Boats while they were at sea. Powerful antennas were positioned on the British and American coasts for this purpose.Early DF (direction finding) systems had serious flaws. For instance, if the analyst tracking where the transmission origin point was off by just a little, he could be dozens of miles away from the actual point of origin. For instance, British DF analysis at the Battle of Jutland was off due to user error, meaning that the Royal Navy was not able to accurately track the movements of the German fleet. This improved slowly as more accurate antennas were designed and constructed in greater numbers.In the interwar period, High Frequency Direction Finding, or Huff-Duff, was designed by Robert Watson-Watt. He demonstrated that intercepting radio signals at a high frequency could mean that thunderstorms could be tracked. It also meant that long-range transmissions could be identified more easily. His Adcock antenna, developed in 1924, was one of the strongest DF antennas created to date. By the time WWII broke out, the British had an advantage in Huff-Duff capabilities. This was very beneficial when Germany attacked Britain during the Battle of Britain.SIGINT in WWIIMarian Rejewski, a Polish mathematician who helped break Enigma, 1939. Source: US Department of DefenseThe trajectory of WWII was shaped in large part by the massive use of SIGINT by all the major combatants. The Allies had a major advantage in human and material resources, which helped them develop machines that could crack Axis codes. They also benefited from intelligence networks in occupied Europe and Asia that helped them gain access to Axis transmissions. When Poland was occupied in 1939, much of its Cipher Bureau escaped to France and Britain, bringing with them vital information about the Germans Enigma machine. Among these men was Marian Rejewski, who brought details about Enigmas construction to the British.Developed in 1919, the British Government Cypher and Communications School was one of the preeminent SIGINT agencies in the world at the outbreak of war. In 1939, GC&CS was run by Royal Navy Commander Alistair Denniston. He brought in a team of scientists and mathematicians to crack Axis codes, especially Enigma. This included men like Alan Turing, who helped develop the Bombe, a machine that could decipher every one of Enigmas code changes, and Gordon Welchman, who deciphered German Luftwaffe codes.Thanks to the efforts of the Bletchley Park codebreakers, the British had broken German and Italian codes and set up a system called Ultra. Ultra provided critical intelligence to Allied commanders while fighting in North Africa and Europe. While it could not provide a complete picture, it did provide useful information on Axis plans and orders of battle. Ultra was essential to Allied war planning in Europe.Japanese carrier Hiryu on fire shortly before sinking during the Battle of Midway, 1942. Source: US Naval History and Heritage CommandThe Americans main SIGINT success in WWII was the cracking of Japans codes, especially JN-25. In 1942, Commander Joe Rochefort managed to crack JN-25 and identified where and when the Japanese fleet was sailing to attack Midway Atoll. The subsequent battle gave the US Navy a decisive advantage in the Pacific for the rest of the war. Like Germany, Japan did not realize that their codes had been broken for the rest of the war. Tokyo believed that Purple, its main encoding machine, was impregnable. Japans own codebreaking efforts were limited and it never achieved the penetration that the Allies did.Rocheforts Station Hypo, also called Fleet Radio Unit Pacific, continued to decode Japanese messages for the rest of the war. Australian SIGINT personnel worked with their American counterparts in a station in Melbourne. The purpose was to gain intelligence on Japanese fleet and convoy movements. The US Navys submarine fleet targeted Japanese convoys based on SIGINT intercepts. The system for disseminating intercepts was called Magic and it had the same effect as Ultra: reducing Allied losses by supplying much needed intelligence.The Soviets never had the large-scale SIGINT capabilities that the Western Allies had. Nevertheless, the military intelligence service GRU gained some success intercepting low-level German radio transmissions. They gained much of their SIGINT from Allied sympathizers in Europe and America, including the Cambridge Five. Additionally, Washington and London agreed to share some intelligence from Ultra to Moscow. This ended when it became clear that the Soviets saw the Allies as future competitors and adversaries.SIGINT in the Cold WarNSA SIGINT antennas in Sinope, Turkey, that were used to track Soviet missile launches, 1964. Source: National Security Archive, George Washington UniversityThe Cold War saw a major increase in the use of SIGINT to track nuclear launch tests in addition to its more traditional uses. In 1952, President Harry Trumans administration created the National Security Agency to amalgamate the US Army and Navy SIGINT units. The NSA became one of the largest SIGINT agencies in world history. For the rest of the Cold War, it tracked Communist Bloc military and nuclear activity. For instance, NSA radio antennas in Turkey identified Soviet ships headed to Cuba during the Cuban Missile Crisis. It also tracked Soviet troop movements during the Hungarian Uprising of 1956 and the Prague Spring of 1968.US Navy submarines played a crucial role in assisting with the collection of SIGINT. They patrolled in areas off of major communist ports to track Soviet naval movements. Radio operators on board these submarines listened in to rival radio traffic. Submarines also attached wiretaps to communications cables to increase the NSAs reach. On the surface, several spy ships such as the USS Liberty and USS Pueblo undertook surveillance missions off the coasts of Soviet satellite states. These were risky missions: the Pueblo was captured by the North Korean navy and the Liberty was accidentally attacked by Israel during the 1967 Six-Day War.The United States worked closely with its allies around the world, especially in NATO, to combat the communist threat. This involved sharing SIGINT. The creation of the Five Eyes Alliance between the Anglosphere countries meant that SIGINT collection became interoperable between the member nations.SIGINT in the War on TerrorFort Meade, the headquarters of US Cyber Command and the NSA, 2013. Source: C4ISRNetThe end of the Cold War meant that many countries began reorienting their security priorities. SIGINT collection was increasingly directed towards stopping terror plots. This became especially important after the 9/11 attacks. Because terror suspects targeted for SIGINT gave off fewer transmissions than a countrys military, it meant that SIGINT agencies had to be more precise about how they collected information. This meant that certain plots, such as the Khobar Towers bombing or attack on the USS Cole, could not be stopped in time due to lack of foreknowledge of the plots.Using SIGINT to target small groups or individuals caused Western intelligence agencies to run afoul of civil liberties laws. In 2013, NSA contractor Edward Snowden revealed documents that show that the US and several of its allies used illegal surveillance measures to spy on their own citizens as part of an effort to quell terror plots. This led to a serious backlash and questions about how SIGINT collection could affect the lives of regular people.In the Middle East, Israel became a pioneer in the use of SIGINT to fight the Arab states and Palestinian militant groups. Israeli military intelligence AMAN was able to track radio traffic of the Arab armies before the 1973 war. During the 1st and 2nd Intifadas, the Israelis used SIGINT to track militant groups before they could launch attacks against Israeli civilians. SIGINT also helped Israel defeat Hezbollah in 2024. Going forward, SIGINT will remain an essential tool and likely incorporate new technologies like AI software that helps with voice recognition on radio and phone messages.