In addition to the tank and aircraft, another piece of technology came of age during World War II. Signals intelligence, or SIGINT, was yet one more instrument or arm that the commander had to integrate and coordinate with others. Recent histories of the war probably has overstated the strategic importance of SIGINT, while they have understated its tactical role. An army's ability to plan for future operations and concentrate the different arms at the decisive location depended in part on such intelligence.
Ultra, the British codeword for intelligence based on decoding highly classified German radio messages, gave the western Allies only limited access to German military intentions and capabilities. The German Army normally used secure landline communications for high-level messages, except when fluid operations forced them to make radio transmissions. Even then the Allies did not necessarily intercept, let alone decode in a timely manner, every German message. The Germans changed their code every twenty-four hours and periodically made major shifts in codes or equipment. The Allies might go for days or even months without being able to decode transmissions on specific radio networks. On 1 May 1940, for example, Germany changed virtually all its codes, blinding the Allies" SIGINT effort until 22 May, by which time the German offensive through the Ardennes had succeeded. Similar problems recurred during most of the War.
Nor were the deciphered messages of Ultra always illuminating for the tactical and operational situation. Only rarely did the most senior German commanders communicate their specific plans, except where Hitler was personally interfering in operations and required detailed reports. Intelligence analysts pieced together much of the most valuable Ultra information over long periods, or inferred capabilities on the basis of logistical messages. Moreover, few Allied commanders below field army level had access to this information.
The worst drawback of Ultra-level SIGINT was that it discouraged the use of other sources of intelligence collection that might confirm or deny Ultra information and blinded Allied commanders to threats that were not discussed in German radio traffic. In early 1943, for example, the Allied forces in Tunisia relied heavily on Ultra; their other intelligence collection means were improvised and largely ineffective. The German offensive of Sidi-bou-Zid-Kasserine Pass in February 1943 surprised the Allies because available SIGINT indicated that higher German headquarters had disapproved such an operation in favor of an attack elsewhere. Of course, SIGINT could not know that Rommel and other German commanders had met face-to-face on 9 February and had developed a plan that led to the attack on Sidi-bou-Zid. This attack mauled a dispersed U.S. armored division. Lack of SIGINT and misinterpretation of available intercepts also had a considerable effect on Allied failure to predict the scale and intensity of the German counteroffensive in the Ardennes in December 1944.
Although the western Allies held a priceless asset in the strategic intelligence they received from Ultra, for much of the war German SIGINT was more effective at the tactical level. From 1940 to 1942, for example, a single Horch (listening or intercept) company in North Africa skillfully interpreted the unencrypted tactical communications of British units, giving Rommel a complete picture of enemy dispositions and intentions during battle. When the British finally became aware of this unit's activities in July 1942, an Australian battalion raided and captured the company. German replacements could not replace the expertise of the analysts lost in that company and thus had more difficulty detecting later British deception operations.
By contrast, relatively little information is available concerning Allied tactical SIGINT, including the British "Y” Service and American "Radio Intelligence." German tactical communications were often unencrypted, or used easily deciphered code systems. From a miniscule prewar basis, the Allies had to develop their knowledge of German tactical radio networks and procedures. In terms of offensive electronic warfare, the Allies had a number of notable successes. During the evacuation of Dunkirk in 1940, the British effectively jammed German bomber communications, hampering Luftwaffe attacks on the retreating British forces. Two years later, when Montgomery launched the second Battle of Alamein, airborne jammers disrupted German tactical radio communications for hours.
The development of effective tactical radio communications was the basis for controlling fluid, mechanized operations as well as the raw material for tactical SIGINT. The demand for such communications greatly accelerated research and development in this area. In particular, the U.S. Army pioneered the use of frequency modulation (FM) radios for short-range tactical communications, and both very high frequency (VHF) and ultra high frequency (UHF) radios for longer range communications. Unlike the European armies, the U.S. Army used FM extensively, because it provided static-free signals over a wide variety of channels without using a separate crystal for each frequency.
The combination of reliable radio communications with efficient tactical signals intercept services also provided a new opportunity for senior commanders to follow the course of battle without delays in the communications system. Both the British and American armies developed means for senior headquarters to receive battle reports by radio without waiting for the messages to be processed through intermediate layers of command. That is, the senior headquarters could monitor tactical unit radio networks directly, or else assign a radio-equipped liaison detachment to each forward unit to report the situation to the senior headquarters. The British GHQ Liaison (Phantom) units and the American Signal Information and Monitoring (SIAM) companies performed this service admirably during 1944-45, and in the British case as early as 1942. The danger with such a monitoring system, as Gen. Dwight D. Eisenhower noted after the war, was that the senior commander might be tempted to bypass the intermediate headquarters and interfere directly in the battle, using the system for command rather than as a source of timely operational and intelligence information. In the latter role these monitoring services enabled much more effective coordination of the battle, allowing the commander to react through his subordinate commanders to situations as they developed.