Production of signal intelligence during World War II for use by American armed forces was a process more elaborate than the creation of a set of oriental rugs. Looking back today, patterns are recognizable but variations abound. The interdependence of different participants in production is as evident as that of the shepherds, spinners, loom-makers, dyers, and weavers, whose common product, like SIGINT, might end up in an office with a parquet floor under a handsome chandelier or in the tent of a nomad, or in something in between those extremes. But in any setting, it would be highly prized.

During World War II, Americans preferred the term "communications intelligence" (COMINT) as a near equivalent to the British term "signal intelligence," but they accepted the abbreviation, SIGINT, and used it. After World War II, the United States armed forces distinguished electronic intelligence (ELINT) from COMINT, and for several years reserved control over ELINT matters from the province of the U.S. Communication Intelligence Board. When that segregation ended, the term SIGINT soon displaced COMINT in general practice. Without wishing to predate American use of the term "SIGINT" instead of COMINT, I have used it in this account of events in World War II.

American and British units, both together and separately, produced SIGINT used by the armed forces of both countries, either separately or in combined actions. Each country had a SIGINT organization with a center at its capital and tributary stations elsewhere in the country and overseas throughout the world. The British had a unified organization and three separate Service organizations. The Americans had no unified organization; each Service had its own and coordinated with each of the other's. In the overseas theaters they had centers in rear areas and mobile units in combat zones. The latter could be teams, parties, platoons, sections, detachments, companies, or groups – anywhere from 3 to more than 200 men. The ground and air components of the U.S. Army developed related but distinctive operations and units.

A theater's Signal Intelligence Service (SIS) provided tactical SIGINT to commanders. Dissemination was subject to security regulations appropriate for a product of high value and precarious availability. Enemy communications from which tactical SIGINT was derived were those passed between low and intermediate levels of command in low-grade or medium-grade cryptographic systems, or in plain language. When enemy communications at high military levels and in high-grade cryptographic systems could be read, the product – special intelligence – took form after elaborate processing at the British center in England; it was forwarded under stringent controls to eligible recipients, including commanders in the theaters of operations. Thus, there was a dual Allied SIGINT operation-production and dissemination of tactical intelligence ("Y") and special intelligence ("Ultra" or "U").

This narrative, while noting the existence and relevance of special intelligence, makes no attempt to explain the methods of its production, or to show with any precision how it applied to particular operations. Instead, the history centers on American production and use of tactical SIGINT ("Y"), as accomplished in the western Mediterranean and European Theaters of Operation, U.S. Army. It treats Ultra marginally, as it had tactical applications.

Radio communications among American combat units were monitored by the enemy for the same purposes that the Americans were served by radio surveillance of the Germans. American traffic was also monitored by Americans for two purposes: to detect and correct communications insecurities that the Germans could exploit, and to keep track of advance American elements in order to keep American commanders continuously and currently informed of their positions, situations, and intentions. This history does not deal with those operations beyond indicating their claims on radio intercept sources.

Some information about the German Army field SIGINT service is included here. The war was a competition not only between the operators of guns, tanks, aircraft, and other weapons, but between the operators of radio receivers, radio direction finders, and the facilities available to intelligence analysts. Since it is axiomatic that SIGINT emerges from defective COMSEC, instances of German SIGINT success are likely to be examples of American COMSEC failures. To that extent only, is American COMSEC a part of this account.

The Mediterranean theater was only one of many in a conflict often described as "global" in scope and "total" in depth. Military application of technology was accelerated during the conflict. It may be helpful to cite some of the relatively new and distinctive features of World War – features which have become less striking in the light of later and newer developments. Between World Wars I and II, aviation, called "air" or "air power," had become transformed. It relied, however, on single and multimotored propeller aircraft; jet-powered planes were being used, but not widely, as the hostilities ended. Missile systems had been sufficiently developed by the Germans to be used in warfare, but they had not perfected accurate delivery systems. Rockets were widely used by ground, sea, and air forces. The bombsight was sufficiently refined to achieve fair accuracy from great heights. The role of "air" made the acquisition and defense of airfields essential.

During World War II, automotive transportation largely supplanted that by animals; animals were still being used, but primarily as pack animals in mountainous terrain. The newer vehicles rolled on either tires or "tracks." They ranged from small cars and motorcycles to massive, heavily armored tanks equipped with thick armor, 150-mm long-barreled guns, as well as lighter weapons. Artillery was adapted to the new kinds of targets. Shells could be armor-piercing, incendiary, high explosive, white phosphorus, smoke, or high velocity. The "proximity fuse" caused detonation with maximum effect. Bombs, like artillery shells, varied in character as well as in size. Napalm and flamethrowers were used against sheltered positions. Radar and sonar were widely used. Beacon signals assisted ships and aircraft in navigation. Radio direction finding (DF) – goniometry – was a reversing of that procedure.

To offset the military control of ports by hostile forces, the Allies developed the means and the methods necessary to land men, weapons, supplies, trucks, and even armored vehicles through surf and across beaches.

Naval ships bore on their decks small aircraft for reconnaissance and for directing naval gunfire on shore targets. Carriers with flight and hangar decks were used as floating bases for fighter and bomber aircraft.

The control of numerous dispersed units – ground, air, surface, and submarine – required extensive facilities for radiotelegraph and radiotelephone communications.

In Northwest Africa, during 1942–1943, American "commanders began to realize as never before the potentialities of mobile radio, radiotelephone, carrier telephone, and teletype, to say nothing of the immense possibilities of radar, radio intelligence, radio countermeasures, propaganda, and so on. Commanders began to take for granted facilities undreamed of in any previous conflict. They expected to be able to communicate at any time with subordinates, even in moving vehicles widely scattered over a mobile front. They expected to be able to talk with headquarters however distant. In fact, they began to demand facilities not yet developed."^[1]

Technology to meet the COMSEC requirements produced, among other devices, high-speed, automatic Morse telegraphy and non-Morse radioprinters. For security, communicators employed cipher machines and telephone "scramblers." The communications traffic of both sides became voluminous; each sought successfully to derive SIGINT from the other's signals.