The United States Army Security Agency was popularly referred to by its military acronym as the USASA, or simply ASA. During the Vietnam War (1965 to 1972), The ASA provided signals intelligence (SIGINT) directly to separate Army brigades, divisions, corps, the 5th Special Forces Group, and the senior headquarters in Vietnam referred to as the Military Assistance Command, Vietnam, or by its acronym MACV. *2
Signals Intelligence is the exploitation of the enemy’s use of the electromagnetic spectrum in order that friendly forces obtain information and intelligence about the enemy’s communication and other signals and hence about enemy intentions, plans, and capability. Communications Intelligence is a component of signals intelligence.
Communications Intelligence includes cryptology; radio direction finding of transmitters and receivers; exploitation of plain text radio revelation; and analysis of callsign, modal link, traffic pattern, signal strength, terrain, radio frequency, signal duration, time of transmission, length of transmission, transmitter identification and characteristics, type and location of antenna, and other parameters. These techniques are sometimes referred to in military argot as electronic support measures (ESM). Manipulative deception and spot jamming are also signals intelligence techniques and ESM if used to exploit communications. When used for disruption or prevention of communication without regard to exploitation, jamming is referred to as electronic countermeasures or ECM. *3
Officers assigned to these units were trained in the Electronic Warfare Cryptologic military occupational specialty (MOS). Enlisted men were assigned from a family of cryptologic military occupational specialties (MOS) like radio intercept operator, radio direction finding operator, traffic analyst, linguist, and others.
Cryptography, by contrast, is the science of protecting friendly communications from exploitation by enemy cryptology. Cryptography uses codes and ciphers and special communications security (COMSEC) operating techniques to protect against enemy cryptology or signals intelligence.
The ASA provided signals intelligence support to the Army. With one exception, the Army Signal Corps provided cryptographic support. The exception is that the ASA also monitored friendly communications to determine vulnerability to exploitation by enemy signals intelligence. That part of the ASA mission was called communications security or electronic counter countermeasures (ECCM).
Toward the end of the Vietnam War, the term electronic warfare (EW) once exclusive to actions involving noncommunication emitters like radar, telemetry and jamming or deceiving those emitters, became the colloquial term used by the United States Army to define tactical attack and defense of the electromagnetic portion of the battlefield.
The ASA was deactivated in 1975 and its resources and experience integrated into Army Combat Intelligence and Electronic Warfare (CEWI) units and designated Military Intelligence units.
Most communications intelligence success occurs because an enemy uses poor security practices, poor equipment design, or carelessness. A cryptologic axiom is, “Protect source and success!”
The “source” is the targeted enemy electronic emitters and systems, their emissions, and the way enemy operators use them.
“Success” is the result of exploiting electronic emissions for meaningful intelligence. If source and success are revealed, the enemy will take steps to prevent exploitation. The emphasis on protecting source and success evolved into special security practices, need to know restrictions, and limited access code words within the cryptologic community referred to as Special Intelligence or routinely by the acronym SI.
2 - In Vietnam the term Army Signals Intelligence referred mostly to intelligence from communication signals not from all electronic signals as the term may imply.
3 - From unclassified sources.