The Army Security Agency existed from 1945, when it absorbed the Signal Corps' Radio Intelligence Companies, until 1977, when it was consolidated with existing military intelligence units under the new Intelligence & Security Command.
In the beginning, the ASA set up a chain of listening posts around the Soviet Union. These posts, which performed signals intelligence missions against the Soviet Union and its satrapies in the Warsaw Pact, were called field stations. (Later stations targeted other Communist nations in Asia.) The primary chain of command for the ASA ran outside the Army's structure to the National Security Agency; the ASA units were part of the Army only insofar as the Army supplied trained personnel to the field stations and took care of their pay, supplies and (occasional) disciplinary actions. Early in the Cold War, the primary targets of the ASA were Morse code signals in the AM band, but the tasking later shifted to voice communications in the FM band, non-voice signals such as teletype and radar, and multiplexed voice signals. Radio direction finding was also a large part of the ASA mission, literally so in the case of the AN/FLR-9 "Elephant Cage" RDF system. This antenna array was 1400 feet in diameter and 120 feet high, and eight of them were erected.
ASA enlisted personnel were mostly draftees, although due to the stringent mental requirements many ASA enlisted men were either college graduates or college-bound; it was not unusual to have ASA soldiers with more education than their platoon leaders. At first this posed no problem, since the training course for 05H Morse code interceptors (also known as "ditty-boppers" or "Hogs") was only a few weeks long, but later as the mission focused more on voice intercepts, this became a serious problem since many of the target languages required a year-long course at the Defense Language Institute, followed by four months of technical training at Goodfellow AFB and Fort Devens, and draftees were only in for two years. Moreover, re-enlistment rates during the era of the draft were only 12 per cent. This obstacle was sometimes overcome by asking draftees whether they would prefer four years in Germany with the ASA or two years in Vietnam with the infantry. Until 1967, there was no separate military intelligence branch in the officer corps, and officers detailed to the ASA mostly came from the Signal Corps.
The direct control of ASA units by the NSA became an irritant for the army during the Vietnam War. ASA units were required to pass all information up the reporting chain to the NSA at Fort Meade, and were forbidden to share any classified intelligence with the units they coexisted with. The NSA, which was primarily focused on strategic intelligence, often neglected valuable tactical intelligence or relayed it to field commanders too late for the intel to be of any use. Frequently, the only intel firebase or garrison commanders got from ASA units was the inference that an attack might be underway when the local ASA -er, "Radio Research"- listening post began hurriedly tearing down its antennae and preparing for a move. This ongoing intel failure became known as the "Green Door Syndrome", and was part of the reason General Charles Denholm was tasked with the job of bringing the ASA back into the Army.
Tactical ASA units had existed during Vietnam, attached to various division headquarters, the 11th Armored Cavalry Regiment, and to select Special Forces and Special Operations Group teams, but the Green Door Syndrome prevented them from rendering much help to the commanders they were ostensibly supporting. This changed after the war, when ASA units became organic to corps, division, and regimental headquarters, and ASA troops began being trained in the arcane (and pointless) art of "sanitizing" signal intelligence so as to conceal its origin and speed its delivery to supported commands.
Eventually, with the abolition of the ASA, ASA battalions at corps headquarters were combined with MI assets into a corps MI group; similarly, the division support companies were folded into divisional Combat Electronic Warfare & Intelligence battalions, which also included a ground surveillance radar company and a LRRP company. ASA companies in armored cavalry regiments became part of oversized MI (CEWI) companies. This process continued into the 1990s as Reserve Component ASA units were converted to the new CEWI organization.
ASA MOS codes:
05B - Linguist (Not used after the Vietnam War)
05D - Radio Direction Finding operators (a/k/a "Duffies")
05G - Communications & signal security specialists (a/k/a "Buddyfuckers")
O5H - Morse code interceptors ("Ditty-boppers" or "Hogs")
05K - Non-Morse/non-voice (teletype & fax) intercept operators
98B - Cryptanalysis technician
98C - Signal intelligence analysts
98G - Voice intercept operators
98J - Non-communications (radar/telemetry) intercept & analysis techs
98K - Signal collection/ID analyst
For most of the history of the ASA, linguists were nicknamed "Monterey Marys" for the high number of homosexual men who seemed to find their way into the MOS. Allegedly, someone in the Army got the bright idea that the DLPT, which tested for language learning aptitude, might also be (somehow) testing for homosexuality. They farmed this notion out to the University of Michigan, and eventually the results came back: there was a high, if not 1:1, correlation between high scores on the DLPT and homosexual tendencies. A crash project began to develop a revised version of the DLPT, and at roughly the same time, the 98 CMF was opened to women. These days the 98G population is no more gay (or lesbian) than the average Army unit.
The James Crumley novel One to Count Cadence is, I am told, a fairly accurate depiction of ASA units in the early Vietnam War period.