The United States Army's Security Agency (ASA) was, from 1945 through 1976, the United States Army signal intelligence branch. Its motto was "Vigilant Always." The Agency was the successor to a number of Army signal intelligence operations dating back to World War I. As well as intelligence gathering, it also had responsibility for the security of Army communications and for electronic countermeasures operations. In 1976, the USASA was merged with the US Army Military Intelligence component in a process which formed the United States Army Intelligence and Security Command (INSCOM).

Composed primarily of soldiers with the very highest scores on Army intelligence tests, the ASA was tasked with monitoring and interpreting military communications of the Soviet Union, the Peoples Republic of China, and their allies and client states around the world. ASA was directly subordinate to the National Security Agency and all field stations had NSA tech reps on site.

All gathered information had time-sensitive value depending on its importance and classification. Information was passed through intelligence channels within hours of intercept for the lowest-priority items, but in as little as 10 minutes for the most highly critical information.

ASA personnel were stationed at locations around the globe, wherever the United States had a military presence -- publicly acknowledged or otherwise. In some cases such as Eritrea, it was the primary military presence. Although not officially serving under the ASA name, cover designation being Radio Research, ASA personnel were among the earliest U.S. military advisors in Vietnam.

The first battlefield fatality of the Vietnam War was Specialist 4 James T. Davis (from Livingston, Tennessee) who was killed on December 22, 1961 on a road near the old French Garrison of Cau Xang. He had been assigned to the 3rd Radio Research Unit at Tan Son Nhut Air Base near Saigon, along with 92 other members of his unit. Davis Station in Saigon was named after him. President Lyndon Johnson later termed Davis "the first American to fall in the defense of our freedom in Vietnam".

All ASA personnel processed in country through Davis Station. ASA personnel were attached to Army infantry and armored cavalry units throughout the Vietnam war. Some select teams were also attached to MACV/SOG and Special Forces units.

ASA military occupational specialties (MOS's) included linguists, morse code  intercept operators ("Ditty Boppers" or sometimes "Hogs" for their 05H designation), non-morse (teletype and voice) intercept operators, communications specialists, direction-finding equipment operators ("Duffy's" for their 05D designation), cryptographers (crippies), communications traffic analysts, and electronic maintenance technicians and a 42 man Special Operations Detachment to conduct clandestine combat operations, among others. ASA had its own separate training facilities, MP corps, communication centers and chain of command.

These occupations, which required top secret clearanece, were essential to U.S. Cold War efforts. ASA units operated in shifts, 24 hours a day, seven days a week. ASA troops were not allowed to discuss their operations with outsiders — in fact, they could not talk among themselves about their duties unless they were in a secure location. Even today, decades after they served, some of the missions still cannot be discussed. Owing to the sensitivity of the information with which they worked, ASA soldiers were subject to travel restrictions during and long after their time in service. The activities of the U.S. Army Security Agency have only recently been partially declassified. This turn of events has been accompanied by the appearance of a small number of ASA memoirs and novels.