Next year marks the 50th anniversary of the US sending combat troops to Vietnam. However, there had been US personnel in the troubled region for many years prior to that. Americans had been sending supplies to South Vietnam and had assumed responsibility for training its army when France pulled out of the country in 1954. John F. Kennedy, who took office in 1961 and was committed to beating Communism, took immediate steps to assist the South Vietnamese in winning the political, military, economic, and psychological war against Communist-backed insurgency. It wasn't long, however, before the Republic of Vietnam needed more specific assistance. For example, the South Vietnamese army was trying to monitor the communications of the North Vietnamese-backed guerilla forces but needed help. The US Army supplied radio receivers and AN/PRD-1 direction-finders to support this effort. Shortly thereafter, US Army intelligence personnel began planning for a deployment to Southwest Asia to conduct small-scale communications intelligence (COMINT) operations.The Army Security Agency, or ASA, was tasked with this mission. The agency quickly put together two Operation Plans: WHITEBIRCH, which established a 78-man ASA operational element to target local Communist guerillas, and SABERTOOTH, which would field a 15-man team to train South Vietnamese COMINT specialists. Troops and equipment were identified and assembled at Fort Devens, Massachusetts within three days of presidential approval of the plans. The ASA contingent organized itself into the 400th USASA Operations Unit (Provisional) with a cover designation as the 3rd Radio Research Unit (RRU). The ASA personnel were required to carry US passports and wear civilian clothes. For the latter requirement, the Fort Devens Post Exchange remained open late one night so the Soldiers could select their "civvies." Since the exchange had only a limited variety to choose from, it became a standing joke that they might as well have been issued uniforms since they were all wearing clothes of similar style and color.On May 13, 1961, the 92 personnel of the 3rd RRU arrived at Tan Son Nhut Air Force Base, just west of Saigon. Its deployment marked the first time that an entire Army unit had deployed to Vietnam--previously, individuals had been assigned to the Army's advisory group individually. Within two days of arrival, the 3rd RRU had operations up and running. The unit established its headquarters in an empty warehouse on the Air Base. In nearby vans, RRU Soldiers tuned their receivers in search of manual Morse signals. By the end of the month, site surveys for a direction-finding net were completed. Stretching along the South Vietnam coastline, this net was fully operational by the end of June. In July, the unit began training South Vietnamese soldiers.The 3rd RRU's monitoring mission quickly moved from 17 hours per day to round-the-clock coverage. An additional 52 spaces were authorized. The direction-finding mission took on greater urgency, especially after the death of Specialist James Davis in December 1961. The 3rd RRU mourned the loss of their brother-in-arms, named their area of operations at Ton Son Nhut Air Base "Davis Station," and looked for a better way to find the elusive enemy.The answer turned out to be airborne radio direction finding. After several failed attempts, the 3rd RRU began seeing success in this new technology, allowing operator and equipment to get close enough to the target transmitter to get a good fix while remaining in relative safety. Airborne Radio Direction Finding would become one of the most important intelligence assets of the Vietnam War and in February 1963, the 3rd RRU became the first unit in Vietnam to receive the Meritorious Unit Commendation for its contributions during the escalation of the ground war.The 3rd RRU continued to grow in numbers in response to expanded requirements. A mobile detachment was established at Da Nang, much further north, manned by 21 officers, 4 warrant officers, and 356 enlisted men. When the mobile detachment relocated to Phu Bai, even closer to the northern border, it became known as Detachment J. Phu Bai became a second permanent base for air operations -- 450 miles north of Davis Station in Saigon. ASA began plans for a major field station to be constructed at the new location that would accommodate nearly a thousand personnel and a hundred operational positions. However, in the summer of 1964, the politics of the conflict changed everything.On August 4, 1964, President Lyndon Johnson asked Congress to approve a joint resolution that gave him broad authority to expand the US military's role in Vietnam beyond an advisory and into a defensive one. Specifically, the Gulf of Tonkin Resolution stated that, "Congress approves and supports the determination of the President, as Commander in Chief, to take all necessary measures to repeal any armed attack against the forces of the United States and to prevent any further aggression." The Resolution became the legal basis on which President Johnson, and later President Nixon, based their military policies in Vietnam. In the short term, it meant a massive build-up of troops, one consequence of which was the 3rd RRU ceased to be the only ASA asset there.By mid-1965, ASA had a total of 1,487 personnel distributed among three units: the 3rd RRU, the 7th RRU, and the 8th RRU. The 3rd RRU served as the command element over all three. However, over the next nine months, the creation of direct support units forced the 3rd RRU Commander into the untenable position of having to divide his attention between his own operational mission and ensuring that the direct support units were meeting the needs of their field commands. Therefore, ASA discontinued the 3rd RRU on June 1, 1966 and redistributed its mission and resources among four new organizations under the administrative control of the 509th Radio Research Group. Within six years of the 3rd RRU's initial arrival, ASA's in-country strength would be nearly 6,000 personnel, divided into 20 to 30 units that were further broken down into detachments and teams located at remote sites and fire support bases. At any given time more than 100 such elements could be scattered throughout South Vietnam -- all commanded by the 509th Radio Research Group.Following the cease-fire in October 1972, the 509th RR Group was discontinued on March 7, 1973 and the handful of its remaining soldiers boarded the last plane for home, bringing ASA's 12-year tour of service in Vietnam to a close.