The nature of the enemy and the terrain, together with the peculiar political circumstances in which the wars in Southeast Asia were fought, soon highlighted the need for precise control of air attacks.  During the Korean War the USAF had developed a system of airborne forward air control using specially trained pilots and observers who controlled air and artillery strikes in T-6 Texans, often in close proximity to friendly troops.  By the early 1960s the aircraft of choice for the forward air control mission was the little Cessna O-1 Bird Dog.  Several O-1s were supplied to the VNAF even before the US forces began to take over the air war.  The first USAF FACs were brought in with the Jungle Jim detachment and flew T-28s and borrowed O-1s from the VNAF to perform FAC duties.  The US Army also employed FACs in their own Bird Dogs in the early years but after 1965 the FAC mission was turned over to the USAF and the Army’s O-1s specialised primarily in artillery observation.  The USAF’s first FAC squadron, the 19th TASS, was formed at Bien Hoa in June as part of the 34th TG and became operational in September.  The Squadron was equipped with the O-1F model of the Bird Dog and was only intended to spend six months in Vietnam after which it would pass on its aircraft to the South Vietnamese and deactivate.  The O-1F was a FAC conversion of the Army’s O-1D light liaison aircraft.  Supposedly the Squadron’s prime mission was to train the VNAF in forward air control and visual reconnaissance techniques but it also provided operational forward air control for the Farm Gate strike aircraft.  Four O-1s had arrived by the end of July and a further 18 arrived in August on board the USS Card.  Six aircraft were later deployed to Can Tho.  Eventually four Tactical Air Support Squadrons were activated in South Vietnam, each one attached to a Corps which covered a geographical region of the country.  The 19th TASS based at Bien Hoa was attached to III Corps; the 20th at Da Nang was attached to I Corps; the 21st at Pleiku was attached to II Corps; and the 22nd at Binh Thuy was attached to IV Corps.  By August 1965 the four Squadrons had a total of about 120 O-1Fs on charge.  In 1966 a fifth squadron, the 23rd TASS, was formed at Nakhon Phanom for FAC duties over Laos.