The idea of cavalry being used as a mobile, mounted force dates back centuries, and had been used very effectively in warfare all the way up to the early 20th century. The days of mounted cavalry charges and an army primarily using horses as a mode of transport, however, was long gone by the time of the Vietnam War.
The idea of capitalizing on troop mobility, rapid reaction forces and versatile reinforcement capability are still quite valid today. In Vietnam, it made sense that the newly emerging technology of helicopter troop and gunships fit well with the cavalry concept, and the terrain the US faced in that conflict (rice paddy and jungle) also seemed well-suited to the widespread use of helicopter-borne troops in self-contained units.
The Army began testing the air cavalry/air mobile concept in 1963 with the 11th Air Assault Division. By 1965, the 1st Air Cavalry Division had been formed and detailed to Vietnam. Tactics were adjusted to fit the counterinsurgency role, and to provide close air gun support to troop insertions and extractions. In addition, air mobile units were equipped to evacuate wounded to well-equipped hospitals much more quickly than had been possible before, thus cutting down greatly on battlefield losses.
Air Cav became legendary during the war, was part of eight separate campaigns through 1972, and the lessons learned during the Vietnam War with Air Cavalry are still applied and refined in the Iraq and Afghanistan Wars we are currently involved in.