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Image downloaded from https://rarehistoricalphotos.com/vietnam-war-the-early-years-1965-1967/
According to the Meriam-Webster dictionary air mobility is defined as: The ability to transport military troops and supplies in and out of combat areas by means of aircraft.
On 11 December 1961 the United States aircraft carrier USNS Card docked in downtown Saigon with 82 U. S. Army H-21 helicopters and 400 men. The 57th Transportation Company (Light Helicopter) from Fort Lewis, Wash., and the 8th Transportation Company (Light Helicopter) from Fort Bragg, N. C., had arrived in Southeast Asia; it was the beginning of a new era of airmobility in the United States Army.
Just twelve days later these helicopters were committed into the first airmobile combat action in Vietnam, Operation CHOPPER. Approximately 1,000 Vietnamese paratroopers were airlifted into a suspected Viet Cong headquarters complex about ten miles west of the Vietnamese capitol. The paratroopers captured an elusive underground radio transmitter after meeting only slight resistance from a surprised enemy. The events of December 1961 prefaced a decade of unparalleled growth of air mobility.
From LG John J. Tolson, Vietnam Studies: Airmobility 1961-1971.