General William C. Westmoreland, the commander of U.S. forces in South Vietnam between 1964 and 1968, called helicopter air assault warfare “the most innovative tactical development to emerge from the Vietnam War.” Nothing symbolized the American war effort in South Vietnam more than the helicopter in air assault mode, and no unit was more closely associated with that way of war than the 1st Cavalry Division (Airmobile). What was striking and original about that division at its creation in 1965 was the unusually large number of organic helicopters—428—and their multiple functions. As an integral component in the division’s operations, helicopters delivered troops to the battle area to attack, reinforce or block and returned them to camp; supplied them in the field; carried out reconnaissance and screening missions; and provided aerial artillery for the division’s ground troops. Such activities represented a radical rethinking of the Army in combat, allowing planners to design action in terms of vertical movement on the battlefield as well as along the lines of the more traditional two dimensions. Since the division did not emerge full-grown and mature at the start of the Vietnam War, where did the notion of air assault, also called airmobile, warfare originate?3 How did it evolve? What, on the eve of the commitment of American ground forces to Vietnam, had it produced?4 And finally, what lessons does this study hold for the Army regarding transformation?