From 1966 to 1968 the 4th U.S. Infantry Division operated in the Central Highlands of Vietnam, where the indigenous population consisted primarily of Montagnard tribesmen with near-Neolithic cultures living in the countryside, and secondarily of a ruling group of Vietnamese living in the cities and a few villages. Personnel of the 4th Division conducted civic action programs among the Montagnards and the Vietnamese for the purpose of enhancing the security of the division, but the nature of these programs had to be adapted to the needs and mores of the two indigenous cultures. Officials of the Vietnamese government also conducted programs among the Montagnards to maintain governmental control and ethnic dominance over them. American civic action policies varied over time as officers with different perceptions were in charge of them, and implementation of the policies affected the Montagnard and the Vietnamese cultures in ways that were sometimes constructive and sometimes contradictory. This article is an analysis of psychological, cultural, and military relationships among members of the three cultures, and of the consequences of American civic action policies for the two indigenous populations and for the security of the division. It is based on archival materials from the 4th Infantry Division and letters from American officials who participated in the civic action programs.