After presenting a chronological account of United States military documentary films from World War I through the Vietnam War, this paper reports on a study that compared the effectiveness of films prepared during World War II, the Korean War, and the Vietnam War. The first section, the chronological account, touches on such subjects as the production of World War I films by the Division of Pictures, established in 1918; the growth of the American documentary tradition that contributed to later U. S. military films; the development by Frank Capra, a Hollywood producer, of a series of World War II films entitled "Why We Fight" in response to the Pentagon's needs for propaganda; and the production of two subsequent propaganda films, "Why Korea" and "Why Vietnam." The second section discusses the research approach used in comparing results of analyses of "Why Korea" and "Why Vietnam" with results of a previous analysis of the "Why We Fight" series. In its third section, the paper reports observations made regarding visual, audio, and thematic aspects of the three films. It concludes that from an artistic point of view, "Why We Fight" is superior to both of the later films, providing a far richer visual experience and complementing the visual elements more effectively through music and narration. It also reports the generally negative response to "Why Vietnam." (GT)