This term was popularized by Richard Slotkin. The traditional Westerncowboys-versus-Indians myththat is the basis of a large percentage of Westerns stems from a basic fear of Native Americans that white settlers have held since the earliest colonial days. Savage battles did in fact take place historically, though the Native Americans were far less often the aggressors than were the white U.S. Cavalry and other white Indian hunters. In cinema Westerns, however, these brutal encounters have been used to reflect cultural fears from outside aggressors.
While 20th- and 21st-century American audiences no longer had to fear battles with Native Americans, the reenactment of Indian battles allowed filmmakers to reflect contemporary fears of such things as Nazism, communism, and other invading outside forces. When the whites fought the Indians on-screen, they always won and they always overcame evil through savage war. Savage war has been necessary in order to purify white America of traces of nonwhite culture. Westerns thus have always appealed to this basic cultural fear and have appeased it. After the classic Western era, filmmakers turned from Native Americans to Mexicans for waging savage war.
Historical Dictionary of Westerns in Cinema. Paul Varner. 2012.