For the United States Army, as Robert M. Utley proves in this excellent study, ""the two decades that began with the Mexican War and closed with the Civil War hold much greater significance than implied by the attention devoted to it by historians."" As the nation moved westward, its emigrating citizens first came into meaningful contact with the Indians of its vastly enlarged western domain. With nearly all the tribes during this period the blue-clad frontiersmen of the Regular and Volunteer armies skirmished inconclusively, and with a few they fought conclusively. In protecting the settlements, in policing the trail routes, and in warring with the Indians, their achievements and failures between 1848 and 1865 contributed importantly to a major chapter in the history of westward expansion. Mr. Utley's book is scholarly without being pedantic, authoritative without being overbearing, convincing and entertaining. A promising first volume in Macmillan's A History of the United States Army series (twelve more are in preparation).