“The drums they roll, upon my soul, for that’s the way we go,” runs the chorus in a Harrigan and Hart song from 1874. “Forty miles a day on beans and hay in the Regular Army O!” The last three words of that lyric aptly title Douglas C. McChristian’s remarkable work capturing the lot of soldiers posted to the West after the Civil War. At once panoramic and intimate, Regular Army O! uses the testimony of enlisted soldiers—drawn from more than 350 diaries, letters, and memoirs—to create a vivid picture of life in an evolving army on the western frontier.