The Civil War in New Mexico began in 1861 with the Confederate invasion and occupation of the Mesilla Valley. At the same time, small villages and towns in New Mexico Territory faced raids from Navajos and Apaches. In response the commander of the Department of New Mexico Colonel Edward Canby and Governor Henry Connelly recruited what became the First and Second New Mexico Volunteer Infantry. In this book leading Civil War historian Jerry Thompson tells their story for the first time, along with the history of a third regiment of Mounted Infantry and several companies in a fourth regiment.

Thompson's focus is on the Confederate invasion of 1861–1862 and its effects, especially the bloody Battle of Valverde. The emphasis is on how the volunteer companies were raised; who led them; how they were organized, armed, and equipped; what they endured off the battlefield; how they adapted to military life; and their interactions with New Mexico citizens and various hostile Indian groups, including raiding by deserters and outlaws. Thompson draws on service records and numerous other archival sources that few earlier scholars have seen. His thorough accounting will be a gold mine for historians and genealogists, especially the appendix, which lists the names of all volunteers and militia men.