Most histories of the American Civil War divide the fighting between the Eastern theater—the battles along the Potomac, Gettysburg, and finally Sherman's March—and the Western theater, with Shiloh, Vicksburg, and the Union seizing control of the Mississippi. But the struggle to control what later became the forty-eight contiguous states extended all the way from Richmond to Los Angeles. In The Three-Cornered War, historian Megan Kate Nelson draws welcome attention to the Civil War as it was fought in (here and in what follows) present-day West Texas, New Mexico, Arizona, and California. She also includes both women and Native Americans in unexpected ways in her account of how complex the war could be when fought beyond the Mississippi.

Nelson's narrative centers on nine individuals who left traces in the historical record: three US Army soldiers, one of them Kit Carson; a noncombatant (Union) government surveyor and an officer's wife; two (Confederate) Texans, one an officer; Mangas Coloradas, a famous Apache chief; and Juanita, a Navajo woman. Being literate, the Anglos left letters or diaries. Proceeding chronologically, Nelson devotes a short chapter to each of the nine, beginning in the East in 1861 and ending with the ceremonial gold and silver spikes that sealed the transcontinental railroad in 1869.

Unlike historians of the war in the East, Nelson must begin by describing the landscape and ethnic map of the Western theater. When John Baylor led his Confederate troops west from San Antonio to El Paso, he feared dying of thirst or at the hands of Comanche or Apache Indians far more than from any Union ambush; the railroads that moved soldiers to and from battlefields in the East were still hundreds of miles, and several years, away. Similarly, when a Union column marched from California to Tucson and New Mexico, they never saw a Confederate soldier but did lose several men to chiefs Mangas Coloradas and Cochise and their warriors—the third side of Nelson's three-cornered war.

Native Americans, of course, did not care whether the soldiers wore blue or gray. Slavery was part of the picture in the West as well—but in a completely different context. One of the three ethnic groups that dominated New Mexico Territory was the Hispanos, descendants of Spanish settlers who had arrived before Mexico achieved independence. For centuries, they had farmed, ranched, and traded in this land, and suffered raids from both Navajos and Apaches. In fighting back, they took and enslaved captives. Aware that the new Confederacy supported slavery and that the Union would probably abolish it, many Hispano slave-owners naturally sided with the South. Some left Santa Fe when Union troops arrived early in the war. But the combination of slavery and mutual Anglo vs. Native American hostility led to events unlike anything seen in the East. Consider this episode which came between the two battles fought in New Mexico: John Baylor, who had led his men from Texas, returned to Mesilla, New Mexico, after defeating Union forces at Valverde (20 Feb. 1862). He then led a hundred men on a search for Chiricahua Apaches in Mexico:

The interactions, hostile and otherwise, between Anglos and Indians that are at the heart of Nelson's book will be unfamiliar to most readers. But she limits herself to just two tribes. The Comanches who controlled much of West Texas are briefly mentioned, while the Apaches and their wars with both Confederate and Union soldiers are examined at length. In northern New Mexico, the saga of the Navajo nation and its forced removal to a reservation and subsequent return to its homeland occupies the longest section of the book, while the thousands of Pueblo Indians in the Rio Grande Valley are omitted altogether.

The Three Cornered War is a fine history of the Civil War period in New Mexico, extending up to the Navajos's July 1868 migration back to their homeland in the Four Corners, where Arizona, Utah, Colorado, and New Mexico now meet. Nelson does not complete the history of Apache hostilities: Geronimo and his band fought on until 1886. As military history, the book has strengths and weaknesses. Its author's use of contemporary letters and diaries give us an up-close-and-personal, if uneven, view of the two battles—Valverde and Glorieta Pass (26–28 Mar. 1862)—fought in New Mexico. The former is thoroughly described, while significant parts of the latter are glossed over. In both cases, maps would have helped readers better understand the fighting. (The book's lone map shows the entire United States in 1860.) In any case, the "military history" of the Civil War in this region did not last very long:

Combat between Union and Confederate soldiers in the West ended two months before the Battle of Antietam (17 Sept. 1862). The war, and Nelson's story, both continue. The rest of The Three-Cornered War concerns the efforts of Union forces to control Apaches and Navajos. An epilogue describes the fate of each of Megan Nelson's nine subjects.[1] Juanita outlived all the Anglos to die in 1910.