A Brotherhood of Valor is the story of the men who served in two of the most famous combat units of the Civil War, the Stonewall Brigade of the Confederacy and the Iron Brigade of the Union. They fought in some of the most famous and bloody engagements of the war, from First and Second Manassas (Bull Run) to Sharpsburg (Antietam), Chancellorsville, and Gettysburg. Jeffry D. Wert offers a visceral depiction of the Civil War from the perspective of the ordinary soldiers who fought in it.

Virginia's Stonewall Brigade got its name from its legendary commander, General Thomas (Stonewall) Jackson. Made up mainly of men from the Shenandoah Valley, it fought with distinction even after its commander suffered fatal wounds at Chancellorsville. The Iron Brigade was formed in what were then the western states of Wisconsin and Indiana.

Most of the soldiers on both sides were literate, and many wrote touching letters home to their families. Wert quotes liberally from these moving letters, which bring an immediacy to the horrors of the Civil War that no other source can match.