Virginia's Great Valley, prosperous in peace with a rich soil and an enslaved workforce, invited destruction in war. Voracious Union and Confederate armies ground up the valley, consuming crops, livestock, fences, and human life. Pitched battles at Gettysburg, Lynchburg, and Cedar Creek punctuated a cycle of vicious attacks and reprisals in which armies burned whole towns for retribution.

North of the Mason-Dixon line, in the Pennsylvania portion of the valley, free black families sent husbands and sons to fight with the U.S. Colored Troops. In letters home, even as Lincoln...