Most Americans are familiar with major Civil War battles such as Manassas (Bull Run), Shiloh, and Gettysburg, which have been extensively analyzed by generations of historians. However, not all of the war's engagements were fought in a conventional manner by regular forces. Often referred to as "the wars within the war," guerrilla combat touched states from Virginia to New Mexico. Guerrillas fought for the Union, the Confederacy, their ethnic groups, their tribes, and their families. They were deadly forces that plundered, tortured, and terrorized those in their path, and their impact is...