Battle Flag by Bernard Cornwell

Battle Flag (The Starbuck Chronicles, Book 3) - As the US Civil War moves forward Nathaniel Starbuck and his friend Adam Faulconer find themselves facing their own fathers across front lines.  Adam is in a Northern regiment of Southerners who are scouts. Nate, fighting for Adam’s father, is guarding a ford.  When General Faulconer finds out, he removes the troops from the ford, arresting Nate.  Adams troops use the ford to raid the Legion’s camp and capture the Legion’s flags.  Later he raids his father’s estate for horses.  Nate also faces his father when he finds him at Galloway’s farm.