Ghost Brigade

Ghost Brigade is a 1993 supernatural horror film set during American Civil War and directed by George Hickenlooper. Starring Corbin Bernsen, Adrian Pasdar, and Martin Sheen, the film was also released under the alternate titles The Killing Box and Grey Knight.

The film centers around Captain John Harling, a Union army man that has been awaiting the end of his parole it is cut short when there are unusual murders of Union troops. To discover the culprits behind this Harling seeks Colonel Nehemiah Strayn, a former Confederate regiment commander, now sitting in a Union prison at Bowling Green, who was once Harlings teacher. The two officers, along with Colonel George Thalman supervising, set out with a small detachment of troops and a runaway slave named Rebecca. When they reach the site where several Union troops were murdered, Strayn tells about what happened to his regiment, whose gruesome end happened at the same place where the Union corpses were found.After leaving the site and wandering deeper into Tennessee, they come across a group of Confederate soldiers, who quickly surrender, much to the surprise of the Union troops. The Confederate in charge tells Thalman that their group was attacked by a band of undead troops who took many of his men away, yet he was unaware of why he and his people were taken. Thalman, deciding that they require reinforcements, takes off leaving Harling in charge. But during the night, while Strayn and Rebecca are talking just a few yards away from the camp, they are assaulted by Stayns old regiment, now a bunch of walking zombies. His former secondincommand, Major Josiah Elkins, tells him to lead this new regiment. Strayn refuses the offer, however, and escapes with Rebecca to the camp. Strayn warns everyone at the camp and they all hide behind the carts that the Confederates had with them and wait until the zombie soldiers approach them. After holding them off, they are surprised to see Thalman reappear again, but he has been changed. After being burned by Rebecca he returns to his former self, But before he dies, he puts Harling in charge, in hopes that Harling will see the men out. ........

Source: Wikipedia