Grey Knight AKA The Killing Box AKA Ghost Brigade (2nd American Film Festival, 1993)

During the American Civil War, mysterious slaughters of Union soldiers occur repeatedly in places that are actually pacified. The concerned general sends his adjutant Harling with the prisoner of war Strayn into the area to get to the bottom of what is happening. The search party comes across a band of Confederates who met their deaths at a cult site of African slaves and are now wreaking havoc as murderous zombie soldiers. It's up to Harling and Strayn to put an end to the haunting.