The Twilight Zone - The 7th Is Made Up Of Phantoms

A National Guard tank crew on war games suddenly find themselves back in time to June 25, 1876 the day General Custer fought and lost to the Sioux at the battle of the Little Big Horn. They report what they've seen and heard but the officer-in-charge is more than a little dubious about what they claim. They return to the area and when the attack begins they join the fight. When the commander goes to locate them, they find something else entirely.