You are right, something as recent as the American Civil War would not be carbon datable. Radiocarbon dating is one good for 300 to 50K years. (although dates of up to 75K BP have been recorded, things are changing daily) Mostly I think it's because of the Industrial Revolution which started shortly after the Civil War and really changed the amount of carbon in the atmosphere which skews the C14 absorbtion and decay in objects that absorb it.
(no subject)
Sorry, it's the archaeology training... :)