If you found this written on the back of a photograph, a document, or a storage box, it likely follows a standard archival format:
Could represent Box 10 or Folder 10 in a specific collection. Civil War: The primary subject matter of the contents. 10 Civil War - 8d
It might refer to the 10th Cavalry or 10th Infantry , with "8d" being a specific platoon or squad notation used in modern historical gaming or reenactment databases. If you found this written on the back
Less commonly, this may refer to a specific military designation, though it doesn't fit the standard U.S. Army "Regiment/Company" format perfectly. or a storage box