William G. Middleton had prior service as captain in Capt. W. G. Middleton's Company, Mississippi Cavalry which was assigned in April/1862 to the 1st. Miss. Cavalry and became Company C. He was slightly wounded at the Battle of Shiloh.
Copy & paste for a capsule history of this unit.
http://mississippiscv.org/MS_Units/1st_MS_CAV%28AW%29.htm