Complex sentence: He later served as the assistant adjutant general with the rank of captain on the staff of major general William S. Rosecrans in the Army of the Cumberland, and later was on the staff of Major General George H. Thomas.
Split into simpler sentences: He later served as the assistant adjutant general with the rank of captain on the staff of major general William S. Rosecrans in the Army of the Cumberland. Later he was on the staff of Major General George H. Thomas.