Complex sentence: Harlan served in the Western Theater of the American Civil War until the death of his father James in February 1863, whereupon Harlan resigned his commission as colonel and returned to Frankfort in order to support his family.
Split into simpler sentences: Harlan served in the Western Theater of the American Civil War until the death of his father James in February 1863. At that time Harlan resigned his commission as colonel and returned to Frankfort in order to support his family.