Don Carlos Buell
Fitzhugh was appointed to the United States Military Academy in 1859, but left in September 1861 before finishing to join the Union war effort. He was commissioned as a first lieutenant in the 4th United States Regular Artillery in October 1861, and served as an Aide-de-Camp to major general Don Carlos Buell between 1861 and 1863, including during the April 1862 Shiloh Campaign.