Original: After the battle he was assigned to the 5th U.S. Cavalry, with which he served through the early days of the Peninsula Campaign in 1862. As a staff officer for Major General George B. McClellan, Custer was promoted to the rank of captain.
Simplified: Custer graduated last in his class and served at the First Battle of Bull Run as a staff officer for Major General George B. McClellan in the Army of the Potomac's 1862 Peninsula Campaign.