 Though long associated with engineering and artillery, Gillmore's first independent command came at the head of a cavalry expedition against Confederate General John Pegram.Gillmore defeated the Confederates at the battle of Somerset for which he was given a brevet (military) promotion to colonel in the U.S. Army.Gillmore was assigned to replace Maj. Gen. Ormsby M. Mitchel in charge of the X Corps after that officer's death from yellow fever.