On February 15, 1862, the Army of the Tennessee attacked Fort Donelson in Tennessee. During the battle, Twombly picked up and carried his regiment's national colors after the color sergeant and two other corporals had been killed or injured by the enemy. Twombly was knocked to the ground by cannon fire, but managed to carry the flag for the duration of the battle. For his actions during the battle, Twombly was promoted to sergeant and later awarded the Medal of Honor in 1897. His citation reads: