At the outbreak of the Civil War, Smith was commissioned colonel of the Third Vermont Volunteers; he fought at First Bull Run (First Manassas) on the staff of General Irvin McDowell. On 13 August 1861 he was promoted to brigadier general of volunteers and given command of the Second Division, VI Corps, Army of the Potomac. Participating in the Peninsula campaign of spring 1862, he led his command in the battle of Williamsburg and later in the Seven Days' battles around Richmond. For his services protecting the crossing of White Oak Swamp, he was brevetted lieutenant colonel in the regular army in June. A month later (4 July) he became a major general of volunteers and was brevetted colonel in the regular army for his service at the battle of Antietam.
Chris Calkins, "Smith, William Farrar," American National Biography Online, February 2000, http://www.anb.org/articles/05/05-00728.html.