At the beginning of the American Civil War in 1861, he resigned his clerk position to volunteer in the Union Army. On November 2 he was commissioned a
major in the
3rd Indiana Cavalry. Chapman briefly led the cavalry division of the
Army of the Ohio in May of the following year. Transferring to the infantry, he commanded a
brigade in the
Army of the Potomac until June 27, 1862. Chapman was promoted to
lieutenant colonel on October 25, 1862. Shortly after the battle, Chapman was given brigade command in the cavalry of the Army of the Potomac until March 1864. Chapman led a cavalry brigade in the Union
Army of the Shenandoah from August 6 until September 19, when he was wounded during the
Third Battle of Winchester. Recovered by the following month, Chapman resumed leading a cavalry brigade in the Shenandoah Valley. After the
Battle of Waynesboro, Virginia, on March 2, he was ordered to remain in the Shenandoah Valley while the rest of the Union forces headed for
Petersburg, Virginia. Chapman had with him now three small regiments and a few artillery pieces to hold the Valley. ==Postbellum==