Commissioned as a colonel in the 13th Ohio Infantry, Smith first served in the West Virginia campaign in 1861. He was transferred to the Army of the Ohio, with which he fought at the Battle of Shiloh. Subsequently, he was promoted to brigadier general. He led his new command into action at the Battle of Perryville. In the Vicksburg campaign, he led a division of the XVI Corps. He was made chief of cavalry in the western theater. In March 1864, he led a raid into Mississippi, only to be driven back into Tennessee by General Nathan B. Forrest. In July, Smith resigned due to ill health.