At the outbreak of the Civil War, Smith became a colonel of the 2nd California Volunteer Cavalry in the Union Army, rising early in 1862 to the rank of brigadier general in the United States Volunteers and to the chief command of the cavalry in the Department of the Missouri. From March through July, he served in the same capacity in the Department of the Mississippi. Assigned afterwards to the Army of the Tennessee, he took part in the Battle of Chickasaw Bayou and the capture of Arkansas Post. He commanded a division of the XIII Corps in the Vicksburg Campaign. Later, he led a division of the XVI Corps in the Red River Campaign of Maj. Gen. Nathaniel Banks. He received the brevet rank of colonel in the regular army for his services at the action of Pleasant Hill. In July 1864, Smith became a lieutenant colonel in the regular army, to rank from May April 10, 1864. On May 14, 1864 President Abraham Lincoln appointed Smith a major general in the volunteers, to rank from May 12, 1864, the date of the United States Senate's confirmation of President Lincoln's May 9, 1864 nomination of Smith for the appointment. On July 14-15, 1864, Smith defeated Confederate Lt. Gen. Stephen D. Lee at the Battle of Tupelo, where he, after General Nathan Bedford Forrest refused to take charge of the Confederate forces on the field. During the autumn of 1864, Smith lead Union troops against Confederate Maj. Gen. Sterling Price during Price's Raid into Missouri. Smith was then summoned to join forces with Maj. Gen. George Henry Thomas at Nashville, Tennessee, then threatened by the advance of Confederate Lt. Gen. John Bell Hood. Smith bore a conspicuous share in the crowning victory at the Battle of Nashville leading his troops past the Confederates' south flank. He commanded the XVI corps in the final campaign against Mobile, Alabama, in 1865. On April 10, 1866, President Andrew Johnson nominated Smith for appointment to the grade of brigadier general in the United States Army, to rank from March 13, 1865, for his services at the Battle of Tupelo and the United States Senate confirmed the appointment on May 4, 1866. On the same dates, President Johnson nominated and the United States Senate confirmed Johnson's appointment of Smith as brevet major general in the regular army, to rank from March 13, 1865 for his success in leading his men at the Battle of Nashville. The Senate reconfirmed this appointment on July 14, 1866 after recalling the confirmation and return of the nomination to President Johnson for possible readjustment of the date.
Postbellum life
Smith resigned his volunteer commission and was mustered out of the volunteers on January 15, 1866. He then became colonel of the U.S. 7th Cavalry Regiment, serving in the American West. He retired from the military service in April 1869 to become postmaster of St. Louis, Missouri, where he died on January 30, 1897. He was buried at Bellefontaine Cemetery in St. Louis.
