At the start of the Civil War Stoneman was in command of
Fort Brown,
Texas, and refused the order of
Maj. Gen. David E. Twiggs, a southern sympathizer, to surrender to the newly established Confederate authorities there, escaping to the north with most of his command. Returning east, he was reassigned to the 1st US Cavalry and promoted to major on May 9, 1861. Stoneman then served as adjutant to General
George McClellan during his campaign in Western Virginia during the summer. After McClellan became commander of the newly formed Army of the Potomac, he assigned Stoneman as his chief of cavalry; Stoneman was promoted to brigadier general of volunteers on August 13. Stoneman had a difficult relationship with McClellan, who did not understand the proper use of cavalry in warfare, relegating it to assignment in small units to infantry brigades. On November 22, 1861, Stoneman married Mary Oliver Hardisty of
Baltimore. They had four children. Following the failures of the
Peninsula campaign, Stoneman was reassigned to the infantry, and received command of the 1st Division of the
III Corps on September 10 after its former commander, Maj. Gen.
Phil Kearny, had been killed a week earlier. The III Corps remained in Washington, D.C., during the
Maryland campaign. On October 30, Stoneman was placed in command of the entire III corps. At Fredericksburg, it formed part of Maj. Gen.
Joseph Hooker's Center Grand Division and helped drive back a Confederate assault during the battle. Following Fredericksburg, Hooker became commander of the Army of the Potomac and decided to re-organize the cavalry into a single corps with Stoneman at its head.
Stoneman's raids The plan for the
Battle of Chancellorsville was strategically daring. Hooker assigned Stoneman a key role in which his Cavalry Corps would
raid deeply into
Robert E. Lee's rear areas and destroy vital railroad lines and supplies, distracting Lee from Hooker's main assaults. However, Stoneman was a disappointment in this strategic role. The Cavalry Corps got off to a good start in May 1863, but quickly bogged down after crossing the
Rapidan River. During the entire battle, Stoneman accomplished little, and Hooker considered him one of the principal reasons for the Union defeat at Chancellorsville. Hooker needed to deflect criticism from himself and relieved Stoneman of his cavalry command, sending him back to Washington, D.C., for medical treatment (chronic hemorrhoids, exacerbated by cavalry service), where in July he became a Chief of the U.S. Cavalry Bureau, a desk job. A large cavalry supply and training depot on the
Potomac River was named Camp Stoneman in his honor. In early 1864, Stoneman was impatient with garrison duty in Washington and requested another field command from his old friend Maj. Gen.
John Schofield, who was in command of the
Department of the Ohio. Although originally slated for an infantry corps, Stoneman assumed command of the Cavalry Corps of what would be known as the
Army of the Ohio. As the army fought in the
Atlanta campaign under Maj. Gen.
William T. Sherman, Stoneman commanded an unsuccessful raid of the infamous
Andersonville Prison. In the course of the raid he and his aide,
Myles Keogh were captured by Confederates outside of
Macon, Georgia. However, the
5th Indiana Cavalry Regiment under
Col. Thomas Butler made a valiant stand, allowing the rest of his forces to retreat. They were surrendered as well, despite protest by Col. Butler. Stoneman became the highest-ranking Union
prisoner of war, In March 1865, Stoneman took roughly 4,000 troops out of
Knoxville, Tennessee, and led them on a raid of Virginia and North Carolina, the intent being to cripple Confederate infrastructure and demoralize the population. Within a week, they had sacked the towns of
Hillsville,
Asheville, and
Christiansburg, among others, and destroyed several bridges, lead mines and railroads. ==Postbellum politics==