California, to participate in the Mexican–American War, though by the time the battalion arrived, California was controlled by the United States, and his unit never actually saw combat. Stoneman was assistant quartermaster for the march. He fought in the Yuma War and was responsible for survey parties mapping the Sierra Nevada range for railroad lines. After promotion to captain of the 2nd U.S. Cavalry in March 1855, he served mainly in Texas until 1861. Civil War service 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.