Upon leaving Alabama with his small force, Reno was temporarily assigned to command the Fort Leavenworth Arsenal until he was appointed brigadier general of volunteers in the fall of 1861. He transferred to Virginia, took command of the 2nd Brigade, Burnside Expeditionary Force, and soon had organized five regiments. The 2nd Brigade fought in Maj. Gen. Ambrose Burnside's North Carolina Expedition from February through July 1862. Reno became a division commander in the IX Corps, which had become part of the Army of the Potomac. In the Northern Virginia Campaign, Reno actively opposed his friend and classmate Stonewall Jackson during the Second Battle of Bull Run and the Battle of Chantilly. Reno was appointed a major general on August 20, 1862. (This promotion was confirmed posthumously on March 9, 1863, with date of rank established as July 18, 1862.) Burnside became commander of the Army of the Potomac's right wing for the start of the Maryland Campaign in September, elevating Reno to command of the IX Corps from September 3.