At the start of the American Civil War, Sedgwick was serving as a colonel and assistant inspector general of the Military Department of Washington. He missed the early action of the war at the First Battle of Bull Run, recovering from cholera. Promoted to brigadier general on August 31, 1861, he commanded the 2nd brigade of Maj. Gen. Samuel P. Heintzelman's division in the Army of the Potomac, then his own division, which was designated the 2nd division of the II Corps for the Peninsula Campaign. In Virginia, he fought at Yorktown and Seven Pines. During the Seven Days Battles, Sedgwick's division fought at Savage's Station and Glendale, being wounded in the latter engagement. After the Seven Days Battles, he was promoted to major general. The II Corps and Sedgwick's division were not involved in the Northern Virginia Campaign.