At the start of the Civil War, Steinwehr raised a regiment, consisting primarily of German immigrants, the 29th New York Volunteer Infantry Regiment, which he commanded at the First Battle of Bull Run. The regiment was in reserve during the battle, but served an important screening role during the Union retreat. He was promoted to brigadier general on October 12, 1861, and commanded the 2nd Brigade of Louis Blenker's division of the Army of the Potomac. This brigade was moved into Maj. Gen. John C. Frémont's Mountain Department on April 1, 1862, and it fought in the Valley Campaign against Maj. Gen. Thomas J. "Stonewall" Jackson. Fremont's command was expanded into an army corps, which was soon commanded by Maj. Gen. Franz Sigel, another German immigrant. Steinwehr was given the 2nd Division in that corps. It was assigned to the Army of Virginia, under Maj. Gen. John Pope, and participated in the Northern Virginia Campaign, but had little role in the Second Battle of Bull Run. Although the corps joined the Army of the Potomac, the division did not fight at the Battle of Antietam or the Battle of Fredericksburg.