Promoted to major of artillery shortly after the start of hostilities between the Union and the Confederacy, Barry served as Brig. Gen. Irvin McDowell's chief of artillery during the First Battle of Bull Run, where his position was overrun after mistaking advancing Confederates for retreating Union forces. Barry was appointed brigadier general by President Abraham Lincoln on August 20, 1861, to rank from August 20, 1861. President Lincoln submitted the nomination to the U.S. Senate on December 21, 1861 and the Senate confirmed it on March 17, 1862. Barry came up with the concept that became the U.S. Horse Artillery Brigade in the Army of the Potomac.