Irvin McDowell was a career American army officer. He is best known for his defeat in the First Battle of Bull Run, the first large-scale battle of the American Civil War. In 1862, he was given command of the I Corps of the Army of the Potomac. He fought unsuccessfully against P.G.T. Beauregard's troops during the Valley Campaign of 1862, and was blamed for contributing to the defeat of United States troops at the Second Battle of Bull Run in August.