The Battle of Thoroughfare Gap, also known as Chapman's Mill, took place on August 28, 1862, in Fauquier County and Prince William County, Virginia, as part of the Northern Virginia Campaign of the American Civil War. Confederate forces under Maj. Gen. James Longstreet successfully drove back Union forces under Brig. Gen. James B. Ricketts and Col. Percy Wyndham, allowing his corps to unite with that of Thomas J.