The Battle of Ball's Bluff was an early battle of the American Civil War fought in Loudoun County, Virginia, on October 21, 1861, in which Union Army forces under Major General George B. McClellan suffered a humiliating defeat.

Loudoun County is in the northern part of the Commonwealth of Virginia in the United States. In 2020, the census returned a population of 420,959, making it Virginia's third-most populous county. The county seat is Leesburg. Loudoun County is part of the Washington–Arlington–Alexandria, DC–VA–MD–WV Metropolitan Statistical Area.