The First Battle of Bull Run, called the Battle of First Manassas by Confederate forces, was the first major battle of the American Civil War. The battle was fought on July 21, 1861, in Prince William County, Virginia, just north of what is now the city of Manassas and about thirty miles west-southwest of Washington, D.C. The Union Army was slow in positioning themselves, allowing Confederate reinforcements time to arrive by rail. Each side had about 18,000 poorly trained and poorly led troops. The battle was a Confederate victory and was followed by a disorganized post-battle retreat of the Union forces.

Prince William County lies beside the Potomac River in the U.S. state of Virginia. At the 2020 census, the population was 482,204, making it Virginia's second most populous county. The county seat is the independent city of Manassas.