The battle of bull run was in Fairfax County and Prince William County, VA on Jul 21, 1861. It was the first full scale battle of the civil war between the union and the confederacy. In the war the confederacy won, and it forced the union army to retreat back to Washington. Out of 60,000 troops there was almost 5,000 casualties between both sides. The confederacy had about the same number of troops. 

Bull Run Battle Facts and Summary | American Battlefield Trust (battlefields.org)