Manassas National Battlefield Park is a unit of the National Park Service located in Prince William County, Virginia, north of Manassas that preserves the site of two major American Civil War battles: the First and Second Battles of Bull Run.
This is a phenomenal museum with an IMAX theater and other great features. Note: as part of the Smithsonian Institute, admission is free but PARKING is $15 per car. It is NOT metro accessible.