Here’s a really well done video by Fairfax County, only two minutes long, highlighting the one major battle that was fought in Fairfax: The Battle of Ox Hill, fought in the rain on Sept. 1, 1862.
The preserved site is now a park on West Ox Road and Monument Drive, in the Fair Oaks area near the Fairfax Towne Center. It is also called the Battle of Chantilly, depending on whose side you were on, and it was the end of the Second Bull Run campaign. An estimated 1,000 Union and 500 Confederate troops were killed, and the Union had to retreat back to Washington.
The Chantilly Battlefield Association worked for many years to preserve this park, and it was reopened on Sept. 1, 2008. For more information on the battle and the park, go here. Fairfax County has many other Civil War sites, which are in a very detailed PDF here.