Prince William County, named for William Augustus, Duke of Cumberland and third son of King George II, was officially formed from Stafford and King George Counties in 1731. Manassas was designated the county seat in 1892. Previously the county seat had been located at Occoquan Creek, Cedar Run, Dumfries, and Brentville. The two battles of Manassas took place here on 21 July 1861 and 28-30 August 1862. Both battles resulted in Confederate victories over the Union army. Several sites here are listed on the National Register of Historic Places, including Rippon Lodge, the Weems-Botts House, and Bel Air.
Stafford County was formed in 1664 from Westmoreland County and was part of the Northern Neck Proprietary of Lord Fairfax. It was named for Staffordshire, England. The county seat was located at Stafford in 1780; previously it had been at Marlborough Point. During the Civil War, the Union Army of the Potomac's more than 100,000 troops camped in the county during the winter of 1862-1863. Aquia Church, a National Historic Landmark, and the childhood home sites of George Washington and George Mason are located here.