The Treaty of Richmond, in which the United Canadian and American States agreed to the peaceful secession of the Confederation of American States, required the state of Virginia surrender ten counties1 to act as a buffer around the Federal District of Columbia against potential CAS invasion.
Additionally, neighboring Fauquier and Loudoun counties were completely de-militarized by the CAS as a means of avoiding their inclusion in North Virginia.
North Virginia is a thin strip of land between the Potomac and Rappahannock rivers up to Fauquier County. Additionally, Accomack and Northampton counties on the Delmarva Peninsula were offered up as recompense for the CAS preserving sovereignty over Fauquier and Loudoun.
1 Specifically, the counties of Fairfax, Prince William, Staford, King George, Westmoreland, Richmond, Lancaster, Northumberland, Accomack and Northampton.