What made Loudoun useful to the armies was its position on the Potomac, the numerous fords which made that river unusually easy to cross, and the intersection of two major thoroughfares in Leesburg. These were the north-south running Old Carolina Road (today’s Route 15) and the east-west running Alexandria to Winchester Turnpike (Rt. 7). The first could serve as an invasion route for whichever side controlled the river crossings. The second was critical for communication into the Shenandoah Valley. Other major roads were the north-south Berlin Turnpike (Rt. 287) west of Leesburg and the paved road that ran westward from Leesburg to Hillsborough and Harpers Ferry (Rt. 9). Together, these routes and river crossings made Loudoun County very inviting for any large army and therefore one of the most strategically significant pieces of ground in North America for several months early in the war.
Most of the military incursions into Loudoun were deliberate. Ball’s Bluff, however, was not. That most famous of Loudoun fights resulted from the mistaken report of an inexperienced officer who confused a row of trees with the tents of a Confederate encampment while leading a reconnaissance patrol on the night of October 20, 1861. His error led to the quick organization of a raiding party which, having crossed the river, found that it had nothing to raid but inadvertently encountered a small Confederate force. From this, there evolved a battle. Very few of the Union troops, however, either got, or had intended to get, more than about a mile into Virginia.
Later troop movements were more purposeful. Gen. Robert E. Lee and his Army of Northern Virginia passed through the county, using the nearby Potomac fords, on their way to Antietam in 1862. There was a small skirmish at Mile Hill, north of Leesburg on the Carolina Road just prior to Lee’s arrival in the town where he finalized his plans for the invasion of Maryland. After the Battle of Antietam, the Union Army of the Potomac crossed from Berlin into Loudoun, with J.E.B. Stuart's Confederate cavalry attempting to slow them up in sharp fighting at Philomont, Unison, and Upperville on November 1-3, 1862.