Ordered to uncover the location and movements of Robert E. Lee and his Army of Northern Virginia, Brig. Gen. Alfred Pleasonton sent his Army of the Potomac cavalry forces westward into the Virginia counties of Loudoun and Fauquier. By mid-June 1863, there was much confusion and concern as to where Lee and his vaunted army were positioned. Were the Confederates headed back towards Richmond or were they being transferred to the Western Theater? Or was Lee headed toward the Potomac and another invasion of the North?
On June 17, 1863, Pleasonton’s troopers clashed with Confederate cavalry near the town of Aldie, Virginia, a strategic site where the Snickersville and Ashby Gap Turnpikes met. JEB Stuart, still under a dark cloud for his performance at Brandy Station, had been ordered by Lee to shield his army's movement north towards Maryland. The sharp, bloody battle at Aldie did not deter the Federal forces from continuing with their probes to the west.
Additionally on the 17th, Pleasonton had ordered a patrol out towards the town of Middleburg. 280 men of the 1st Rhode Island Cavalry, under the command of Col. Alfred Duffié, brushed aside the pickets outside of town and almost captured Maj. Gen. JEB Stuart and some of his staff who were unaware of the approach of the Federal cavalry.
The 1st Rhode Island, following its orders to hold the town, ambushed Confederate reinforcements sent towards Middleburg, but by the next morning they were overwhelmed by the 4th and 5th North Carolina cavalry regiments. The isolated Rhode Islanders lost 225 of its ranks in that action.
On June 19th, Col. J. Irvin Gregg’s brigade of Federal cavalry was ordered to retake the town of Middleburg and continue to press westwards up the Ashby Gap Turnpike. Gregg’s men easily drove off the Confederate forces in town and then stopped one mile west of town near a new Confederate defensive line placed atop Mount Defiance.
Stuart’s position atop Mount Defiance was a strong one. Not only were his 3,200 men positioned upon the high, commanding ridge, but the line was further augmented by the placement of several horse artillery batteries.
By mid-morning Brig. Gen. David McMurtrie Gregg, J. Irvin Gregg’s cousin and superior, had ordered an attack upon Stuart’s Mount Defiance position. Supported by additional cavalry forces from Kilpatrick’s brigade and Federal artillery, Gregg’s forces began their attack after a brisk artillery duel.
Cavalry troopers from the 4th and 16th Pennsylvania Cavalry engaged in a “carbine assault” against the green 4th and 5th North Carolina cavalry regiments south of Mount Defiance. Troopers from the 1st Maine and 10th New York attacked up the Ashby Gap Turnpike and along its flanks. The powerful Union assault led to the momentary capture of the Confederate guns near the summit, but a quick counterattack by the 9th Virginia Cavalry drove back the Federal troopers.
On the Confederate southern flank, the hard-pressed North Carolinians had begun to give way and JEB Stuart and several of his staff officers rode down amidst the retreating troopers. As they emerged from a wood line the mounted group was taken under fire and Maj. Heros von Borcke, Stuart’s popular Prussian staff officer, was grievously wounded in the neck and taken from the field.
With the arrival of additional Federal cavalry under the command of Brig. Gen. John Buford threatening his left flank, and his right flank in a perilous state, Stuart smartly pulled his forces back to another ridge line along the Ashby Gap Turnpike, nearer the town of Upperville.
While the Union attack was successful in taking the Mount Defiance position, Stuart’s expert parrying of various Federal probes and attacks prevented Pleasonton’s forces from directly observing or interdicting the movement of the Army of Northern Virginia towards Maryland.