Gen. Robert E. Lee established his headquarters here on the evening of 17 June 1863 as the Army of Northern Virginia marched north. Lt. Gen. Richard S. Ewell, who had replaced Stonewall Jackson as corps commander after Jackson’s death on 10 May, had crossed the Potomac River into Maryland after defeating Union Maj. Gen. Robert H. Milroy on 15 June at Winchester. The way was then clear for Lt. Gen. James Longstreet’s corps to enter the Shenandoah Valley at Snicker’s Gap and Lt. Gen. A.P. Hill’s at Chester Gap. Maj. Gen. J.E.B. Stuart’s cavalry screened the army’s flank near Aldie as Lee prepared to invade Pennsylvania.