After his victory at Chancellorsville, Confederate Gen. Robert E. Lee prepared to invade the North and ordered his cavalry into Culpeper County to screen the secret withdrawal of his army from Fredericksburg and protect the movement of infantry and artillery toward the Shenandoah Valley. Federal cavalry north of the Rappahannock River discovered the Confederate horsemen, and Union Gen. Joseph Hooker ordered the Cavalry Corps under Gen. Alfred Pleasonton to disperse and destroy them. At dawn on June 9, 1863, Federal cavalry attacked Gen. J.E.B. Stuart's cavalry division around Fleetwood Hill and came close to defeating the Confederate mounted forces, but by day's end the Union cavalry withdrew across the Rappahannock. More than 20,000 soldiers fought in the largest cavalry action of the war, about 1,500 men were killed, wounded, or captured, and the clash marked the beginning of the Gettysburg Campaign.