Here on 9 June 1863, the largest cavalry battle in North America occurred when 9,500 troopers fought 8,000 cavalrymen under Union Brig. Gen. Alfred Pleasonton. This daylong battle, the opening engagement of the Gettysburg campaign, erupted when the Federal attack surprised Stuart and his men. The Confederates prevented the Union cavalry from learning the intentions of Gen. Robert E. Lee, who had begun marching his infantry to the Shenandoah Valley to invade the North. For the first time in the Civil War, however, the Federal cavalry proved itself a match for its opponents.