American Battlefield Trust’s map of the Battle of Shepherdstown
On September 19, a detachment of Porter’s V Corps pushed across the river at Boteler’s Ford, attacked the Confederate rearguard commanded by Brig. Gen. William Pendleton, and captured four guns. Early on the 20th, Porter pushed elements of two divisions across the Potomac to establish a bridgehead. Hill’s division counterattacked while many of the Federals were crossing and nearly annihilated the 118th Pennsylvania (the “Corn Exchange” Regiment), inflicting 269 casualties. This rearguard action discouraged Federal pursuit. On November 7, President Lincoln relieved McClellan of command because of his failure to follow up Lee's retreating army. Maj. Gen. Ambrose Burnside rose to command the Union army.