On September 18th, the day after the battle at Antietam, both sides remained on the battlefield too bloodied to resume fighting. That evening, lead elements of Gen. Robert E. Lee's army began to withdraw across the Potomac River back into Virginia at Boteler's Ford near Shepherdstown. Artillery on the south side of the river placed by Brig. Gen. William Pendleton covered the crossing of the remainder of Lee's army that night. On September 20th, a detachment of Maj. Gen. Fitz John Porter's Fifth Corps pushed across the river, attacked the Confederate rearguard and captured four guns. That evening, pressing the Rebel defenders, Porter sent forward elements of three brigades across the river to establish a bridgehead. Maj. Gen. A. P. Hill's division counterattacked while many of the Federals were crossing and nearly annihilated the 118th Pennsylvania (the "Corn Exchange" Regiment), inflicting heavy casualties. Porter withdrew his force back to the Maryland side of the river. The action at Shepherdstown discouraged Federal pursuit of the Confederates across the Potomac and marked the end of Robert E. Lee's Maryland Campaign. On November 7th, President Abraham Lincoln relieved Maj. Gen. George McClellan of command due in part to his failure to aggressively follow up Lee's retreating army. Maj. Gen. Ambrose Burnside relieved him as commander of the Army of the Potomac.