After taking command of General Joseph Johnston’s army following the Battle of Seven Pines, General Robert E. Lee conducted operations that defeated General George B. McClellan on the Virginia Peninsula and General John Pope in Northern Virginia. These operations relieved Richmond and shifted the focus of the war from Central to Northern Virginia. Now, with no forces to bar his way, Lee crossed the Potomac River and invaded the North. On September 4, 1862, he began a campaign designed to bring relief to Virginia, draw recruits from Maryland, and score a victory decisive enough to bring European recognition to the Confederate cause. It did not go as planned.
As Lee advanced down the Shenandoah Valley, McClellan took command of the Federal forces recently defeated at Second Manassas. Knowing that Lee was north of the Potomac, he set out in pursuit with 70,000 men. McClellan received a gift when a copy of Lee’s orders was brought to his headquarters providing detailed information of divided Confederate forces. As McClellan tried to exploit his new intelligence and moved towards Lee, General Thomas Jackson besieged and captured Harper’s Ferry on September 12-15.
McClellan engaged General D.H. Hill’s Division at South Mountain on September 14. With Federal forces closing in, Lee concentrated his army near Sharpsburg on September 16-17. The following day McClellan attacked Lee at the Battle of Antietam. With more than 22,000 casualties, it was the bloodiest day in American history. Although a tactical draw, General Lee withdrew across the Potomac during the night of September 18. McClellan attempted to pursue the Confederates, but was repulsed at Shepherdstown on September 20. President Lincoln utilized the battle as the impetus to issue his preliminary Emancipation Proclamation.