With the Federal forces around Washington demoralized, Lee realized he could march the Army of Northern Virginia north into Maryland and Pennsylvania. If he could defeat a major Union army on its own territory, the North would become even further discouraged about an eventual victory, and England and France might well recognize the Confederacy as a new nation, which would be a big boost to the Confederate cause.
Lee's immediate goal was the capture of the Union railroad center in Harrisburg, Pennsylvania, also the state capital. He hoped to take the small city by dividing his army. While Lee moved into Pennsylvania, Stonewall Jackson would capture the Union garrison at Harpers Ferry, then hook up with Longstreet's three divisions and join Lee near Harrisburg.
The plan might have worked, considering the lackluster and sluggish history of the Union Army of the Potomac. Moreover, Pope had been reassigned and McClellan placed in charge of the now united Federal armies in and around Washington. McClellan was known to be a slow mover and very cautious. But, through an incredible quirk of fate, McClellan came into possession of a copy of Lee's plans, which had been found wrapped around some cigars near an abandoned Confederate camp. At first McClellan thought the orders might be a trap and failed to act on them for nearly sixteen hours. Lee, meanwhile, was told that his plans were in the hands of the enemy and worked hard to protect his three vulnerable flanks.
Antietam Bridge, Maryland Photo courtesy of the National Archives (165-SB-19)
Fighting broke out in a number of locations along the planned route, and the Confederates experienced heavy casualties. On September 15, Lee was planning a retreat back into Virginia when he learned that Jackson had taken Harpers Ferry and collected some much-needed supplies. Lee quickly changed his mind and ordered all of his divisions to meet in Sharpsburg, Maryland. McClellan, again overly cautious, allowed Lee's forces to converge. On September 17, he finally attacked; 75,000 Union troops faced just 40,000 Confederates.
The Bloodiest Day
Lee's left flank, led by Stonewall Jackson, was almost annihilated during the Union onslaught, led by Joseph Hooker, Joseph Mansfield, and Edwin Sumner. The Rebel soldiers took a horrible pounding until they were reinforced by two fresh Confederate divisions, who fought back with amazing vigor. Within just twenty minutes, an astounding 2,200 Union soldiers were killed or wounded in an area known as the West Woods, and Sumner was forced to retreat.
General Ambrose Burnside attempted for hours to get his men across a stone bridge over Antietam Creek. Georgians on heights beyond the bridge kept the Federals at bay until a swift rush got the first men across. The bridge is still intact and known as Burnside's Bridge.
Later in the day, the Union army focused on decimating the Confederate center, led by Major General Daniel Hill. Sumner's remaining divisions attacked Hill's line for three brutal hours, resulting in such carnage that the narrow road on which it occurred became known as Bloody Lane. The assault broke the Confederate center, and Union forces under the command of Burnside crossed Antietam Creek to attack Lee's right flank. Lee's army probably would have been destroyed during the ensuing battle if it hadn't been reinforced by a division led by A. P. Hill, which arrived after a grueling thirty-mile march from Harpers Ferry. Upon arrival, Hill's men launched a blistering counterattack that helped keep the Union forces at bay.
Had McClellan continued his assault, it's very likely he could have completely devastated Lee's army. But instead of making one final push, he let his troops rest, as did Lee. Then Lee retreated back to Virginia on the evening of September 18, having lost nearly a quarter of his army at Antietam.
The Battle of Antietam was the single bloodiest day of fighting in the Civil War, with a combined total of more than 27,000 casualties. It was also a decisive victory for the Union and gave Abraham Lincoln the confidence he needed to issue his Emancipation Proclamation, which effectively changed the Union's war aims to include the abolition of slavery. The Emancipation Proclamation also paved the way for increased Union manpower by allowing African Americans to be become soldiers.