The Antietam or, Maryland, Campaign began in early September 1862, when General Robert E. Lee led his Army of Northern Virgina across the Potomac River and for the first time into Union soil. Even though he had only 40,000 men at most in his army, Lee believed that the Union Army of the Potomac was demoralized after the defeat at Second Manassas in late August and now was the time to take the offensive. His plan was to live off the land of Maryland and then springboard from Hagerstown into Pennsylvania, with the vital junction at Harrisburg as his main target. As soon as Lee's army crossed the Potomac, problems began to plague the Confederates. Straggling was at its all time high and the pro Union Western Maryland gave the Confederates a cold reception. Lee falsely assumed that the slave state Marylanders would provide him with recruits for his tired army.But the biggest problem for Lee was that the Union garrison at Harpers Ferry refused to abandon the armory once the Confederates entered Maryland. This forced Lee to divide his army into five pieces spread over 20 miles in order to capture Harpers Ferry. General Stonewall Jackson led his corps in the capture of Harpers Ferry, while General Jame Longstreet and Lee led the rest of army Northward. Lee ordered Special Orders 191 for this separation.
Meanwhile, The Army of the Potomac was demoralized and defeated after Second Manassas, but not for long because President Abraham Lincoln reappointed General George B. McClellan command of the Army of the Potomac. The effect was immediate and McClellan reorganized and energized the Union troops in a matter of weeks. Then, McClellan received a copy of Special Orders 191 that was found in a camping field in Fredrick, MD. With this information in hand, McClellan began to move his army from Washington cautiously Westward toward the spread out Confederate Army. Lee sensed this increased activity in the Union Army and especially after the Battle of South Mountain on September 14 when McClellan broke through the small Confederate detachments assigned to protect the South Mountain gaps. At this point, Lee contemplated ending the campaign but the next day Jackson successfully captured Harper's Ferry and Lee decided to make a stand on the good defensive ground east of Sharpsburg. All through the 15th and 16th the two armies gathered around Sharpsburg for the ensuing conflict. Jackson moved his entire wing from Harpers Ferry expect for A.P. Hill's division who was left in charge of collecting the supplies captured and the 11,500 prisoners. On the night of the 16th, McClellan sent the First Corps under General Joesph Hooker and the Twelfth Corps under General Joesph Mansfield across the unguarded upper bridge and thereby revealed to Lee where the first attack would strike.
The Battle of Antietam on September 17 can be though of in three separate stages and four separate Union Corps level attacks. It began early in the morning when Hooker's First Corps attacked straight south against Lee's left wing commanded by Jackson. The Union objective was a small white building later identified as the Dunkard Church. To get to the Dunkard Church, the Union troops had to traverse a 30 acre cornfield just to the north of the Confederate battle line. By the end of the morning phase, this cornfield would forever be known as The Cornfield. After Hooker's troops began to break through the thin gray line, Jackson ordered in his reserves; General John B. Hood's division. Their vicious counterattack pushed the Union troops back across The Cornfield to their original starting position. At this point, the First Corps was spent and the Twelfth Corps under Mansfield attacked from the Southeast through the East Woods. However this attack quickly broke down after Mansfield was fatally wounded, one of the six generals who would be killed due to the battle. However, General George Greene's division was able hold high ground south of The Cornfield where the visitor center stands today. At this point, the huge Second Corps, 15,000 men under the command of General Edvin V. Summer attacked from East. The first of Summer's three division under General Sedgewick entered the West Woods, but suffered heavy casualties when the Confederate divisions of General McLaws and General Walker countered attack along with the rest of Jackson's command. The "Disaster in the West Woods" concluded the morning phase of battle. The Confederate line wavered but held. About 10,000 men have been killed or wounded in the area of The Cornfield and the East and West Woods.
While Sumner's first division engaged in the West Woods, his other two divisions under General French and General Richardson spotted Greene's division on the high ground and falsely identified that as Sedgewick. They deployed to the left of these Union troops and shifted the attack to the south directly toward a Sunken Road connecting the Haggerstown and Boonsboro Turnpikes. Waiting for them was about 2,500 Confederates under the command of General D.H. Hill. While Hill's men were outnumbered, they held a wonderful defensive position in the Sunken Road. During the midday phase the Confederates were able to hold the piecemeal attack of the two Federal division for over three hours. The tide turned when Federal troops were able to shoot down on the Confederates and casualties began to mount in the road. Then the command structure began to falter and the breakthrough occurred when an order to retreat was misinterpreted. At this point the Confederate center was in pieces, the situation so desperate that both Generals Hill and Longstreet became personally involved with the combat. However, after General Richardson was fatally wounded, the Union attack fizzled to a halt and the Confederate center barely held on. McCellen had the troops to exploit this breakthrough, but he chose not to commit them. Another 5,000 men lay killed or wounded in area that would know forever after as Bloody Lane.
The third phase of the battle, known as the afternoon phase, took place in isolation of the other three attacks. The lowest of the three bridges across the Antietam Creek was the only one that Lee protected with troops, because of its great defensive positions. However, due to the heavy fighting on the other areas of the battlefield, Lee was forced to shift troops that he originally intended to guard this position to where they were needed. This meant that only 400 Georgians under General Robert Toombs were left to defend this critical position. It was General Ambrose Burnside's task to capture this bridge, which would soon become know as Burnside Bridge. He had 12,500 troops of the Union Ninth Corps under his command to accomplish this task. Burnside believed that his mission was a diversionary attack, while the main assault would occur to his right. Since Burnside believed this, he took his time attacking the bridge. He sent the main assault around to the left to cross a ford downstream and flank the Confederates. However this took a long time, so with McCellen's urgent messages, Burnside sent three separate regiments across the bridge starting at 10:00. The first two failed, but the third one succeeded. It then took him two hours to get his entire corps across the bridge and ready for battle. By this time it was 3 in the afternoon and the battle shifted focus to this southern area. There was just a thin line of Confederates to defend Sharpsburg against the Ninth Corps and if Burnside succeeded, then Lee's escape route would be cut off into Virgina. At this critical point, just as the Ninth Corps was moving on Sharpsburg, A. P. Hill's division arrived on the battlefield. They had started from Harper's Ferry early in the morning and had made the grueling 14 mile march to make it in the nick of time. Hill slammed into the inexperienced Union troops and Burnside's attack crumpled at around 4:30. After 12 hours of brutal combat 23,000 men had fallen on the bloodiest day of the Civil War. Thus ended the Battle of Antietam.