Following the smashing defeat of John Pope and his army at Second Bull Run, George B. McClellan was restored to his command of the Army of the Potomac. McClellan’s skills as an organizer and morale booster made him a good fit for the disheartened Union forces, even if he lacked speed and audacity.
Meanwhile, Lee decided it was the right time to launch an offensive into the North. Lee had taken command of the Army of Northern Virginia following the wounding of Joseph Johnston at the Battle of Seven Pines. Hoping to relieve pressure and gain the intervention of European powers, Braxton Bragg and Earl Van Dorn were to launch offensives into Kentucky and Tennessee while Lee made a move through Maryland to Pennsylvania.
Lee not only hoped to gain foreign support, he also hoped that Marylanders would rise up enmasse and join the Confederate ranks. Lee also hoped to relieve some of the pressure that was being placed on the Virginia countryside, procure supplies and threaten the supply lines of the North.
The rebels splashed across the Potomac river on September 5, 1862, and moved into Maryland. Lee, however, was disappointed by the poor welcome they received. They did not get thousands of volunteers as they had expected and reactions to their invasion were mixed. While the Army of Northern Virginia moved North, Lee sent Thomas “Stonewall” Jackson to capture a Union force of about 13,000 men at Harper’s Ferry.
These orders were written on September 9 and named Special Orders 191. Lee was breaking a fundamental rule of warfare: never divide your army in the face of a larger foe. McClellan and his Army of the Potomac began chasing the rebels on September 7, moving faster than was typical for McClellan. Meanwhile, on September 12-13, Jackson was successful in his mission to capture the home of John Brown’s infamous raid.
McClellan’s First Mistake
However, fate intervened and on September 13, a copy of Special Orders 191 was found by Union soldiers wrapped around three cigars.These orders quickly made their way to McClellan. One would imagine this would spur McClellan to move quickly. However, he delayed, believing that Lee had a larger force than he in fact did. This was the first of McClellan’s mistakes.
McClellan had the answer sheet right in front of him and Lee’s army was divided and ready to be crushed. However, he delayed and the Union army did not move till the following day. If McCellan had not hesitated, he might have captured the passes in the South Mountain. Then, J.E.B. Stuart learned that the Federals had captured Special Orders 191. Lee did not hesitate like his counterpart did and he quickly blocked three passes in the mountains. On the 14, fighting ensued and the rebels were forced to relinquish control of the passes to the Federals.
Lee was unsure whether or not to abandon his offensive when he learned, on the 15, that Jackson had captured Harper’s Ferry. Lee decided to make a stand at Sharpsburg, Maryland.
Fighting Breaks Out – McClellan’s Second Mistake
On the 15, Lee deployed 18,000 troops at Sharpsburg. That afternoon, Union forces too began to arrive at the Maryland town. However, here we see another of McClellan’s blunders. McClellan did not cross Antietam Creek and instead gave the rebels time to defend it. He also failed to reconnoiter the far bank of the river. McClellan’s cautiousness again gave Lee time to deploy, regather his troops and set up defenses as he had at the South Mountain passes.
McClellan’s Third Mistake
By the 16, the rebel force was about 25,000 while McClellan had more than double that. However, instead of pressing the advantage with his 55,000 men and 14,000 nearby, McClellan spent the entirety of the 16th planning. McClellan had a perfect chance to crush a portion of Lee’s army and to capture Sharpsburg before the rebels could truly secure it.
By the following day, the 17th, most of the Confederate army had gathered at Sharpsburg, minus A.P. Hill’s division. Lee had his rebel force fan out north and east of Sharpsburg along a ridgeline. Both wings of the line were protected by water, making it good defensive ground. The left of the line was anchored on the Potomac and the right on Antietam Creek.
The extreme left was held by Stuart, the main body of the left Jackson and the right was held by Longstreet. On the Federal right McClellan had Joseph Hooker and the I Corps and JOseph K. Mansfield’s XII Corps. These corps had crossed Antietam creek on the 16th. The bulk of McClellan’s force remained on the east side of the creek. Ambrose Burnside’s IX Corps made up the Federal left.
McClellan planned to have Hooker, supported by Mansfield and Sumner, move down the Hagerstown Turnpike and attack the left flank of the Confederate line. Then, Burnside was to attack the right flank by moving over the Rohrbach Bridge.
The Battle Begins – McClellan’s Fourth Mistake
When the sun rose on the 17th, the fighting began. Hooker made the initial movement with the I Corps moving along the Hagerstown Pike. The Federal soldiers began to pour into the cornfield that sat to the east of the Hagerstown Pike. Here, the Federals fought furiously with Confederates under “Stonewall” Jackson. The artillery played a major role in the fighting in the cornfield with the Union artillery firing point-blank into the fighting.
The fighting in the cornfield continued for two hours until 7:00 a.m. when Mansfield’s XII Corps was sent into the fight. Here, McClellan made his fourth mistake. He failed to coordinate the attacks between Hooker and Mansfield, missing the opportunity for his superior numbers to be of advantage. This also gave Lee time to move soldiers to the left flank. The XII Corps did breakthrough but, with a lack of support, failed to capitalize much on this advantage. The XII Corps did receive one division from Sumner’s Corps, however, they went blindly into the fight and were badly hurt.
The Bloody Lane – McClellan’s Fifth Mistake
The other two of Sumner’s divisions began moving at about 9:00 a.m. These divisions traveled south to hit the center of the Confederate line. The Confederates were stationed at a 800m road that was worn away from years of use. The Confederates used this natural trench as a defensive position and for about four hours they tried to stave off Union attacks. The Union finally did capture what became known as the “Bloody Lane.”
Here, we see McClellan making his fourth mistake. Here, McClellan did not send in the reserve corps of Porter and Franklin. McClellan believed that Lee had troops in reserve and thus needed to save these troops to counter Lee’s reserve force, which did not exist.
Burnside Bridge – McClellan’s Sixth Mistake
While the attacks in the cornfield, the West Woods, and the Bloody Lane were occurring, Burnside still failed to make his move which he was to make early in the day. This gave Robert E. Lee time to move units that were opposing Burnside to parts of the battlefield that they were needed. Finally, at 10:00 a.m., Burnside began his movement following a direct order to attack.
However, Burnside, even with superior numbers to the Confederates, failed to make a powerful attack. Burnside began sending units across Rohrbach Bridge, which would become known as Burnside bridge. By early afternoon, some Federals began to turn the Confederate position by crossing the creek downstream. However, it was not till nearly 3:00 p.m. that Burnside prepared for a general advance against the right of the Confederate line.
Because of Burnside’s delay, it gave A.P. Hill time to arrive from Harpers Ferry at 4:00 p.m. This allowed the Confederates to counterattack the Federals left flank and collapse it. Even with his left flank attacked, Burnside continued to press his attack and began pushing the rebels back towards Sharpsburg. However, here McClellan made his sixth mistake. He failed to send in supporting troops, believing that Lee had many more in reserve.
Lee’s Escape – McClellan’s Seventh Mistake
The fighting for the 17th ended and the next day, Lee had about 30,000 effective troops. McClellan, already outnumbering Lee greatly, received 13,000 more reinforcements. This meant that McClellan had more troops in reserve than Lee had available in his entire army. However, McClellan believed that Lee outnumbered him and thus he let the Army of Northern Virginia get away during the night of the 18th. Thus, McClellan let go of a perfect opportunity to crush and destroy Lee’s army because of his belief that Lee outnumbered him.
While Antietman was still a strategic victory for the Union, McClellan let the perfect opportunity to crush Lee slip through his hands. His inability to coordinate attacks allowed Lee to move troops along his interior lines and reinforce his lines where need be. McClellan’s belief that Lee’s army was much larger than it was also meant that he failed to play the best card that he had in his hand. McClellan could have easily crushed Lee but let the Army of Northern Virginia live to fight another day, especially because Lee refused to retreat the night of the 17 and stayed in place till the night of the 18. This gave McClellan a perfect opportunity for a crushing, Waterloo Esque victory.