The Battle of Antietam: Background

This entry is part 7 of 11 in the series The Maryland Campaign

The Battle of Antietam

After the various actions at the gaps along South Mountain, General Robert E. Lee withdrew his army to the west side of Antietam Creek around the town of Sharpsburg. This position was to be the location of the Battle of Antietam that took place on September 17, 1862.

The dividing barrier between the two armies was Antietam Creek, a tributary of the Potomac River that ran generally north and south from Pennsylvania and through Maryland. In the Sharpsburg area, the creek was traversed by three bridges, appropriately named the Upper Bridge, the Middle Bridge and the Lower Bridge. The latter was to become famous as Burnside’s Bridge after the battle.

General Lee had organized his Army of Northern Virginia into two large infantry corps. The First Corps was commanded by Maj. Gen. James Longstreet, considered to be Lee’s second-in-command. The 42-year old South Carolinian commanded five divisions and an independent brigade. His division commanders were Maj. Gen. Lafayette McLaws, Maj. Gen. Richard H. Anderson, Brig. Gen. David R. Jones, Brig. Gen. John G. Walker and Brig. Gen. John Bell Hood. The independent brigade was commanded by Brig. Gen. Nathan G. “Shanks” Evans.

The Battle of Antietam overviewLee’s Second Corps was commanded by Maj. Gen. Thomas J. “Stonewall” Jackson. The 38-year old Virginia commanded four divisions that were led by Brig. Gen. Alexander R. Lawton, Maj. Gen. A.P. Hill, Brig. Gen. John R. Jones and Maj. Gen. D.H. Hill. During the fight at Harpers Ferry, McLaws’ and Walker’s divisions had been attached to Jackson’s force.

Click Map to enlarge.

All of the cavalry units were grouped in the Cavalry Corps under the command of Maj. Gen. J.E.B. Stuart. The reserve artillery was under the command of Brig. Gen. William N. Pendleton. The Second Corps was organized with component artillery attached to each division while the First Corps had placed all of its artillery assets at the corps level.

The Army of the Potomac, commanded by Maj. Gen. George B. McClellan, included six infantry corps. They were commanded by Maj. Gens. Joseph HookerEdwin V. SumnerFitz John PorterWilliam B. FranklinAmbrose E. Burnside and Joseph K. Mansfield. There were a total of 18 divisions among the six corps. The cavalry division of Brig. Gen. Alfred Pleasonton consisted of the brigades of Maj. Charles J. Whiting and Cols. John F. Farnsworth, Richard H. Rush, Andrew T. McReynolds, and Benjamin F. Davis. All of the Union artillery assets were incorporated with the divisions.

There are various accounts of the overall strength of both armies. The number of Confederates who were engaged ranges from 38,000 at the low end to 51,800, including A.P. Hill’s Light Division which arrived during the afternoon. Likewise, the number of Union soldiers engaged ranges from 71,500 up to 87,164. The operative word is engaged. At least 30,000 Union soldiers were held in reserve by McClellan because he had overestimated Lee’s strength.

Beginning on September 15, 1862, Lee deployed his men on a low ridge west of the creek. He did not order the building of fortifications. Instead his troops were ordered to used the available terrain features for defense. The terrain consisted of rail and stone fences, outcroppings of limestone, General Robert E. Lee, 1862and little hollows and swales.

The creek did not present much of a barrier for a determined army yo advance over.ranging from 60 to 100 feet in width, and was fordable in places and crossed by the three stone bridges each a mile apart. The Confederates had there backs to the Potomac River with only a single crossing point, Boteler’s Ford at Shepherdstown, nearby should retreat be necessary.

On September 15, the force under Lee’s immediate command consisted of no more than 18,000 men, only a third the size of the Federal army. Jackson’s force was still at Harpers Ferry, accepting the surrender of the Union garrison that morning.

Two Union divisions had arrived on the afternoon of the 15th while the rest of the army arrived during the evening hours. McClellan trademark caution delayed the Union attack through September 16th. This allowed Lee to concentrate the bulk of his forces at Sharpsburg. Longstreet’s corps arrived from Hagerstown and Jackson’s corps, minus A.P. Hill’s division, arrived from Harpers Ferry on the 16th.

Lee placed Jackson in command of his northern flank, anchored on a bend of the Potomac River. Longstreet was in command of the southern flank, anchored on Antietam Creek. The Confederate line was about four miles long.

McClellan positioned Hooker’s I Corps on his right at the northern end of his line. Mansfield’s XII Corps was to his left and Franklin’s VI Corps in their rear as a reserve. Moving south, Sumner’s II Corps was in the center and Burnside’s IX Corps was at the southern end of the Union line. Porter’s V Corps was on the east side of the creek around McClellan’s headquarters as the Union reserve.

General George McClellanDue to the nature of the opposing terrain, McClellan planned a powerful assault on the Confederate left with his objective being to turn the Confederates. He intended to commit half of his army to this attack with a two-corps assault at the start and a third corps in support. If necessary, McClellan intended to use his reserves to bolster the attack.

McClellan intended to use his forces in the south as a diversionary attack to draw Confederate forces from the north, weakening it  for the main assault. The troops in the center would be available to exploit any successes on either flank.

At the onset, McClellan made several key mistakes. He ordered Hooker to probe the Confederate defenses in the north on the evening of September 16th with infantry supported by artillery. This alerted Lee to McClellan’s intentions and he immediately strengthened that part of his line.

McClellan only issued orders to each corps commander that pertained to their corps. He did not lay out his entire battle plan as other commanders would do. This did not allow the corps commanders to understand the entire plan, only their part in his plan. This made the Battle of Antietam a series of uncoordinated battles rather than a coordinated whole.





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