Lee recognized their lack of supplies, which would hinder their effort further into enemy territory. However, he would not let his disadvantages hold him back, and he threw caution to the wind by choosing to execute his invasion. His words to Davis in his proposal letter were: "Still, we cannot afford to be idle, and though weaker than our opponents in men and military equipments, must endeavor to harass if we cannot destroy them."

The victory on behalf of the Union lifted spirits which had recently been dampened from prior successful Confederate campaigns. Local reports claim "Gen. McClellan immediately proceeded to the right, where he was enthusiastically received, and by his presence added much to our success in recovering the ground lost."

McClellan was a source of inspiration to his troops throughout the duration of the battle. The soldiers under him praised him and looked to him for motivation. According to a first-hand account shows this admiration; "What strength of character George Brinton McClellan brought to his mission was untampered by adversity."

Thus, McClellan pumped his troops up during even the height of the battle.

Review Tactical Situation

The two sides had opposing missions when they met at Antietam Creek. The Union aimed to intercept Lee on his retreat South. Thus, several brigades were deployed to meet Lee at Antietam Creek in a move to block his march towards Washington D.C. The Union force was comprised as follows:

The disposition of the troops for the impending battle was as follows: Gen. (Edwin Vose) Sumner's corps, with Banks' division, to occupy the center; Gen. Hooker's corps, with the Pennsylvania Reserves, and Franklin's corps, on the right, and (Fitz John) Porter's corps on the left of Sumner, and Burnside on the extreme left, with the view of turning the enemy's right flank. Gen. Pleasanton supported the centre with 2,500 cavalry and four batteries.

The Confederate goal was to conquer, fulfilling a principle of war against their Northern enemy. Lee believed that the time to invade was at hand, and wrote to President Davis "If it is ever desired to give material aid to Maryland and afford her an opportunity of throwing off the oppression to which she is now subject, this would seem the most favorable."

And so, he took a defensive position against McClellan's approaching forces. See Appendix B for a detailed map of each of the two's positions as the battle began and progressed.

Terrain

There were several aspects of the terrain that affected the outcome of battle. According to first hand accounts, "At Antietam it was a low, rocky ledge, prefaced by a corn-field."

Miller's Cornfield was at the center of the two forces, and saw some of the bloodiest fighting in the morning, the earliest stages of the battle. The Confederacy used the cover of the cornfield strategically to counter the oncoming Union forces; "Beyond this road stretched a plowed field several hundred feet in length, sloping up to the battery, which was hidden in a corn-field. A stone fence, breast-high, enclosed the field on the left, and behind it lays a regiment of Confederates."

Lee was in defensive positions directly behind Antietam Creek; "Lee's army then fell back behind Antietam Creek, a few miles above its month, and took a position admirably suited for defense."

This position proved beneficial, with infantrymen being provided sufficient cover by stone fences, limestone, and small hallows.

However, the creek which they used as their front barrier was only 60-100 feet in width, depending on the location. Accounts of generals on the field even note this limitation in terms of tactical defense cover. The creek itself was not enough to fully hamper the coming Union forces; "Antietam Creek was not so much a barrier to the Federal army as it was a nuisance, limiting troop movements and hampering a close look at the enemy's positions."

There were bridges only a mile apart, allowing the Union to spread out their forces effectively. Behind Lee's forces was the Potomac, which was much more difficult to cross in comparison to the creek. There was only one bridge at Shepherdstown, which is where the battle actually ended on September 18 with Lee's inability to effectively cross over in time.

Troops Available

Lee had the lesser number of troops in the situation. His troops numbered around 55,000 when McClellan finally decided to attack,...

On the other hand, McClellan had a force of over 90,000 Union men. Accounts from the field acknowledged this large disproportion between; "To oppose man against man and strength against strength was impossible; for there were four lines of blue to my one of gray."
The primary source of a fighting general states that "McClellan gives his force at Sharpsburg at 87,164; Being seriously outnumbered, Lee had to commit his entire stock of soldiers in the fight, and was therefore at a disadvantage. McClellan, having the edge in terms of troop numbers, did not have to commit his entire force, and so held the strong advantage.

Disposition of Forces

The two were placed strategically at the beginning of the battle. Lee had batteries in the front of his position, in the front of Dunker church.

McClellan had reserve artillery east of Antietam.

McClellan held the east side of Antietam Creek, with Burnside and Porter heading up the left flank. Sumner, Mansfield, Hooker, and Franklin led garrisons towards the north of the site. See Appendix B for their starting positions, and the ground gained throughout the first day. Sumner, Mansfield, and Hooker made the force that pushed Confederates out of the cornfields back towards the Potomac. Yet, General McClellan over-estimated the enemy, but also had other reasons for not so quickly pursuing Lee included McClellan's belief that Union supplies were running low, "On several occasions General McClellan has telegraphed to me that his army was deficient in certain supplies."

As supplies were delayed, McClellan made the decision to wait and not attack earlier than the morning of September 17th. According to Henry Halleck's defense of McClellan's decision, "General McClellan stated that it would require at least three days to supply the First, Fifth, and Sixth Corps; that they needed shoes and other indispensable articles of clothing, as well as shelter-tents."

Initial Movements

The battle began around dawn. Major General Joseph Hooker attacked Lee's left flank and began the battle. First hand Confederate reports state that "There was an ominous lull on the left."

From 6 am to later in the morning, around 9, the battle was primarily focused around Dunker Church and the Cornfield. Additional fighting broke out in the woods north and east of Sharpsburg. The Union forces began pushing the Confederates back westward. Sumner, Mansfield, and Hooker took the most ground, forcing the Confederates to retreat to Sunken Road by the afternoon of the first day. Here at Sunken Road, the battle waged for another four long hours, racking up a total of over 5,000 casualties. Union forces focused on attacking Lee from Sunken Road, and eventually broke through the Confederate lines. General Ambrose Burnside and his troops captured a bridge over Antietam Creek and successfully moved toward the right flank of the Confederates. Fighting wrapped up towards the end of the day, with the final attack taking place at the lower portion of the bridge, where Burnside effectively cut off Lee's path of retreat; "His troops got into action about dusk, which lasted two hours, during which the enemy were driven about half a mile, with considerable loss."

Major General a.P. Hill led a counter attack which ended the fighting for the day. Lee retreated south while still fighting of McClellan's forces and suffered immense loses. The Confederates then retreated to Bunker Hill and were pursued by the Union army over the next few days.

The Northern pursuit followed the Confederate army south; "The next morning (the 19th) shells began to come from over the river, and we were started on the road to Richmond with a mixed guard of cavalry and infantry."

For a detailed description of the time line with a visual aide see Appendix a.

Outcome

What resulted was the bloodiest battle in American history. All parties involved saw huge losses. First-hand accounts state that "The effect was appalling. The entire front line, with few exceptions, went down in the consuming blast."

Both the North and South saw these huge losses. Southern accounts show nearly all of their already few numbers being massacred in the span of the first major day of fighting, with "Nearly one-fourth of the troops who went into battle were killed or wounded."

Soldiers were mowed down by the entire row, as commanders forced line after line to gain the advantage. According to a Confederate soldier, "I was not at the front when, near nightfall, the awful carnage ceased; but one of my officers long afterward assured me that he could have walked on the dead bodies of my men from one end of the line to the other."

The carnage was immense. Newspaper reports from the Washington Star reported that "It is impossible at this writing…