As the sun rose on the foggy morning of September 16, General Robert E. Lee had assembled less than 20,000 men of his Confederate Army of Northern Virginia. These forces were positioned on the hills between Antietam Creek to the east and Sharpsburg, Maryland, to the west. The Potomac River was at their backs.
Major General George B. McClellan, commanding the Federal Army of the Potomac, outnumbered Lee by more than two to one. He wrote his superiors at 7 a.m., “Will attack as soon as situation of the enemy is developed.” McClellan told Major General William B. Franklin, commanding the Sixth Corps (and a good friend of his), “I think the enemy has abandoned the position in front of us, but the fog is so dense that I have not yet been able to determine.” He then sent another message to Franklin: “If the enemy is in force here, I shall attack him this morning.”
McClellan continued to boast about his “great victory” at South Mountain two days prior. He wrote his wife Ellen on the morning of the 16th that he had “no doubt delivered Penna & Maryland.” Writing to General-in-Chief Henry W. Halleck, McClellan could “ascertain that some of the enemy are still there,” but he would not attack until he learned how many there were.”
Reports continued to arrive at McClellan’s headquarters stating that Lee was returning to Virginia. He shared these with Halleck, who responded, “I think you will find that the whole force of the enemy in your front has crossed the river. I fear now more than ever that they will recross at Harper’s Ferry or below, and turn your left, thus cutting you off from Washington.”
McClellan seemed poised to attack that morning. But then the morning sun burned the fog away, the enemy lines in the distance were revealed, and in typical McClellan fashion, he suddenly thought it imprudent to attack a force that he estimated to number 100,000 men. Additionally, McClellan had been reinstated to army command solely “for the defense of Washington,” so if he waged an offensive battle outside the capital that failed, he risked being excoriated by the Lincoln administration and the Republicans in Congress, or even court-martialed for exceeding his authority. For these reasons, a McClellan staffer wrote, “nobody seemed to be in a hurry… Corps and divisions moved as languidly to the places assigned to them as if they were getting ready for a grand review instead of a decisive battle.”
As McClellan spent the day guessing how many enemy troops he faced, Major General Thomas J. “Stonewall” Jackson led two divisions along with Brigadier General John G. Walker’s division from Harpers Ferry to reinforce Lee at Sharpsburg. The men had conducted a grueling 17-mile march through the night to get there. This raised Lee’s strength to nearly 40,000. Lee hurriedly positioned his force:
- Jackson’s divisions took positions on the left (north) flank.
- Walker’s division took up the extreme right (south).
- The divisions of Major Generals James Longstreet and D.H. Hill comprised the center and right.
Major General A.P. Hill’s division remained at Harpers Ferry paroling prisoners after capturing the Federal garrison. That night, Lee ordered Hill to hurry his men to join the main force as well, but they would not be able to join the main army until the next day.
The Confederates could see the enormous Federal army gathering in the hills across the fields; Longstreet called it “an awe-inspiring sight.” Lee’s army remained vastly outnumbered, but the Federals only threatened him with a light artillery barrage. President Abraham Lincoln, trying to gather all the information he could, wired Pennsylvania Governor Andrew Curtin, “What do you hear from Gen. McClellan’s army?”
About 60,000 Federals had arrived by the end of the 16th, with the rest en route. The Federal army was short of medical supplies and personnel, having left most of their equipment and ambulances on the Virginia Peninsula. Hospital tents were also a rare commodity, prompting the army’s medical director, Jonathan Letterman, to commandeer homes and barns in the vicinity and prepare them to receive wounded troops.
McClellan finally drafted a plan of attack around 2 p.m., or 24 hours after arriving in the Confederates’ front. He did away with the three-wing configuration he had created before leaving Washington, instead having each of the six corps commanders report directly to him. This decentralized the command structure at a time when centralization could have been much more effective. Under McClellan’s plan:
- Major General Joseph Hooker’s First Corps, on the far right (north), would begin the general assault and “attack the enemy on his left flank” at dawn.
- Major General Fitz John Porter’s Fifth Corps would hold the center and join the assault on the left once that flank was turned.
- Major General Ambrose E. Burnside, new to the army and just put in command of the Ninth Corps the previous day, would move against the enemy’s right (south) to secure the Potomac River crossing, thereby cutting off Lee’s escape. This was the flank closest to Harpers Ferry, where Confederate reinforcements were expected to arrive from the next day.
- Franklin’s Sixth Corps would be held in reserve and deployed wherever it was needed most.
The attack, which McClellan had said would happen on the 16th, was postponed until the morning of the 17th. Hooker’s Federals advanced west and took up assault positions around 4 p.m. They briefly exchanged fire with Brigadier General John Bell Hood’s Confederates in the East Wood. As the Confederates fired their cannon to measure the range, a Federal soldier recalled, “Nothing could have been more grand. The red glare of flame along the Rebel line for more than a mile, the bright streams of light along the track of the shell, and the livid clouds of smoke as the shell burst in the air, constituted a spectacle brilliant beyond comprehension.”
This exchange of fire revealed Hooker’s position to the Confederates, and he called on McClellan for reinforcements. McClellan assured him that he would be supported by the Twelfth Corps, which Major General Joseph K.F. Mansfield had just taken command of the previous day. Major General Edwin V. Sumner’s Second Corps would then join in as well, and they would overwhelm the Confederate left flank. This was to be where the bulk of the fighting would take place.
Major General Alpheus Williams, who had been replaced by Mansfield as commander of the Twelfth Corps, returned to his division command and wrote that the night before the battle was “so obscure, so mysterious, so uncertain… but with a certain impression that the morrow was to be great with the future fate of our country. So much responsibility, so much intense, future anxiety!”
The arrival of Federals in the northern sector of the field indicated to Lee that McClellan would focus most of his energies there. In one of the rare instances of the war, both sides camped within sight of each other across an open field, both knowing that a major battle would take place the next day. A “dismal, drizzling rain” fell throughout the night as troops on both sides readied themselves for the fight.
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