General Robert E. Lee, commanding the Confederate Army of Northern Virginia, ordered the portions of his army that had held South Mountain in Maryland to withdraw. Those divisions, commanded by Major Generals James Longstreet and D.H. Hill, fell back west to Sharpsburg, a hamlet among various hills and ridges between Antietam Creek and the Potomac River.
Lee planned to concentrate the forces at hand before returning to Virginia, but he changed his mind when he received word that Major General Thomas J. “Stonewall” Jackson’s Confederates had captured Harpers Ferry. Lee read the message and said, “That is indeed good news. Let it be announced to the troops.”
Lee had just 18,000 men, but he relied on Jackson and the rest of the Confederates at Harpers Ferry to hurry and join his army at Sharpsburg, which would give him close to 45,000. Then, pointing to the hills outside Sharpsburg, Lee told his staff, “We will make our stand on those hills.”
Lee needed to make a stand in Maryland. Retreating without a fight would demoralize the Confederacy and possibly even destroy hopes of foreign recognition. Major General George B. McClellan, commanding the Federal Army of the Potomac, had folded in the face of aggression during the Seven Days’ Battles, and Lee was confident that McClellan would fold again.
Even with all his 45,000 men, Lee’s army was about half the size of McClellan’s, which was closing in. But McClellan had never waged an offensive battle before, and Lee counted on his adversary’s tendency to caution. Lee would not be disappointed. McClellan informed his superiors, “The enemy disappeared during the night. Our troops are now advancing in pursuit of them.” But the pursuit did not begin until 8 a.m. on the 15th, giving the Confederates plenty of time to withdraw and prepare defenses.
Brigadier General Alfred Pleasonton’s Federal cavalry led the pursuit, riding down the western slope of South Mountain and driving off any remaining Confederates in the area. Some Federals stayed at South Mountain to collect scattered equipment and bury the dead. A Federal soldier wrote, “How all feeling of enmity disappears in presence of these white faces, these eyes gazing upward so fixedly in the gray of the morning hour.” Marching troops noted that Major General Joseph Hooker, who had led some of the hardest fighting in the battle on the 14th, was “in the saddle taking his brandy and water, looking as clean and trim as though he had just made his morning toilet at Willard’s (Hotel in Washington).”
Rather than hunting Lee down, McClellan took time to reorganize his command:
- Major General Ambrose E. Burnside had commanded the right wing consisting of Hooker’s First Corps and the Ninth Corps, but since Hooker’s corps had been “separated from you for the present by force of circumstances,” Burnside would command just the Ninth Corps, taking over for Major General Jesse Reno, who had been killed in action.
- A division of Major General Edwin V. Sumner’s Second Corps would lead the advance, with the tired, battle-worn First Corps following. The remainder of the Second Corps would round up enemy stragglers around Boonsboro.
- The exhausted Ninth Corps stayed back in reserve, but this was not communicated to the rest of the army, so these troops blocked the way for Major General Fitz John Porter’s Fifth Corps to advance. McClellan scolded Burnside, who grew distrustful of Porter for telling on him, and of Hooker because he thought the detachment of the First Corps from his command had been Hooker’s idea. Major General Jacob D. Cox, the Kanawha Division commander who had temporarily replaced Reno in command of the Ninth Corps, was “disturbed and grieved at the course things had taken.”
- The left wing, led by Major General William B. Franklin, remained intact, but Franklin led a short advance into Pleasant Valley and then halted for the day, even though the enemy forces in his front were less than half the size of his own.
- In the center, Major General Alpheus Williams was replaced as interim commander of the Twelfth Corps by Major General Joseph F.K. Mansfield. Mansfield had been in the army for nearly 40 years, and to the men he seemed to be a “calm and dignified gentleman,” who rode “with a proud, martial air and was full of military ardor.” Mansfield had long been requesting a field command, and now it was finally his.
The Federal forces methodically adjusted to their new roles and set out west in pursuit. The victory at South Mountain had boosted Federal morale, and the men advanced with a vigor they had previously lacked. The advance elements reported meeting very little enemy resistance, and McClellan wrote, “Information this moment rec’d completely confirms the rout & demoralization of the rebel Army.” He added, “General Lee admits they are badly whipped.”
But President Abraham Lincoln and his advisors were wary of these boastful claims; Navy Secretary Gideon Welles questioned the source who delivered Lee’s confession to McClellan. Welles said, “A tale like this from (General John) Pope would have been classed as one of his lies.” Lincoln chose not to question McClellan’s sources. He simply replied, “Your despatches of to-day received. God bless you, and all with you. Destroy the rebel army, if possible.”
McClellan ecstatically wrote his wife Ellen that he had won “a glorious victory,” and that he was pursuing Lee “with the greatest rapidity, and expect to gain great results.” He continued, “If I can believe one-tenth of what is reported, God has seldom given an army a greater victory than this. How glad I am for my country that it is delivered from immediate peril.”
In early afternoon, McClellan received word from a signal station atop South Mountain that “A line of battle–or an arrangement of troops which looks very much like it–is formed on the other side of the Antietam creek and this side of Sharpsburg.” This information was confirmed by Captain George A. Custer shortly thereafter. Sumner, leading the Federal advance, learned of this large force up ahead and wrote McClellan, “Shall I make the necessary dispositions to attack? And shall I attack without further orders?”
McClellan did not trust Sumner’s judgment and rode up to see things for himself. He did not arrive until around 5 p.m., which he deemed too late in the day to attack. He estimated enemy strength at 50,000 men, even though Lee had no more than 18,000, with his back to the Potomac River. By this time, the Federals had begun crossing Antietam Creek, east of Sharpsburg. The entire army was across by nightfall, gathering in the hills opposite the Confederates. Lee correctly guessed that McClellan was in no hurry to attack.
As his men camped for the night, McClellan continued to receive reports on the Battle of South Mountain. Some reported that Lee had been wounded, and others claimed that the entire Confederate army had been pushed back into Virginia, which was now obviously false. McClellan forwarded a report from an unknown source to General-in-Chief Henry W. Halleck: “Lee last night stated publicly that he must admit they had been shockingly whipped.” McClellan then boasted to his old adversary, former General-in-Chief Winfield Scott, “R.E. Lee in command. The rebels routed, and retreating in disorder.” Scott replied, “Bravo, my dear general! Twice more and it’s done.”
But McClellan could have done much more to capitalize on Lee’s “lost order.” He had not rescued the Federal garrison at Harpers Ferry, and he had not delivered the fatal blow to destroy the Confederate army before it could be reunited. And now McClellan was waiting until the next morning before making a move, thereby giving Lee even more time to concentrate his force.
McClellan had expected Lee to retreat, but to his surprise, Lee continued to hold his ground in the hills around Sharpsburg. Had McClellan ordered any reconnaissance to determine enemy strength, he might have learned that the Confederates were hopelessly outnumbered and vulnerable to annihilation (which he should have already known from Lee’s “lost order”). McClellan instead vowed to attack the next day.
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