Major General George B. McClellan’s Federal Army of the Potomac began pouring into Frederick, Maryland, on September 13, two days after the main body of General Robert E. Lee’s Confederate Army of Northern Virginia had left.
McClellan’s arrival to the town thrilled the largely Unionist populace. Brigadier General John Gibbon, commanding the Iron Brigade, wrote, “When Genl. McClellan came thro’ the ladies nearly ate him up, they kissed his clothes, threw their arms around his horse’s neck and committed all sorts of extravagances.” McClellan wrote his wife, “I was nearly overwhelmed & pulled to pieces. I enclose with this a little flag that some enthusiastic lady thrust into or upon Dan’s bridle. As to flowers!–they came in crowds!”
As the Federals continued marching into Frederick, McClellan still did not know where Lee was. He told President Abraham Lincoln that he feared Lee might go back to Virginia before the Federals could give battle. But Major General Ambrose E. Burnside, whose Federals comprised McClellan’s right wing and whose cavalry had been skirmishing with the Confederates, noted that the enemy seemed to be moving in various directions. He wired General-in-Chief Henry W. Halleck, “I can hardly understand how they can be moving on these two latter roads at the same time. If they are going into Pennsylvania they would hardly be moving upon the Harper’s Ferry road, and if they are going to recross, how could they be moving upon Gettysburg?”
As the word spread that Confederates may be invading Pennsylvania, panicked residents of Harrisburg and Philadelphia rushed to leave town. City officials shipped their state documents and archives to New York.
On the Confederate side, Major General J.E.B. “Jeb” Stuart, commanding the cavalry, informed Lee that the Federals had reached Frederick. This alarmed Lee because he had not expected McClellan to move so quickly. Lee was also troubled by having heard nothing from Harpers Ferry, which he expected to have been captured by today.
Meanwhile, soldiers of the 27th Indiana (Twelfth Corps) set up camp in the same meadow south of Frederick used by Major General D.H. Hill’s Confederates four days prior. Corporal Barton W. Mitchell found a long envelope laying in the grass. Looking inside, he saw that it contained three cigars and an official-looking Confederate document. It read, “Headquarters, Army of Northern Virginia,” and was labeled, “Special Orders No. 191.” It was signed, “R.H. Chilton, Assist. Adj.-Gen,” and it was addressed to “Maj. Gen. D.H. Hill.” This was Lee’s special order describing how the Confederate army was to be divided, and now it was in enemy hands.
The document was forwarded up the chain of command all the way to McClellan’s headquarters, where one of McClellan’s staffers had served with Chilton in the old army and verified his handwriting. This was the greatest security breach in American military history. Not only did McClellan now have Lee’s plans, but he also knew that Lee’s army was divided and scattered across 25 miles.
When Gibbon came into McClellan’s tent to discuss troop strength, McClellan held up the order and said, “Here is a paper with which, if I cannot whip Bobbie Lee, I will be willing to go home. I will not show you the document now, but here is the signature, and it gives the movement of every division of Lee’s army. Tomorrow we will pitch into his center, and if you people will only do two good, hard days’ marching I will put Lee in a position he will find it hard to get out of.”
But to capitalize on this intelligence coup, McClellan would have to cross South Mountain to confront the Confederates before they could reunite, and he would have to do something he had never done before: move quickly. He initially doubted the authenticity of the order, so he contacted his cavalry commander, Brigadier General Alfred Pleasonton, to confirm that the Confederates were indeed marching according to what was written.
McClellan did nothing until Pleasonton responded at 6:15 p.m., over six hours after the “lost order” was discovered: “As near as I can judge the order of the march of the enemy that you sent me has been followed…” McClellan finally issued general marching orders, but these called for the Federals to start moving out next morning. All told, the army would not be put into motion until 18 hours after McClellan had learned of Lee’s plans. As Confederate Major General James Longstreet later wrote:
“The Confederates were dispersed and divided by rivers, and drifting thirty and forty and fifty miles apart. Under similar circumstances General (Winfield) Scott, or General (Zachary) Taylor… would have put the columns at the base of South Mountain before night, and would have passed the unguarded gaps before the sun’s rays of next morning could have lighted their eastern slopes.”
To downplay his discovery, McClellan informed Halleck that he had only received this new piece of intelligence on the night of the 13th, and that it only contained “some plans of the enemy.” McClellan believed that the order, which named eight Confederate generals, vindicated his claim that he was facing at least 120,000 enemy troops. When Lincoln wired, “How does it look now?” McClellan sent a response that reached Washington at 2:35 a.m. on the 14th:
“I have the whole rebel force in front of me, but am confident, and no time shall be lost. I have a difficult task to perform, but with God’s blessing will accomplish it. I think Lee has made a gross mistake, and that he will be severely punished for it. The army is in motion as rapidly as possible. I hope for a great success if the plans of the rebels remain unchanged. We have possession of Catoctin. I have all the plans of the rebels, and will catch them in their own trap if my men are equal to the emergency. I now feel that I can count on them as of old. All forces of Pennsylvania should be placed to co-operate at Chambersburg. My respects to Mrs. Lincoln. Received most enthusiastically by the ladies. Will send you trophies. All well, and with God’s blessing will accomplish it.”
Under McClellan’s plan, nearly 70,000 Federals would march down the National road and confront the Confederates at Boonsboro, while the other 19,000 moved through Crampton’s Gap in South Mountain to rescue the Harpers Ferry garrison and add those troops to McClellan’s Potomac army. McClellan wrote his wife Ellen, “I have the mass of their troops to contend with & they outnumber me when united.” McClellan told her, “I feel as reasonably confident of success as any one well can who trusts in a higher power & does not know what its decision will be.”
The Federals began pushing west from Frederick, leaving Lee confused as to how such a supposedly demoralized enemy with a notoriously sluggish commander would be closing in on him so fast. A pro-Confederate resident of Frederick informed Stuart that McClellan knew Lee’s plans. Lee hurried to consolidate his army to meet the growing Federal threat, ordering Longstreet to move eight of his nine brigades from Hagerstown to Turner’s Gap.
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