September 12, 1862 – General Robert E. Lee’s Army of Northern Virginia was divided into multiple sections as Federals entering Frederick discovered a document that threatened to destroy the Confederates.
By September 12, Major General James Longstreet’s Confederates were at Hagerstown, Major General D.H. Hill’s were at Boonsboro, Major General Jeb Stuart’s cavalry was at South Mountain, and three Confederate forces were converging on Harpers Ferry. Stuart reported that advance Federal units drove him from the Catoctin Mountain, about seven miles east of South Mountain, that night.
Troops of Major General George B. McClellan’s Army of the Potomac began entering Frederick on the 13th, two days after the Confederates had left. The largely Unionist populace cheered the Federals’ arrival. Stuart 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.
As the Federals entered Frederick, McClellan still did not know where Lee was. He wrote his wife, “From all I can gather, secesh is skedaddling & I don’t think I can catch him unless he is really moving into Penna… I begin to think that he is making off to get out of the scrape by recrossing the river at Williamsport.” McClellan 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 Confederates 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 morning of the 13th, soldiers of the 27th Indiana (XII Corps) set up camp in the same meadow south of Frederick used by D.H. Hill’s men four days ago. Private Barton Mitchell found three cigars wrapped in official-looking Confederate documents laying in the grass. These papers were Hill’s copy of Lee’s Special Orders No. 191, written by Colonel Robert H. Chilton, the Confederate army adjutant general.
The documents were forwarded up the chain of command until they reached McClellan’s headquarters. One of McClellan’s staffers had served with Chilton in the old army and verified his handwriting. Not only did McClellan now have Lee’s plans, but he also knew that Lee’s army was divided and scattered over 25 miles apart. When Lincoln wired, “How does it look now?” McClellan responded:
“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… My respects to Mrs. Lincoln. Received most enthusiastically by the ladies. Will send you trophies.”
McClellan showed one of his subordinates Lee’s “lost order” and said, “Here is a paper with which if I cannot whip Bobbie Lee, I will be willing to go home.” But McClellan did not issue general marching orders for the army until 6:20 p.m., six hours after receiving the “lost order.” And the Federals would not be moving until the next morning, another 10 crucial hours later.
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.
The Federals began pushing west from Frederick, leaving Lee confused as to how such a supposedly demoralized enemy would be closing in on him so fast. A pro-Confederate Frederick resident 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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