General Robert E. Lee was ready to move his Confederate Army of Northern Virginia north from Frederick, Maryland, but first he needed to open lines of supply and communication to Virginia. A line through Manassas Junction could be easily disrupted by Federals guarding nearby Washington, so Lee opted to open a line to the Shenandoah Valley. To do this, he needed to capture the 12,000-man Federal garrison at Harpers Ferry, which protected the Baltimore & Ohio Railroad and the Chesapeake & Ohio Canal.
Under Lee’s Special Orders Number 191, Major General James Longstreet would lead three divisions out of Frederick toward Hagerstown, shielded by South Mountain. The rest of the army, split into three commands, would turn and cross the Potomac River, cut the B & O, and advance on Harpers Ferry from three separate directions. The garrison was to be captured within three days, at which time the three commands would join forces and rejoin the main army at either Hagerstown or Boonsboro. From there, the entire army would advance into Pennsylvania.
For the third time in his first three campaigns with the Army of Northern Virginia, Lee would defy military logic by dividing his army in the face of a larger enemy. But this time he would do it in enemy territory. Brigadier General John G. Walker, one of the three commanders slated to go to Harpers Ferry, expressed surprise that Lee would divide his army into so many pieces while Major General George B. McClellan’s Federals approached. Lee asked, “You doubtless regard it hazardous to leave McClellan practically on my line of communication, and to march into the heart of the enemy’s country?” Walker said yes.
Lee replied, “Are you acquainted with General McClellan? He is an able general, but a very cautious one. His enemies among his own people think him too much so. His army is in a very demoralized and chaotic condition, and will not be prepared for offensive operations–or he will not think so–for three or four weeks. Before that time I hope to be on the Susquehanna (River in Pennsylvania).”
Lee’s aide, Colonel Robert H. Chilton, wrote out copies of the order and delivered them to each division commander. Major General Thomas J. “Stonewall” Jackson, who formerly had Major General D.H. Hill’s division under his command, sent a second copy to Hill, who never received the copy written by Chilton. This would produce important consequences.
Lee wrote President Jefferson Davis, “I believe that the enemy are pushing a strong column up the Potomac River by Rockville and Darnestown, and by Poolesville toward Seneca Mills. I hear that the commands of (Edwin) Sumner, (Franz) Sigel, (Ambrose) Burnside, and (Joseph) Hooker are advancing in the direction above mentioned.”
This was true. Major General William B. Franklin’s corps was near Rockville and Darnestown, Major General Darius Couch was near Seneca Mills, and Burnside and Hooker were east near Cracklintown. McClellan now knew that the Confederates were at Frederick. He had reported on the 8th that they were 100,000 strong, but now, just one day later, he raised that estimate to 110,000.
McClellan wrote, “These are the numbers that are given by the rebel officers & men to citizens as they have passed through & which appear to be consistent… I am pretty well prepared for anything except overwhelming numbers.” The Confederate army actually numbered less than 60,000, and was spread out. McClellan reported to General-in-Chief Henry W. Halleck, “They talk of going to Gettysburg and York” in Pennsylvania.
After advancing further and positioning his troops to descend on the Confederate flank, McClellan wrote, “The army is tonight well posted to act in any direction the moment the enemy develops his movements. I am now in condition to-watch him closely, and he will find it hard to escape me if he commits a blunder.”
The Confederates headed out of Frederick on the morning of the 10th. Longstreet’s three divisions and D.H. Hill’s division moved on the National road toward Hagerstown, when Lee suddenly received word that Federals from Pennsylvania were coming their way. Lee’s army was already divided into four sections, but since Hagerstown was the key to his entire plan, he divided it yet again by sending Longstreet to defend the town and Hill to guard the South Mountain passes near Boonsboro. Longstreet, who routinely protested any order that divided the army, told Lee, “General, I wish we could stand still and let the damned Yankees come to us!”
Meanwhile, the other three Confederate commands approached Harpers Ferry. Military principles dictated that Harpers Ferry should have been abandoned when the Confederates crossed the Potomac River, but the Federals had insisted that it be held. This may have doomed the garrison there, but it served to weaken Lee’s army by forcing him to detach a portion to capture the strategic position. This was an unintended consequence that worked in the Federals’ favor.
Heading one of the three commands en route to Harpers Ferry was Jackson, who led his men west through Turner’s Gap in South Mountain. This required them to move through Martinsburg, but Jackson learned that Federals guarded that town, forcing him to retrace his route. This delayed part of the advance on Harpers Ferry. Meanwhile, Walker approached Loudoun Heights and Major General Lafayette McLaws approached Maryland Heights, two points overlooking Harpers Ferry, as planned.
When McClellan learned that Lee had moved out of Frederick, he ordered his Federals to hurry their northwest advance to catch them before they reached Pennsylvania. McClellan now reported that the enemy could number as many as 150,000 men, while Pennsylvania Governor Andrew Curtin forwarded a local report that the army numbered “not less than 120,000 men.”
On the night of the 10th, McClellan informed Halleck that evidence proved “most conclusively that almost the entire Rebel army in Virginia, amounting to not less than 120,000 men, is in the vicinity of Frederick City… They are probably aware that their forces are numerically superior to ours by at least twenty-five per cent.” McClellan proposed bringing the Federals out of the Washington defenses to join his army in the field. He acknowledged that this could leave the capital vulnerable to capture, but that would be nothing compared to “the ruin and disasters which would follow a signal defeat of this Army.”
By the 11th, the Federals were still 15 miles from Frederick, moving an average of just six miles a day while occasionally stopping to loot civilian property. Governor Curtin, terrified that the Confederates may invade his state, called on up to 50,000 Pennsylvanians to form militia units “for immediate service to repel the now imminent invasion by the enemies of the country.” At Philadelphia, “all able-bodied men” were called on to report to their district precincts, and all businesses were closed at 3 p.m. to give the men time to organize and drill.
Meanwhile, Lee accompanied Longstreet to Hagerstown, where they found no Federals from Pennsylvania as feared. But opposition was coming quickly.
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