(part two of three)
On September 10, 1862, as he advanced deeper into Maryland, Robert E. Lee began splintering his forces, as outlined in Special Orders No. 191. That day, all of his forces, mustered into five separate columns, started their movements to carry out the investment of the Federal garrisons in the Shenandoah Valley. Three of Lee’s columns marched towards Harpers Ferry (“Stonewall” Jackson’s column headed in that direction via a roundabout march through Williamsport, Maryland, and Martinsburg, Virginia). The two other columns—under James Longstreet and D.H. Hill—followed in Jackson’s footsteps, taking the National Road out of Frederick in the direction of Hagerstown.
However, contrary to what is written in Special Orders No. 191, only Hill stopped at the two columns’ appointed destination—Boonsboro. Rumors of Federals approaching Hagerstown from Pennsylvania compelled Lee to further divide his forces by continuing Longstreet’s march until they reached Hagerstown near the Mason-Dixon Line.
Concurrent with Lee vacating Frederick, more information made its way into McClellan’s hands. The Army of the Potomac’s Chief of Staff Randolph B. Marcy repeated twice on the night of September 11 that he (and McClellan) received reports of the enemy leaving Frederick on the road to Hagerstown, two days before the Lost Orders came into the Federals’ possession. Confident enough in these reports, Marcy ordered Ambrose Burnside’s command to occupy Frederick the next day.
Despite the certainties in the enemy movement out of Frederick, the fog of war still existed over the enemy’s movements as a new fold entered the intelligence sphere arriving at army headquarters. At 10:00 am the next morning, September 12, McClellan reiterated to Halleck his belief in the enemy’s abandonment of Frederick. This time, though, McClellan revealed that the route of the enemy’s movement stretched in two very different directions—one on the road running practically north towards Hagerstown and the other headed south in the direction of Harpers Ferry.
An enemy army in foreign territory moving two separate ways surely must have puzzled George B. McClellan. Ambrose Burnside, in the van of the Army of the Potomac, echoed this: “I can hardly understand how they can be moving on these two latter roads at the same time,” he wondered. “If they are going into Pennsylvania they would hardly be moving upon the Harper’s Ferry road, and if they are going to recross [the Potomac into Virginia], how could they be moving upon Gettysburg?” Regardless of the confusion, the head of Burnside’s column and portions of Alfred Pleasonton’s cavalry command occupied Frederick by the evening of September 12.
The picture of Confederate dispositions percolating in McClellan’s mind did become clearer to the commanding general on September 12. His afternoon dispatches show him favoring the idea that Lee’s main column was marching towards Williamsport, Maryland, to return to Virginia. Even in a nighttime note, McClellan told President Lincoln, “The main body of my cavalry & horse artillery are ordered after the enemy’s main column,” using the National Road out of Frederick.
However, hunches or estimations could not be solely relied upon in a campaign fraught with so many consequences should the United States Army fail. Thus, McClellan ultimately had to admit, “My movements to-morrow will be dependent upon information to be received during the night” but told Burnside to bring his command to Frederick, prepared “to move in any direction that may be required.” The bloodhound caught the scent, but did not yet know which path to follow. Above all of this, the question still hovered: what were the enemy’s intentions by moving on two roads in opposite directions?
By September 12, the momentum of the campaign began to shift away from Lee towards McClellan. Now in a position of relative strength with the Army of the Potomac beginning to mass around Frederick, no longer was the talk in McClellan’s dispatches of garnering reliable information. Rather, indications of striking at the enemy’s main column and having his commands ready to move at a moment’s notice dominate the correspondence of September 12. George McClellan was prepared to leap at the enemy, if only he could decipher exactly what that enemy was up to.
Saturday, September 13 began with Pleasonton’s cavalrymen saddling their horses and organizing for action. By daybreak, the blue horsemen trotted out of their Frederick camps in all directions ready for another day’s hard work. The weight of Pleasonton’s command pushed west on the road to Williamsport and Hagerstown. Alfred Pleasonton set a lofty bar for his command to meet that Saturday—“If possible I shall go to Hagerstown tomorrow,” he reported on September 12. Surely, the Confederate cavalry would have something to say about that.
Only a few miles west of Frederick, the Union horsemen stumbled into Jeb Stuart’s next line of defense atop Catoctin Mountain. A battle ensued there for much of the morning and, with the help of Burnside’s infantry, did not conclude until the early afternoon. “A rapid pursuit was made,” recounted Pleasonton and, following several other skirmishes around Middletown, the Army of the Potomac’s cavalry ended the day at “the foot of [South] mountain.” In the time Pleasonton’s soldiers further developed the enemy and its whereabouts all day on September 13, developments behind them carried the Maryland Campaign into its next stage.
The Confederate roadblock at Turner’s Gap in South Mountain proved “to be too strong a position to be carried by my force,” Pleasonton admitted, but help was on the way. Sometime after noon on September 13, a lost copy of Lee’s Special Orders No. 191 came into McClellan’s hands.
(to be concluded)
 The full text of Special Orders No. 191, not the Lost Order, is found in OR, vol. 19, pt. 2, 603.
 Report of Robert E. Lee, OR, vol. 19, pt. 1, 145.
 Randolph B. Marcy to Edwin Sumner, September 11, 1862, 7:00 pm, OR, vol. 51, pt. 1, 815; Randolph B. Marcy to Ambrose Burnside, September 11, 1862, 10:00 pm, ibid., 818.
 George B. McClellan to Henry Halleck, September 12, 1862, 10:00 am, GBM Papers, 448.
 Ambrose Burnside to Henry Halleck and George B. McClellan, September 12, 1862, 5:30 am, OR, vol. 19, pt. 2, 272-73.
 See George B. McClellan to Mary Ellen McClellan, September 12, 1862, 3:00 pm, GBM Papers, 449.
 George B. McClellan to Abraham Lincoln, September 12, 1862, 9:00 pm, GBM Papers, 452; Randolph B. Marcy to Ambrose Burnside, September 12, 1862, 11:00 pm, OR, vol. 51, pt. 1, 823.
 George B. McClellan to Henry Halleck, September 12, 1862, 5:30 pm, OR, vol. 19, pt. 2, 271; Randolph B. Marcy to Ambrose Burnside, September 12, 1862, 6:15 pm, OR, vol. 51, pt. 1, 823; Randolph B. Marcy to Ambrose Burnside, September 12, 1862, 8:30 pm, ibid.
 Report of Alfred Pleasonton, OR, vol. 19, pt. 1, 209.
 Alfred Pleasonton to Randolph B. Marcy, September 12, 1862, 9:15 pm, George B. McClellan Papers, Library of Congress (hereafter cited as GBM Papers, LOC) reel 31.
 Alfred Pleasonton to Randolph B. Marcy, September 13, 1862, 1:00 pm, ibid.
 Report of Alfred Pleasonton, OR, vol. 19, pt. 1, 209.
 The timing of this being in the “evening” as compared to sometime before noon is clearly established in George B. McClellan to Henry W. Halleck, September 13, 1862, 11:00 pm, OR, vol. 19, pt. 2, 281.