Disunion follows the Civil War as it unfolded.

By early September 1862, having culminated his triumphant Second Manassas campaign with Gen. Thomas J. “Stonewall” Jackson’s postscript victory at Chantilly, Va. — within an easy commute of present-day Washington — Robert E. Lee was laying plans to win the war. He had concentrated his army at Frederick, Md., about 40 miles northwest of Lincoln’s capital. A few months before, the Union had been on the verge of taking the Southern capital of Richmond, Va. Now Lee was a victory away from, if not capturing Washington, demonstrating the South’s military and political viability on the doorstep of the Union.

The ensuing Battle of Antietam, like Gettysburg a year later, was one of the great turning points of the war. Given Union demoralization following the rout of Gen. John Pope’s army and doubts among troops returning from Virginia’s peninsula, it was a fight Lee might have a realistic chance of winning. And yet, events in the days leading up to it helped level the field for the Union, which managed — despite its own errors — to eke out a critical victory. The story of early September 1862 was complex, but much of it revolved around two sheets of paper.

Owing to straggling, aggravated by fast marching, the strength of Lee’s army in and around Frederick is best estimated as a range, somewhere between 40,000 and 50,000. His opponent, Gen. George B. McClellan, had 85,000 troops nearby, with another 72,500 in Washington under General Nathaniel Banks. Lee knew Washington fortifications were too strong to attack directly, but he hoped to lure McClellan into the open where he could have a fair fight to the finish on Northern soil.

Lee reasoned that his army should first travel west, into the northern continuation of Virginia’s Shenandoah Valley, from which he could march into Pennsylvania’s Cumberland Valley, hidden from enemy observers by a line of mountains. To do that, however, he would have to eliminate a 12,000-man Federal garrison at Harpers Ferry that could otherwise easily disrupt Valley supply lines.

Lee issued a special order, No. 191, dividing his army into two parts, starting Sept. 10. Gen. Thomas J. “Stonewall” Jackson would get two-thirds of the troops to capture Harpers Ferry, while Lee would take the remaining troops 24 miles northwest to Hagerstown, Md., just shy of the Pennsylvania line.

Jackson’s command was subdivided into three segments, corresponding to the triangular perimeter of Harpers Ferry, which in turn was shaped by the confluence of the Shenandoah and Potomac rivers. That created a possible problem: if help were required by Lee, or any of Jackson’s segments, each fragment would face at least one difficult river crossing in order to provide support.

Moreover, Lee’s smaller army would be temporarily divided into four fragments. None was large enough to defeat McClellan alone, but Lee calculated that the momentary vulnerability would be brief. This was his first mistake: he expected Harpers Ferry to fall on Sept. 13, but it held out until the 15th. Meanwhile, on Sept. 13, McClellan’s army moved into Frederick, putting it just 15 miles east of the midpoint separating Lee from Jackson. The only thing preventing McClellan from taking advantage of Lee’s temporary weakness was simply that he wasn’t aware of it.

Early on the morning of the 13th, however, Cpl. Barton Mitchell of the 27th Indiana found a bulky envelope on the ground outside a recently vacated Confederate camp, inside of which was an official-looking paper wrapped around three cigars. The document concluded, “By command of General Robert E. Lee” and was signed “R.H. Chilton, Assistant Adjutant General.” The paper was quickly passed up to the corps command, where Staff Col. Samuel Pitman, who knew Chilton from before the war, verified the signature. It was an authentic copy of Special Order 191.

By late morning it was in the hands of General McClellan, who was meeting with a group of local citizens. The general quickly wired President Lincoln: “I have all the Rebel plans and will catch them in their own trap.” McClellan knew immediately that the documents offered him the chance at a master stroke. As he told Gen. John Gibbon: “Here is a paper by which if I am unable to whip Bobby Lee, I will be willing to go home.”

However, among the citizens meeting with McClellan when the order arrived was a Confederate sympathizer. Although he did not know the specifics, the man could judge from McClellan’s reaction that the Federals had unexpectedly learned something significant. The unknown sympathizer rode off to report the news to a nearby rebel officer, who passed the information along to Lee.

Library of Congress

Still, McClellan had the upper hand, and had he moved quickly, he could have caught Lee and destroyed him, piece by piece. Instead, after a quick start on the morning of Sept. 14, he slowed his advance on Lee’s center. He did not launch a full attack until Sept. 17, by which date Lee’s army was mostly reunited along Antietam Creek. Lee’s forces survived the ensuing battle and were able to retreat, bloodied but intact, to Virginia.

The mystery of how the “lost order” got lost has yet to be solved. Three copies of Special Order 191 survive. One was delivered to Richmond and now sits in the National Archives. A second, addressed to Gen. D. H. Hill, is in Stonewall Jackson’s handwriting and was held by Hill until his death in 1889. A third copy, also addressed to Hill, originated in Lee’s headquarters and was the one discovered by the Yankee corporal.

Why were two copies sent to Hill? When Jackson crossed the Potomac, Hill was under his command, but after Lee arrived, Hill’s command was separate. Evidently, inertia in the midst of rapid change left Jackson uncertain about Hill’s status. When Jackson received a copy of Order 191 from Lee, he assumed a duty to get a copy to Hill. Consequently, he wrote one out and had it delivered. But Lee had earlier directed Chilton to send a copy directly to Hill.

For the rest of his life Hill was adamant that Stonewall’s copy was the only version he received. After the war his chief of staff swore an affidavit stating that the dispatch from Lee’s headquarters never arrived. Similarly, in postwar correspondence with McClellan, Hill learned the lost order was found inside an envelope – presumably the delivery one, implying that it had never been opened and therefore never delivered. Hill also obtained letters from two former Union officers opining that the order was found two miles away from Hill’s campsite, raising the possibility that it was dropped inadvertently by a courier.

But Hill’s evidence is not conclusive. The order could have been lost from Hill’s command as the troops were leaving Frederick, just as easily as it could have been lost on its way to him. And the decades-old memory of Union officers as to the state of the orders’ envelope is hardly enough to seal the case.

Chilton, not surprisingly, averred that the orders had indeed been delivered. But because it was half a year before Chilton learned that the dispatch had been lost, he could not recall the courier’s name, so he couldn’t be interrogated.

Since it was customary for delivery envelopes to also function as stock for signed receipts, some historians argue that the discovery of the order inside an envelope is convincing evidence that the copy from Chilton never reached Hill. But that scenario has problems.

For one thing, there’s no evidence the envelope was sealed when discovered; Corporal Mitchell’s first sergeant claimed it was discovered unsealed. Yet the order was probably originally put in a sealed envelope so it could not easily be secretly examined and replaced by unauthorized readers. Moreover, if the envelope had indeed been sealed, it is unlikely Chilton would have wrapped the order around three cigars.

But if responsibility for losing Lee’s order can’t be affixed to one officer or the other, it remains possible, at least circumstantially, that they share the blame. Both men had other incidents of bad judgment and an unwillingness to accept the consequences of their actions.

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Just before the Battle of Chickamauga the following year, Hill was at least partly responsible for the lost opportunity at McLemore’s Cove — his commander, Braxton Bragg, had set one of the best traps of the war to crush an isolated Yankee division, yet Hill failed to attack as ordered. He offered various mysterious explanations, including that a perfectly healthy subordinate, Patrick Cleburne, was too ill to lead the attack. Similarly, Chilton prepared and delivered questionable orders during the Peninsula and Chancellorsville campaigns.

In a way, Lee shares in the blame as well. To hold the initiative and keep his opponent off-balance, Lee’s marches were breathtakingly fast. Straggling among the rank-and-file was only one consequence. The lost order was arguably a statistically predictable result of rushed administration connected with the speedy movements of Lee’s army. Yet Lee was sensitively aware of the rewards that speedy movements might yield, since many Richmond officials felt the failure to quickly occupy Washington after the victory a year earlier at the First Battle of Bull Run was crucial mistake.

Perhaps when Chilton’s courier arrived, he learned that Hill had already otherwise received the orders and thereby assumed his copy was a mere duplicate. With no need to leave the orders and believing them of lesser importance, he may simply have obtained a perfunctory receipt and kept the envelope and enclosure to hold his own cigars. It is the sort of thing anyone might do, and if he were one of the thousands of men killed in the ensuing battle, his story would never be heard.

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Sources: Steven E. Woodworth, “Davis and Lee at War” and “Six Armies in Tennessee”; Donald Jermann, “Antietam: The Lost Order”; Stephen Sears, “Landscape Turned Red” and “Controversies and Commanders”; Joseph L. Harsh, “Taken at the Flood”; Hal Bridges, “Lee’s Maverick General: Daniel Harvey Hill”; Silas Colgrove, “The Finding of Lee’s Lost Order,” in “Battles and Leaders of the Civil War,” Vol. 2; D.H. Hill to McClellan, April 17, 1869, George B. McClellan Papers, Library of Congress; Ezra Carmen and Thomas Clemens, “The Maryland Campaign of 1862,” Vol. 1.

Phil Leigh is an armchair Civil War enthusiast and president of a market research company. He is the author of “Natural Selection at Pea Ridge” and “Brooklyn in Charleston.” He has also annotated an edition of “Company Aytch,” the diary of a Confederate soldier.