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Union commander George B. McClellcm won the Battle of South Mountain in 1862. So why was it such a strategic disaster?
In the annals of warfare, it is beyond rare that the commander of an army is given the enemy's battle plans. Yet that is precisely what happened in September 1862, when a copy of Confederate general Robert E. Lee's Special Orders No. 191 fell into the hands of the commander of the Union's Army of the Potomac, Major General George B. McClellan. It was, in the words of historian Bruce Catton, "the greatest security leak in American military history," and for a moment it gave McClellan the opportunity to end the Civil War--an opportunity that was, tragically, squandered.
Lee had assumed command of the Army of Northern Virginia in June 1862 and had succeeded in driving the Union forces back to the security of Washington. In early September, flush with his victories at the battles of the Seven Days and Second Manassas, he decided to lead his army into western Maryland, taking the war, for the first time, into the Northern homeland. By now, Confederate generals Braxton Bragg and Edmund Kirby Smith were marching through Tennessee into Kentucky; it remained only for Lee to cross the Potomac.
Before his Maryland Campaign, Lee--in concert with Confederate president Jefferson Davis--had declared several objectives. The northward incursion would put an end to the South's defensive war and level some retribution on the Yankees. And, after a year and a half of fighting on Southern soil, the farms, towns, and industries of Northern Virginia and the Shenandoah Valley would get some respite from the war's devastation.
Lee and Davis assumed that Marylanders would welcome a Confederate military presence and hoped to bring them into the Southern fold. Of all the states not in secession, Maryland was the most problematical to the Union, as the earlier Baltimore Riots of 1861 had demonstrated. Frustrated by the situation, President Abraham Lincoln had stretched the limits of his executive power by suspending the writ of habeas corpus and jailing some 31 Maryland legislators as well as Baltimore's mayor. Lee and Davis believed that Maryland needed little encouragement to join the Confederacy.
Mainly, however, it was the Confederacy's hope that a series of defeats on Union soil, compounded on those already suffered during the summer, would drive the now-demoralized citizens of the North to reject both the war and their Republican president, replacing him in the upcoming elections with a Democrat who was more inclined to a settled peace. Neither Lee nor Davis could have known that it was Lincoln's plan to use the next Union victory as a springboard for his Emancipation Proclamation, which would expand the focus of the war to include abolition and unequivocally remove the prospect of a "conquered peace" from the Rebels' plans.
When McClellan became aware of the threat of a Rebel drive across the Potomac River, he moved with atypical swiftness. He had already combined his Army of the Potomac with Major General John Pope's Army of Virginia, and he led his 75,000man force out of the capital's fortifications and marched to intercept Lee's army. Lacking specific information on Lee's exact location, McClellan marched west toward Frederick, Maryland, in the general direction where he expected to find him.
By then, Lee's army had crossed the Potomac near Leesburg, Virginia, and had indeed marched north into Frederick, then west across South Mountain toward Hagerstown, near the Pennsylvania border. Uncharacteristically, Lee--whose intelligence from the normally reliable Major General J. E. B. Stuart was sparse and inaccurate--underestimated his enemy, and on September 8 he wrote to Davis of his expectation that McClellan was still in Washington. In fact, the Union commander was already well on his way to Frederick. Thus far, McClellan had done everything right.
Then the fates rewarded McClellan with an extraordinary gift. A copy of Lee's Special Orders No. 191, drafted on September 9, had been lost by the Rebels and discovered on their campsite four days later by Union soldiers, who passed it up the chain of command to McClellan. It detailed Lee's plans to divide the Army of Northern Virginia. Lee had sent Major Generals Thomas J. "Stonewall" Jackson, A. P. Hill, Lafayette McLaws, and John George Walker to attack the isolated garrison at Harpers Ferry, in western Virginia, from both sides of the Potomac. The rest of the army, which included Major General James Longstreet's command and Major General Daniel Harvey Hill's division, was ordered to march as a rear guard with the reserve artillery and supply trains to Boonsboro, just two miles beyond South Mountain. A rugged continuation of the Blue Ridge Mountains, it formed a natural barrier between the Cumberland and Hagerstown valleys and Eastern Maryland.
By the time McClellan read Lee's lost order, his advance body was already approaching South Mountain. Barely able to contain himself, McClellan reputedly proclaimed: "Now I know what to do! Here is a paper with which, if I cannot whip Bobby Lee, I will be willing to go home!" He resolved to cross South Mountain and--as he later specified in his orders to a subordinate--"cut the enemy in two and beat him in detail."
On September 13, McClellan put Major General Ambrose Burnside in command of the army's strong right wing, consisting of Major General Joseph Hookers I Corps and Major General Jesse Lee Reno's IX Corps. They were to cross South Mountain through Fox's and Turner's Gaps, march on Boonsboro, overwhelm Longstreet and Hill, and drive a wedge between Lee and the rest of his army. The center, under Major General Edwin Vose Sumner, was to be held in reserve.
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McClellan then directed Major General William B. Franklin, commanding the left wing (VI Corps and a division of the IV), to cross South Mountain at Crampton's Gap the next morning, attack the rear of McLaws' undersized division overlooking Harpers Ferry on Maryland Heights, relieve the beleaguered garrison there, and return to assist Burnside.
Although McClellan didn't know it at the time, the prospects for a swift and decisive victory were even better than he had envisioned. Responding to a false report that a large Yankee force was on its way down from Pennsylvania, Longstreet's two divisions had left Boonsboro for Hagerstown to meet them, leaving only D. H. Hill's five-brigade division of battle-weary men to defend South Mountain. Theoretically, all McClellan had to do was whip Hill's exhausted troops, push his army over the mountain to Boonsboro, make short work of Longstreet, and send a contingent to relieve Harpers Ferry and defeat Jackson. A major victory was within his grasp.
Lee was caught off guard when he discovered that elements of the Army of the Potomac were advancing on South Mountain. He had suddenly lost the initiative and, forced to take a stand against superior numbers on ground that was far from ideal, he ordered D. H. Hill's division to establish defensive positions at Fox's and Turner's Gaps, and called Longstreet back to Boonsboro. The gaps--which were in harsh, mountainous terrain, much of it steep, rocky, and covered in thick undergrowth--were now well defended. Any ground taken there by the Yankees would be hard-won.
Still, McClellan's forces vastly outnumbered each of the scattered segments of Lee's army, and his plan was sound: to keep Lee from uniting his forces and destroy his army "in detail," one segment at a time.
The fighting commenced at 9 a.m. on September 14, when Brigadier General Jacob D. Cox's Kanawha division of the IX Corps attacked the Rebels in General Samuel Garland's brigade at Fox's Gap with such force that it drove them to flight from behind a stone wall, killing Garland himself. Cox stopped to wait for reinforcements from Reno, but they were late in coming. The fighting continued off and on throughout the day, with the Rebels, buoyed by the timely arrival of Brigadier General John Bell Hood, doggedly holding their own even as they were driven back a step at a time. When darkness fell, Reno rode to the summit in frustration to see about the holdup and was shot from his saddle and killed--a major loss for the Union army.
Meanwhile, Burnside decided not to risk an all-out assault on Hill's position at Turner's Gap until Hooker's corps arrived. Hooker, however, didn't make it there until 4 in the afternoon-seven hours after the battle had begun--giving Longstreet time to return from Hagerstown and support Hill's beleaguered forces. Eventually, although the Yankees had driven the determined Rebels from the summit, nightfall prevented them from taking the gap. By this time, the Rebels had reinforced both passes, and although badly battered, with the coming of dark they still held Fox's and Turner's Gaps.
Earlier in the day, Franklin's VI Corps, consisting of nearly 13,000 men, set out to capture Crampton's Gap six miles to the south. Given the urgency of the situation, and with Harpers Ferry in imminent danger of falling to the enemy, McClellan should have ordered Franklin to break camp the night before. Barring that, Franklin might well have seized the initiative himself and not waited till morning to pursue his mission. Instead, he let his men get a good night's sleep, losing a crucial 11 hours.
To protect his rear, McLaws had left a small force behind at Brownsville and Crampton's Gaps. It consisted of an artillery battery, three regiments of infantry, an undersized detachment of cavalry, and a single brigade under Brigadier General Howell Cobb--2,100 men in all.
Franklin's VI Corp finally appeared at Crampton's Gap around noon, to a greeting of Rebel artillery fire. Assuming that he faced a large enemy force, Franklin set about making elaborate plans for battle, despite outnumbering the Rebels six to one. Three hours later, he assaulted the enemy position. The fighting was fierce, with the Rebels giving ground only in the face of overwhelming numbers. Finally, the Confederate infantry was driven from the summit, by which time it was dusk.
Stretching out before Franklin's IX Corps lay Pleasant Valley; just beyond it was Maryland Heights, and McLaws's small contingent. Again taking a page from McClellan's book, Franklin convinced himself that McLaws' small force outnumbered his own and halted his advance.
The Rebels at Crampton's Gap had held out for three grueling hours which, combined with Franklin's initial delay in leaving camp, doomed Harpers Ferry. The Union garrison surrendered the next morning. Worse, by failing to attack McLaws as ordered, Franklin gave Jackson and McLaws time to rejoin Lee's army.
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When Lee was first informed that Cramptons Gap had fallen, he ordered Fox's and Turner's Gaps abandoned, intending to lead his men back into Virginia. But word soon reached him of the surrender of Harpers Ferry, and--with the reunification of his army--he instead determined to confront the Army of the Potomac in open battle a short distance from South Mountain.
The Battle of South Mountain is generally viewed as a tactical Union victory. Late on September 14, Lee himself stated matter-of-factly, "The day has gone against us." It was, however, a strategic disaster for McClellan. Through a series of inexcusable delays, the Army of the Potomac's commanders--having squandered so much precious time--failed to follow up their success in the South Mountain passes with a decisive move against Lee.
Historians have long debated the extent of McClellan's culpability in allowing Lee's army to escape and regroup. "The blame ... cannot rest entirely on McClellan's shoulders," John David Hoptak has observed. "It was his subordinates ... who let him down." Some responsibility certainly lies with Franklin. Some also rests with Burnside, who wasted many hours on the 14th awaiting the arrival of Hooker's division. And by delaying the mobilization of the IX Corps the following day, he lost the chance to pursue and destroy Longstreet's and D. H. Hill's retreating columns. McClellan, as Timothy Reese succinctly points out, "was not well served by his wing commanders."
Ultimately, though, the responsibility rests with McClellan. With superior forces and the intelligence in hand to bring the war to an end, he failed to take advantage of the opportunity. Catton, in his timeless Army of the Potomac Trilogy, faults McClellan for a fatal lack of urgency: "With everything in the world at stake, both for the country and for McClellan personally, why couldn't the man have taken fire just once?"
Although the Battle of South Mountain caused Lee to rethink his strategy, a far bloodier confrontation lay just ahead before Lee would abandon his Maryland Campaign. By failing to destroy the Army of Northern Virginia after South Mountain, McClellan gave Lee enough time to solidify his position and ready his forces for the major battle that would follow within days.
The Rebels would remember it as the Battle of Sharpsburg; to the people of the North, it was Antietam, and it would claim some 23,000 casualties, making it the bloodiest single day of combat in the nation's history. And once again, despite possessing superior numbers, McClellan--through an excess of caution bordering on timidity--would fail to seize a second opportunity to destroy Lee's army. The war was destined to last another two and a half years, and to tally a butcher's bill of three-quarters of a million lives. MHI)
Ron Soodalter has written for the New York Times, Military History, Wild West, Smithsonian, and other publications. He is the author of Hanging Captain Gordon: The Life and Trial of an American Slave Trader.