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Lincoln and McClellan met in the general’s tent after Antietam. McClellan thought the battle a “masterpiece”; Lincoln relieved him of command weeks later (Photo: Library of Congress).

From the Summer 2010 issue of MHQ

On the bloodiest day of the American Civil War, Robert E. Lee gambled, George McClellan hesitated, and Abraham Lincoln seized a chance to change history

On September 3, 1862, Robert E. Lee set in motion a chain of events that would culminate two weeks later, along Antie­tam Creek at Sharpsburg in western Maryland, in a day that would see more battlefield carnage than any other in America’s history. When identifying turning points of the Civil War, military analysts frequently point to Gettysburg and Vicksburg. Yet the Battle of Antietam (Sharpsburg, in the Confederate lexicon) resulted in more pivotal changes, across a broader spectrum of events—military, political, diplomatic, societal—than any other battle of the war. This is all the more remarkable for the fact that Antietam, if evaluated in purely military terms, was not decisive at all. Indeed, it took place by happenstance.

To Jefferson Davis in Richmond, Lee’s offensive offered a bright promise. A convincing enough battlefield victory might induce Washington to negotiate a peace, or at least persuade Europe’s powers to recognize the Confederacy and perhaps intervene. The view for Abraham Lincoln was decidedly darker. To meet Lee’s challenge Lincoln was entrusting his principal army to Maj. Gen. George B. McClellan, whom he profoundly mistrusted. Moreover, the president had a major investment in the coming battle: Only with a victory could he follow through on his intention to make the war for preserving the union also a war against slavery.

On that early September day, the game was in General Lee’s hands. He wrote President Davis, “The present seems to be the most propitious time since the commencement of the war for the Confederate Army to enter Maryland.” Since taking command of the Army of Northern Virginia three months earlier, Lee had driven McClellan’s Army of the Potomac from the gates of Richmond in the Seven Days’ Battles, whipped Maj. Gen. John Pope’s Army of Virginia in the Second Battle of Bull Run, and now had the two Yankee armies pinned behind Washington’s fortifications.

Marching into Maryland and beyond into Pennsylvania, as he intended, was Lee’s best option for holding the initiative and exploiting his recent victories. The Washington defenses were too strong to assault, and he lacked the heavy weaponry to besiege the city. Northern Virginia was stripped of food and forage, and his supply line to Richmond was tenuous. Hungry Confederates speculated that north of the Potomac River lay a land of milk and honey.

In writing to Davis, Lee expressed only modest expectations: “Still we cannot afford to be idle, and though weaker than our opponents in men and military equipments, must endeavor to harass, if we cannot destroy them.” But his real ambition, as he explained after the war, was far from modest: “[I] would have had all my troops reconcentrated on Md. side, stragglers up, men rested & I intended then to attack McClellan, hoping the best results from state of my troops & those of enemy.” Robert E. Lee recognized September 1862 as his best opportunity for a major victory, perhaps even a decisive victory if fought on Northern soil.

In Lee’s favor was Washington’s high state of disarray. “Do you know that in the opinion of our leading military men Washington is in more danger than it ever yet has been?” Lt. Charles Francis Adams Jr., 1st Massachusetts cavalry, wrote his father. “Our rulers seem to me to be crazy. The air of this city seems thick with treachery; our army seems in danger of utter demoralization…. Everything is ripe for a terrible panic.”

General McClellan, following the collapse of his Peninsula campaign against Richmond in the spring, engaged in bitter recriminations with the administration and was a hotly controversial figure in the capital. John Pope and his Army of Virginia were outgeneraled at Second Bull Run, but Pope’s task had been rendered all the more difficult by McClellan’s deliberately glacial pace in sending him reinforcements from the Army of the Potomac. The shaken condition of the two Union armies aroused angry debate in President Lincoln’s cabinet. “The thing I complain of is a criminal tardiness,” wrote Attorney General Edward Bates, “a fatuous apathy, a captious, bickering rivalry, among our commanders who seem so taken up with their quick made dignity, that they overlook the lives of their people & the necessities of their country.” A cabinet majority drew up a remonstrance announcing to the president “that, at this time, it is not safe to entrust to Major General McClellan the command of any of the armies of the United States.” But their remonstrance was rendered moot before it could be delivered. On September 2 Lincoln announced that he had appointed McClellan to command the joint armies, with the Army of Virginia being folded into the Army of the Potomac and General Pope sent off to fight the Santee Sioux in Minnesota.

The McClellan decision was as grueling as any Abraham Lincoln took during his presidency. General and president had long been at odds. Lincoln had only grudgingly acquiesced to McClellan’s Peninsula campaign, and its failure widened the breach between the two men. McClellan’s behavior toward Pope infuriated the president. He told Navy Secretary Gideon Welles that he had to have McClellan to reorganize the army and bring it out of chaos. “But there has been a design, a purpose in breaking down Pope, without regard of consequences to the country,” Lincoln explained. “It is shocking to see and know this; but there is no remedy at present. McClellan has the army with him.” That truth dominated the president’s decision. The Army of Virginia, cheered to be rid of Pope, welcomed McClellan warmly; the Army of the Potomac was equally relieved to have “Little Mac” back in command.

A particular irony in this decision cannot have escaped the president. In his White House desk was the draft of the Emancipation Proclamation, its issuance awaiting a propitious moment—a battlefield victory, say—to render a revolutionary change in the war. In his desk as well was a recent letter from General McClellan proposing a reform of Union war policy. “Neither confiscation of property, political executions of persons, territorial organization of states or forcible abolition of slavery should be contemplated for a moment,” McClellan wrote. “A declaration of radical views, especially upon slavery, will rapidly disintegrate our present Armies.” And now it was General McClellan who was entrusted with gaining that propitious moment.

As Lee led the Army of Northern Virginia across the Potomac on September 4, Britain and France watched with close interest. The Federal blockade of Southern ports was crippling the textile industries in both countries. By the summer of 1862, 80,000 English textile workers in Lancashire were jobless and 370,000 more were on half time. In France the cotton famine was equally severe, and Napoleon III directed his foreign minister to ask London “if it does not believe that the moment has arrived when the South should be recognized.” The emperor envisioned dispatching an Anglo-French war fleet to the mouth of the Mississippi to reopen the cotton trade. That was moving too fast for London, but news of Lee’s victory at Second Bull Run, on the heels of his deliverance of Richmond, caused the British prime minister, Lord Palmerston, to observe that the Yankees had suffered “a very complete smashing; and it seems not altogether unlikely that still greater disasters await them.” After another such smashing, he asked, might Britain and France “address the contending parties and recommend an arrangement upon the basis of separation?”

Mountain Maneuvers and the ‘Lost Order’
Lee’s initial objective was Frederick, Maryland, about 50 miles northwest of Washington. There he paused to provision and rest his troops and evaluate the Federal response to his offensive. Local geography shaped his movements. Running through western Maryland are three parallel, north-south ranges that he intended to use to draw McClellan out of the Washington fortifications and toward a prospective battlefield in Maryland or southern Pennsylvania. Catoctin Mountain is the easternmost of these ranges. Next come South Mountain and Elk Mountain. Beyond lies the broad Cumberland Valley, the extension of Virginia’s Shenandoah Valley. By remaining east of Catoctin Mountain, Lee threatened both Washington and Baltimore. By then shifting westward behind rugged South Mountain, he would pull the Yankees ever farther from their Washington base while drawing his own supplies through the Shenandoah Valley.

Casualties for the battle topped 22,000, with more than 3,600 dead, including these Rebels along the Hagerstown Turnpike (Photo: Library of Congress).Lee found that McClellan, as he expected, was moving after him very cautiously. (McClellan insisted to Washington that the “gigantic rebel army before us…amounting to not less than 120,000 men…are numerically superior to ours by at least twenty-five per cent.” He was, as usual, wildly multiplying Confederate strength, on this occasion by a factor of three.) But unexpectedly the Federals still held on to their Shenandoah Valley outposts of Harpers Ferry and Martinsburg, even though both were isolated by the enemy advance. For Lee it was essential that he clear the valley of these garrisons to maintain his supply line, and to do so, on September 9 he composed Special Orders No. 191.

Special Orders 191 detailed a plan to surround and snatch up Harpers Ferry and Martinsburg before McClellan realized what was happening. Maj. Gen. Thomas “Stonewall” Jackson Jr. with three divisions would come in on Harpers Ferry from the west, capturing or driving the Martinsburg garrison before him. Two divisions would seize Maryland Heights, overlooking Harpers Ferry on the north, while a sixth division took Loudoun Heights to the southeast. Lee assigned two-thirds of his army to the operation and estimated three days to carry it out. On the morning of September 10 the Army of Northern Virginia marched west out of Frederick and, utilizing the concealing bulk of South Mountain, divided into four widely separated columns.

Over the next several days, as he advanced very slowly into Maryland, McClellan was inundated with Rebel sightings at virtually every point of the compass, and he was bewildered. On September 12 he wrote his wife, “From all I can gather secesh is skedadelling & I don’t think I can catch him…. He evidently don’t want to fight me.” He earlier had urged the threatened garrisons be evacuated, but General in Chief Henry W. Halleck in Washington insisted they be held. Halleck’s decision would prove costly, but happenstance papered over his blunder.

The happenstance occurred the next morning, September 13, in a clover field on the outskirts of Frederick, where the 27th Indiana Regiment made camp. As the men stacked arms, Cpl. Barton W. Mitchell found a bulky envelope by the road. In it was a sheet of paper, closely written on both sides, wrapped around three cigars. Marked “Confidential,” the paper was headed “Hd Qrs Army of Northern Va Sept 9th 1862 Special Orders No 191.” It was signed “By Command of Gen. R. E. Lee.” Mitchell realized he had something very special indeed, and he immediately started it up the chain of command. Before noon it was in the hands of General McClellan. He read it, threw up his hands, and exclaimed, according to an eyewitness, “Now I know what to do!”

Thus the famous Lost Order, which, wrote soldier-historian Francis W. Palfrey, “placed the Army of Northern Virginia at the mercy of McClellan.” How the Lost Order was lost is not known—most likely a careless courier and a careless headquarters staff failing to monitor the delivery of orders. McClellan, for his part, clearly recognized the importance of the find. At noon that day he excitedly telegraphed Lincoln, “I think Lee has made a gross mistake and that he will be severely punished for it…. I have all the plans of the Rebels and will catch them in their own trap….Will send you trophies.”

From Special Orders 191 McClellan learned the objectives, the timetables, and the makeup of the four columns of Lee’s army. His good fortune was greater than he knew, for Lee had since divided his forces further, taking two divisions with him to Hagerstown, near the Pennsylvania border. The Army of the Potomac, by promptly marching across South Mountain, would be poised to divide and conquer five of the nine Rebel infantry divisions: one at Boonsboro, just across the mountain; two on Maryland Heights, overlooking Harpers Ferry; and the two with Lee at Hagerstown.

The game was now in George McClellan’s hands…and he promptly fumbled. Rather than order his army up to the foot of South Mountain that afternoon, primed to rush the passes at first light the next morning, he frittered the day away. He lingered over intelligence sightings not explained by Special Orders 191—particularly the report of Confederates in force at Hagerstown. The enemy’s vast numbers as McClellan imagined them further excited his native caution. Eighteen hours would pass before the discovery of the Lost Order spurred him to act.

As it happened, 18 hours was just enough time for General Lee to evade outright disaster. While Lee had no inkling that the enemy knew his orders, he did receive a hint that the Federals were, as he later wrote, “advancing more rapidly than was convenient.” When McClellan was handed the Lost Order, he was meeting with a group of Maryland citizens regarding the Federal occupation of Frederick. One of these Marylanders was a Confederate spy who took note of McClellan’s excitement and reported the incident to Lee’s cavalry commander, Maj. Gen. Jeb Stuart. Stuart in turn alerted Lee at Hagerstown that the Yankees seemed to be up to something. As a precaution, Lee determined to start closing up his scattered forces. He ordered the two divisions with him at Hagerstown, under Maj. Gen. James Long­street, to join the division left at Boonsboro, near South Mountain.
But Lee—like McClellan—concluded that the next day would be soon enough to take action.

Lee Bluffs, McClellan Hesitates
September 14 was a day of hairsbreadth escapes for the Rebels. McClellan’s columns finally closed up to the South Mountain passes and late in the day broke through after sharp, bloody fighting. Longstreet’s reinforcements arrived only in time to prevent a complete Confederate rout. Even though Stonewall Jackson had meanwhile closed the ring on Harpers Ferry, Lee felt obliged to give up his campaign and retreat to Virginia. As he put it to one of the generals besieging Harpers Ferry, “The day has gone against us and this army will go by Sharpsburg to cross the river.”

But a dispatch from Jackson changed Lee’s mind. “Through God’s blessing,” the pious Jackson reported, the assault on Harpers Ferry “has been successful thus far, and I look to Him for complete success to-morrow.” The promised capture of the large garrison, with its substantial stock of artillery and arms, revived Lee’s spirits and his hopes for victory on Northern soil. He and the three divisions at South Mountain would march for Sharpsburg. The two divisions on Maryland Heights overlooking Harpers Ferry were to join them. Jackson with the rest of the army would rendezvous at Sharpsburg as well, if a suitable battlefield presented itself.

In many respects this was daring that edged close to foolhardy. Confederate soldier–historian Edward Porter Alexander observed that at Sharpsburg Lee “fought where he could have avoided it, & where he had nothing to make & everything to lose—which a general should not do.” But for Robert E. Lee the compensating factor was the opponent he was facing. He described General McClellan as “very cautious,” even “timid.” Having violently expelled McClellan from the gates of Richmond in the spring, he now welcomed the chance to do battle against him again, whatever the odds.

McClellan indeed proved cautious to a fault. Shortly after noon on September 15 a Federal observation post reported, “A line of battle—or an arrangement of troops which looks very much like it—is formed on the other side of Antietam creek and this side of Sharpsburg.” The enemy, it seemed, had shrugged off its South Mountain defeat and turned defiantly to offer battle. A report that the Harpers Ferry garrison surrendered that morning—some 12,500 men, 13,000 small arms, 73 cannons, and abundant supplies—was received soberly at Army of the Potomac headquarters. McClellan spent the rest of that day as well as all of the 16th drawing in his forces and pondering the situation and getting everything ready. He reasoned that the “gigantic rebel army,” having swept up Harpers Ferry, must be reunited and as formidable as ever. He set September 17 as the day of battle.

In reality, General Lee ran a gigantic bluff on those two days. He had hardly 15,000 men at Sharpsburg. Only at midday on the 16th did the first of Jackson’s troops reach Lee, and they trailed slowly into the lines through the afternoon, leaving one division behind to carry through the Harpers Ferry surrender. The two divisions from Maryland Heights would not reach Sharpsburg until the next morning.

General Lee chose the battlefield of September 17 and thereby shaped the fight. It was neither the battle he had intended nor where he intended it, and because of the Lost Order he fought before he was ready. Still, with his skilled engineer’s eye he picked good ground on which to fight defensively. Lee had confidence in his generals and in his troops (and they in him). Furthermore, he had confidence facing General McClellan as his opponent.

Lee posted his forces in a north-south line some four miles long behind Antie­tam Creek and paralleling the Hagers­town Turnpike that led into Sharpsburg from the north. Two woodlots on opposite sides of the turnpike would feature prominently in the battle: the West Woods, on the Confederate side, and the East Woods, on the Union side. The ground to the north, Lee’s left, was a checkerboard of woods and fenced fields, undulating and marked by numerous rock outcroppings. South of Sharpsburg the terrain tended to be steep and broken and more difficult for military movements. Stonewall Jackson commanded on the left, James Longstreet on the right.

The only practical crossings of Antietam Creek for artillery and wagons were three stone bridges—from north to south, the Upper Bridge, the Middle Bridge, and the Rohrbach Bridge. Three miles to the west lay the Potomac and Boteler’s Ford, Lee’s sole line of retreat to Virginia should the battle go against him. Lee had available to him fewer than 40,000 men, and 200 guns. McClellan’s army was more than twice that size, with 300 guns.

Antietam was the first and only battle George McClellan ever planned and personally directed. He neither issued a battle plan to his lieutenants nor called them into council to explain his intentions; he commanded that day entirely by circumstance. He commanded as well with obsessive caution, holding back, as it were, one fist to parry counterstrokes by an enemy he believed considerably superior in numbers.

McClellan had six army corps (17 infantry divisions) to work with on September 17. He selected Maj. Gen. Joseph Hooker’s I Corps to lead the opening assault on the northern sector of the field against Lee’s left flank. Maj. Gen. Joseph K. F. Mansfield’s XII Corps would support Hooker. These two corps crossed the Upper Bridge late on September 16 and took position to launch the morning attack. Maj. Gen. Edwin V. Sumner’s II Corps was to cross the Antietam to reinforce Hooker and Mansfield when circumstances required. At the center the V Corps of Maj. Gen. Fitz John Porter would act as the army’s reserve, backed up in due course by Maj. Gen. William Franklin’s late-arriving VI Corps. At the southern end of the field, Maj. Gen. Ambrose Burnside’s IX Corps had the task of fighting its way across the Rohrbach Bridge to strike the enemy’s right, either as a diversion or as a full-fledged attack; McClellan did not make the distinction clear.

A Bloody Beginning
September 17 dawned foggy and misty from an overnight rain, but as soon as it cleared “Fighting Joe” Hooker directed his troops southward along the Hagerstown Turnpike. Brig. Gen. James B. Ricketts’s division had the left, Brig. Gen. Abner Doubleday’s the right, and Maj. Gen. George Meade’s was in reserve. Their objective was a mile distant, an open plateau crowded with Confederate batteries and marked by the small whitewashed church of the German Baptist Brethren, whose baptism by immersion led people to call them Dunkers. By moving this flanking column into position the previous day, McClellan lost any advantage of surprise. Stonewall Jackson redrew his lines to face north, in two-division strength. A potentially sharpened flank attack became a blunt frontal attack.

Ricketts sent his three brigades, at intervals, through the East Woods and the 30-acre cornfield of farmer David R. Miller. A New Yorker burst out of the woods and found “[j]ust in front of us a house was burning and the fire and smoke, flashing of muskets and whizzing of bullets, yells of men, etc., were perfectly horrible.” The corn in Mr. Miller’s field—soon to earn grim fame as the Cornfield—was head-high and concealing. When a Yankee line of battle emerged abruptly into the open it was scarcely 200 yards from the Rebel defenders. “Never did I see more rebs to fire at than at that moment presented themselves,” a Federal remembered, and the opposing lines shot each other to pieces. Reinforcements crowded in from both sides, and the artillery of both sides added to the carnage. The Louisiana Tigers brigade from New Orleans limped out of the fighting with 61 percent casualties and its five regimental commanders dead or wounded. Battling the Tigers, the 12th Massachusetts lost 224 of its 334 men, or 67 percent, the highest casualty rate in the Army of the Potomac that day.

As Ricketts’s attack stalled, Abner Doubleday’s division took up the fight, advancing astride the Hagerstown Turnpike with its three brigades well closed up. As Doubleday’s surge gained footholds in the West Woods and beyond the Cornfield, the defending division of Brig. Gen. William E. Starke countered aggressively. Starke’s battle line lying concealed in the pasture south of the Cornfield rose up suddenly and delivered a murderous volley. “Men, I can not say fell,” wrote Maj. Rufus Dawes of the 6th Wisconsin; “they were knocked out of the ranks by dozens. But we jumped over the fence, and pushed on, loading, firing, and shouting as we advanced.” General Starke now personally led a counterattack that drove back the Yankee advance. The firing here was virtually point-blank, the lines barely 30 yards apart, and Starke was mortally wounded. With his fall the counterattack crested and fell back.

General McClellan observed the fighting from the Union center overlooking the Antietam. He was watchful but waiting. He did not order Mansfield’s XII Corps to Hooker’s immediate support, nor was Sumner’s II Corps started across the creek for the fighting front two miles distant. No orders went to Burnside to move against the enemy’s opposite flank.
Lee, by contrast, responded quickly to the Federal offensive. The two divisions from Maryland Heights, under Maj. Gens. Lafayette McLaws and Richard Anderson, had reached the field after a hard night’s march and were readied for action. Maj. Gen. A. P. Hill’s division, left behind at Harpers Ferry to process the captures there, was ordered to Sharpsburg with all possible speed. Finding it quiet on his right, Lee shifted three of Longstreet’s brigades to meet the threat from the north, and he put another of Longstreet’s formations, Brig. Gen. John Bell Hood’s division, at the call of Stonewall Jackson if needed.

Hood was needed. Marching out of the West Woods, his two brigades formed a line of battle from the turnpike and the Cornfield across to the East Woods and knocked the Yankees back on their heels. “In ten minutes the fortune of the day seemed to have changed,” wrote a Northern newspaperman. Hooker threw in his last reserves, from George Meade’s division, but the Confederate tide reached all the way to the northern edge of the Cornfield before it was finally stemmed, in large part by Federal batteries firing double charges of deadly canister. (Afterward, when General Hood was asked for the location of his division, he replied simply, “Dead on the field.”) Over the course of some 90 minutes Jackson’s men and Hooker’s had engaged in a frenzy of mutual destruction.

At last, General Mansfield was ordered up to reinforce Hooker’s decimated corps. Mansfield, a veteran staff officer who had long sought a field command, was in his second day as head of the XII Corps. The corps was a patchwork, containing a half-dozen raw regiments fresh from home; perhaps half had never fired their rifles in anger. As they advanced into the Cornfield and the East Woods there was much difficulty getting them from marching order into line of battle. Mansfield, personally prodding the rookies into position, took a bullet in the chest, a mortal wound. Soon after, Joe Hooker was wounded and left the field.

Lee had dispatched Lafayette McLaws’s division to meet this renewed assault, but until McLaws arrived the defense fell to Maj. Gen. D. H. Hill’s outmanned force from the Confederate center. The fighting grew rapidly in intensity. “The roar of the infantry was beyond anything conceivable to the uninitiated,” wrote Alpheus Williams, a Union brigadier general, in a letter home. “If all the stone and brick houses of Broadway should tumble at once the roar and rattle could hardly be greater, and amidst this, hundreds of pieces of artillery, right and left, were thundering as a sort of bass to the infernal music.” The XII Corps drove Hill’s brigades back almost to the Dunker church, forcing the Rebel batteries there to withdraw. The first objective of the Yankee offensive seemed within reach. A message from General Williams at the front reached McClellan: “Genl Mansfield is dangerously wounded. Genl Hooker wounded severely in foot. Genl Sumner I hear is advancing. We hold the field at present. Please give us all the aid you can.”

When McClellan at last committed Edwin Sumner’s II Corps to the fight—its 15,200 men made it the largest corps in the Army of the Potomac—he continued his too-little, too-late pattern of generalship. Only Maj. Gen. John Sedgwick’s lead division of the II Corps joined the immediate fighting. Brig. Gen. William French’s division started late and lost its direction, and Maj. Gen. Israel Richardson’s division was held back as insurance against a counterattack.

Just then General Sumner was leading from up front and oblivious to such matters. He set Sedgwick’s three brigades to marching due west across the smoldering battlefield, arrayed in three lines, 50 yards apart and 500 yards wide. He intended to cross the West Woods and then wheel left to cut in behind the Rebels. Instead he marched into an ambush. McLaws’s division, along with other troops Lee had called up from Longstreet’s front, smashed into the exposed flank of Sedgwick’s advance. The Yankees had neither time nor space to turn to meet the assault; regiments toppled like dominoes. “We were completely flanked on the left,” wrote a man in the 59th New York. “My men fell around me like dead flies on a frosty morning.”

In minutes Sedgwick’s division lost 40 percent of its 5,400 men and was driven off well to the north. This repulse ended major fighting in the northern sector of the battlefield. By putting the three corps “into that action in driblets,” as General Sumner complained, McClellan had allowed the outnumbered Stonewall Jackson to successfully counter—at terrible cost, to be sure—each stage of the Federals’ early-morning offensive.

‘The Grandest Battle’ and the Sunken Road
At midmorning, by chance rather than design, the battle shifted to the center of the Confederate line. A farm road here, worn down by heavy use and erosion, ran several feet below ground level, and D. H. Hill’s men had enhanced its natural defensive strength with a rough breastwork of fence rails. It would be called the Sunken Road, or more descriptively, Bloody Lane. William French’s division of the II Corps, having earlier strayed from Sumner’s advance, took the Sunken Road as its objective. Without subtlety, French threw his three brigades headlong against this position. The colonel of the 30th North Caro­lina testified that his regiment’s volleys “brought down the enemy as grain falls before a reaper.” There were seven raw regiments in French’s division that had no idea what they were supposed to do; many had yet to master the drill for loading their muskets. One brigade of new troops lost 529 men in its fruitless charge.

General Lee committed the last of his reserves, Richard Anderson’s division, to the struggle for the Sunken Road. The last of the Union’s II Corps divisions, Israel Richardson’s, reinforced the attackers, but still Lee’s center held firm. Then, abruptly, the battle turned. An order to realign the defending Confederate troops was misunderstood and several regiments abandoned their positions, triggering a panic. The Federals quickly overran the Sunken Road. “In this road lay so many dead rebels that they formed a line which one might have walked upon as far as I could see,” a New Hampshire soldier wrote.

It was 1 p.m. now and Lee’s center was reduced to a shadow. A patched-together gun line in front of the Hagerstown Turnpike consisted of some 20 pieces supported by a few hundred disorganized infantry. “It was easy to see that if the Federals broke through our line there, the Confederate army would be cut in two and probably destroyed,” wrote General Longstreet.

At the headquarters of Fitz John Porter’s V Corps, in reserve at the Union center, General McClellan witnessed the Sunken Road fighting scarcely a mile away. As the Federals broke through he exclaimed, “It is the most beautiful field I ever saw, and the grandest battle!” Yet he perceived no opportunity opening before him. The morning’s offensive toward the Dunker church seemed stalemated. He regarded Porter’s V Corps as his only reserve, although William Franklin’s VI Corps was now reaching the field. There was still, he believed, the enemy’s considerable advantage in numbers to be wary of. “Thus far it looks well, but I have great odds against me,” he telegraphed Washington. He decided his best course was to consolidate his position and await events. To the conquerors of the Sunken Road went the order, “Now, men, this place must be held at all hazards!”

McClellan was meanwhile scanning the field to the south with growing anxiety. At 10 a.m. his order had reached Ambrose Burnside to put the IX Corps into action against the Confederate right. If this was intended as a diversion, it came much too late. Lee had already stripped his right flank of troops to defend against the attack on his left. If intended to open a second front, the order was not marked with urgency. In any case, Burnside’s task was not an easy one. The Rohrbach Bridge on his front was the only practicable crossing for artillery and wagons, and it was well defended. On the bluff overlooking the bridge, two Georgia regiments had turned a stone wall and an old quarry into a bastion. Farther up the heights, five batteries commanded the bridge and its approaches.

It took Burnside three tries and 500 casualties to force his way across the bridge. Finally a bold rush supported by heavy covering fire succeeded in driving off the Georgian defenders. As Burnside pushed reinforcements across the creek, McClellan grew frantic in his demands to open a new front south of Sharpsburg. An advance “with the utmost vigor” was vital, he said, “a time when we must not stop for loss of life, if a great object could thereby be accomplished.” That “great object” was to tie down Lee’s hidden battalions and so foreclose any counter assault elsewhere on the field.

Once Burnside gained his bridgehead, McClellan visited the northern sector of the field with the thought of renewing the offensive there. He did so at the urging of William Franklin, whose VI Corps, combined with Porter’s V Corps, meant 20,500 fresh troops to restart the fight. But Edwin Sumner, according to McClellan “expressed the most decided opinion against another attempt during that day.” Clearly Sumner was demoralized by the trials of his II Corps and unfit for further command, but McClellan’s caution overrode his flicker of aggressiveness. He bowed to Sumner and told everyone to hold his position.

It was midafternoon before Burnside began his advance. His battle line was 5,500 strong, with plentiful reserves. Brig. Gen. David R. Jones of Longstreet’s command had about half that number to defend the Confederate right and could fight at best a delaying action. Lee had no reserves left to support him. Unless Burnside was stopped, he would cut Lee’s escape route to the Potomac crossing and trap the Rebel army.

Rugged terrain and stubborn defenders slowed Burnside but did not stop him, and the IX Corps soon reached the outskirts of Sharpsburg. Then, seemingly out of nowhere, the Federals’ flank was struck and smashed in. McClellan had failed to take the fundamental precaution of guarding his flanks with cavalry vedettes, and A. P. Hill’s division, which had marched at a killing pace the 17 miles from Harpers Ferry, caught the Federals by surprise. Burnside’s advance became a retreat all the way back to the bridgehead on the Antietam.

Give Chase?
It was dusk now, and the setting sun cast a blood-red light across the smoking, blasted fields and woodlots. “The flashes are fewer,” a correspondent wrote. “The musketry ceases and silence comes on, broken only by an occasional volley, and single shots, like the last drops of a shower.” The costliest day of the war was at last at an end. By the best estimates, the Union dead, wounded, and missing came to 12,401. The Confederate toll was 10,316. Of the 22,717 total, at least 3,654 were killed. For all that, remarkably little had changed. The Federals had a foothold across what would now be known as Burnside’s Bridge and occupied a few bloodied acres in front of the Cornfield and the Sunken Road. But the Rebels appeared as dangerous and defiant at nightfall as they had at dawn.

And they were still there on September 18, looking just as dangerous and defiant. Running the same bluff he had since he took his stand at Sharpsburg, Lee challenged McClellan to resume the battle. Holding his lines was in part a practical matter—Lee knew he must return to Virginia, but he wanted it to be orderly, carrying off his wounded and not panicking the troops. It was as well a matter of pride and a sure reading of his opponent. “I have always been proud of the fact that Gen. Lee did dare to stand & defy McClellan on the 18th,” Edward Porter Alexander would write. “It…showed his audacity as a commander, & his supreme confidence in his army.”

McClellan wrote that after a “careful survey of the situation and condition of our army, the strength and position of the enemy, I concluded that the success of an attack on the 18th was not certain.” Two additional divisions reached him by that morning; combining them with the V and VI corps, mostly held in reserve on the 17th, gave him 32,000 fresh troops for offensive operations. The I, II, and XII corps were battered but easily capable of holding their lines in support of an attack. Burnside’s IX Corps, more surprised than hurt by its reverse, was well placed to resume its advance. But General McClellan saw only that in battle against a superior foe he had held his own and gained a standoff. He would settle for that. The extent of his delusion is apparent in the letter he wrote his wife that morning: “I hope that God has given us a great success. It is all in his hands, where I am content to leave it. The spectacle yesterday was the grandest I could conceive of—nothing could be more sublime. Those in whose judgment I rely tell me that I fought the battle splendidly & that it was a masterpiece of art.”

By morning on September 19 the Army of Northern Virginia was gone. After a hapless effort at pursuit, McClellan settled his army into a long period of rest and refitting. Despite repeated prodding from Washington, he did not cross the Potomac into Virginia until late October and revealed only indefinite campaign plans. Lincoln’s patience was exhausted; he had, he said, “tried long enough to bore with an auger too dull to take hold.” On November 5 he issued the order relieving McClellan of command of the Army of the Potomac. Ambrose Burnside replaced him.

However frustrating the aftermath, the fact remained that Antietam had forced Lee to give up his offensive—and that in itself was enough for Lincoln. On September 22 he called a special cabinet meeting to announce, as Navy Secretary Gideon Welles entered in his diary, “that he had made a vow, a covenant, that if God gave us the victory in the approaching battle, he would consider it an indication of Divine will, and that it was his duty to move forward in the cause of emancipation.” The Emancipation Proclamation, drafted back in July, was given life by the Battle of Antietam, and the meaning and the course of the war changed thereby.

The echoes of Antietam reached as well across the Atlantic. Upon receiving news of the battle, Prime Minister Palmerston took notice that suddenly the case for intervention “is full of difficulty.” Emancipation closed the case entirely. Lincoln earlier had predicted that Europe would not dare to recognize a Confederacy that stood for slavery if the Union clearly stood for freedom, and his prediction was borne out.

In the last analysis, Antietam easily ranks as the Union’s greatest missed opportunity of the war. With an army twice the size of his opponent’s, with the intelligence coup of the Lost Order, with the Rebels’ escape route problematical, George McClellan missed repeated chances to wreck the Army of Northern Virginia—and to wreck it beyond repair. Soldier-historian Ezra Carman, author of the most detailed tactical study of the battle, wrote that at Antietam “more errors were committed by the Union commander than in any other battle of the war.” Colonel Carman had it exactly right.

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