The Army of the Potomac fought the Army of Northern Virginia. There were about 23,100 total casualties.
Lee knew McClellan was massing to attack. Lee needed time to assemble the army he had so confidently scattered across western Maryland, and picked Sharpsburg as the assembly point. If worst came to worst, he would have a ridge to defend, and a road along that ridge to move reserves. He told his staff 'We will make our stand on these hills'.
His left was loosely anchored by batteries on Nicodemus Heights, only a small bump but one that was in easy cannon range of the Potomac and the main ridge. To support the otherwise isolated batteries there was a cavalry brigade, and JEB Stuart himself. Then on the main ridge, around Miller's farm, were a jumble of men from Longstreet's and Jackson's Corps. Next in line was D.H. Hill's division, in a slightly sunken road, then the line trailed off, with a few men scattered to hold the center (but backed up by the artillery reserve) and pickets dug in overlooking a stone bridge on his left. (There were few entrenchments, even rifle pits. The troops were tired, but they also still thought of fortifications as cowardly.)
McClellan had about twice as many men as Lee could muster. He had two strong Corps (I and XII, Hooker and Mansfield) on his right, II (Sumner), V (Porter), and the artillery reserve linked the center with Burnside's IX Corps on the left. The Cavalry Division was in reserve in the right center, while Franklin's VI Corps and one division of IV Corps were available in the left center. (Franklin never engaged, hardly even moved all day.)
But McClellan's command structure was hardly ideal. First, it started with Little Mac: he was not a dynamic battlefield commander. He had an overwhelming numerical advantage if he'd thrown all his men forward, but he never did. Second, Ambrose Burnside caused some problems. To back up a moment, as the Army of the Potomac grew in size there were too many Corps, so they were grouped in two-Corps 'Grand Divisions'. The idea was that there would be three (or so) Grand Divisions, much easier for Army commander to handle than 6-8 Corps, artillery, and cavalry. But things had broken down. Burnside was supposed to superintend IX and XII Corps - but these were on opposite ends of the battlefield. He gave up on XII Corps, but his deputy at IX Corps was a stickler for orders, and made sure that Burnside saw every order of McClellan's - no matter the delay. In contrast Lee had a flexible command structure, and moved divisions with ease and sometimes detached individual brigades.
The battlefield was mostly open farmland. Some fields were in stubble after the harvest, some corn was still standing. There were moderately dense woods on the northern end of the Confederate line, and overlooking the bridge at the southern end. The Antietam stream was not much of an obstacle, 30-40 feet across and only a few feet deep - especially since there was a dry spell. There were two bridges, one in the Union center and one at the southern end, one ford that was well known in the Union right center, and rumors of others, especially beyond the southern bridge. The Confederate ridge was not high nor steep, just a swale of ground that gave some protection and some defensive advantage. One of its key features was a road, running the length of the Confederate line. It was especially important since McLaws' division would be marching up in the morning. There were a few bumps of higher ground on the northern end of the ridge, only enough to deploy a battery of artillery or so, but there was a spur out from the Dunker Chapel towards the Antietam creek. It had a small plateau on top, and a farm lane that had eroded down a bit, no more than 4 feet or so at the most.
Opening stages of the battle: the Union right wing Hooker's attack opened close behind the dawn. His three divisions were in line-abreast, shoving the Confederates back by weight of numbers. The As the Federals marched toward Miller's Cornfield, the Confederates rose up in the cornfield and blasted the advancing lines. Hooker saw what was happening, halted his infantry and deployed six batteries, three dozen guns, on a slight rise just to the north. After a few minutes with shell and canister, 'In the time I am writing,' Hooker reported, 'every stalk of corn in the northern and greater part of the field was cut as closely as could have been done with a knife, and the slain lay in rows precisely as they had stood in their ranks a few moments before.' That was enough for the advance to resume, but hardly enough for victory. Once the Yankees were through the cornfield, the Confederates opened up again. Veterans to a man, they fought fiercely and clawed down Hooker's troops but, as Jackson reported, his men were 'exposed for near an hour to a terrific storm of shell, canister, and musketry.' The Confederate left was saved only by the flanking fire poured out by the batteries on Nicodemus Hill; Hooker's guns were deployed in his center and the infantry were tender about their open flank. Despite this, despite the heavy losses inflicted, Jackson's line was about to snap. Union infantry could see their objective, the Dunker Chapel and the road junction; it was in their grasp.
From one side Major Rufus R. Dawes of the 6th Wisconsin Volunteers (part of the Iron Brigade) saw 'At the front [south] edge of the corn-field was a low rail fence. Before the corn were open fields, beyond which was the Dunkard church. As we appeared at the edge of the corn, a long line of men in butternut and gray rose up from the ground. Simultaneously, the hostile battle lines opened a tremendous fire upon each other. Men ... were knocked out of the ranks by dozens. But we jumped over the fence, and pushed on, loading, firing, and shouting as we advanced. There was ... eagerness to go forward, and a reckless disregard of life, of everything but victory. . . . 'The Fourteenth Brooklyn Regiment, red legged Zouaves, came into our line. . . . Men and officers of New York and Wisconsin are fused into a common mass, in the frantic struggle to shoot fast. Every body tears cartridges, loads, passes guns, or shoots. Men are falling in their places or running back into the corn. The soldier who is shooting is furious in his energy. The soldier who is shot looks around for help with an imploring agony of death on his face. ... The men are loading and firing with demoniacal fury and shouting and laughing hysterically, and the whole field before us is covered with rebels fleeing for life, into the woods.'
On the opposite side Private Isaac G. Bradwell of the 31st Georgia Infantry remembered 'On they came, crashing down the rank growth of corn, while Hardaway's Battery in rear of our line on a little hill mowed them down with grape and canister, and Stuart's light battery enfiladed their ranks. Wide gaps were torn in the blue lines, but they continued to come on until they reached the fence and began to get over it in great disorder. This was the signal for the Confederates to open. The volley made them stagger and hesitate, but the second line came up, and, despite the fire of the Confederates, they came over and advanced slowly, step by step, and finally halted only a few feet in front of the Confederates, where they kept up the fight for a short while and began gradually to fall back to the fence. ...'
Then the Confederates snatched it back. John Hood's small division, only two brigades, was called away from breakfast (it would have been their first hot meal in days) and launched in a counterattack. They were alone, for every formation Jackson had started with was smashed. Many commanders were down; survivors were dazed. But Hood's men were enough. The Union infantry were already shaken, and this was too much. The grey columns swept through the cornfield, then formed line on the north side and dueled with the Union remnants and their gunline. Not in direct contact with Hood but attacking nonetheless were Early (on the western end) and two of DH Hill's brigades. The three strokes were too much for Hooker's men - Meade's now, since Hooker was wounded - who fell back and consolidated. Their losses had been high, but the greatest damage was to their cohesion. They outnumbered Jackson's torn survivors, but they wouldn't make another attack that day.
McClellan had planned a simultaneous attack, and the second Corps in the north was Joseph Mansfield's XII Corps. They had advanced about the time Hooker started, but they were to Hooker's left rear, and by the time they reached the field Hood's counterattack had finished off I Corps. Mansfield himself went down before there was any real action, as he was surveying the situation and trying to sort out Union from Confederate infantry in the smoke and confusion. His successor, Alpheus Williams, put the two divisions in side-by-side and swept away Hood's remnants. (Hood's men lost a battleflag, their first.) Following Hood back, they too had the Dunker Church in their grasp. And another Confederate counter-attack snatched it back. Lee had switched JG Walker's small division from the right to the left, gambling that McLaws would arrive before anything serious developed on the southern end of the line. They saved the moment in the north, and checked XII Corps' advance. Less than 200 yards apart, the opposing lines fired lead into each other for a half hour. 'They stood and shot each other, until the lines melted away like wax,' reported Isaac Hall, a New York soldier. Fighting continued back and forth over the 20-acre cornfield, with the field changing hands perhaps fifteen times. Still, Walker was too weak, however, to push the Union troops back, and Williams' men consolidated their position. Williams sent word that reinforcements would clinch the victory. Sumner was bringing those reinforcements, leading Sedgwick's 2nd Division of his Corps, the strongest in the Army of the Potomac. He was anxious to get into action, and he kept his men in their marching columns as long as he could to speed them into action. He knew how fragile the Confederate line was: Walker's men were wavering and a new blow would crumple the whole flank. He left it a little too long. The next wave of Confederate reinforcements had arrived, the divisions of Richard Anderson and Lafayette McLaws. Anderson went to reinforce DH Hill in the Sunken Road, while McLaws deployed to bolster Walker's thinning ranks. Sumner couldn't see them, but they could see his men. It was almost too easy a target: the Union troops were caught in column rather than line of battle; they were too densely packed to advance or deploy. The only options were running or dying. Plenty - over 2,000 - died, and the rest followed Sumner's lead: 'MY God, we must get out of this' he called. The battle on the Union right was over. Six Union and four Confederate divisions had been smashed; the survivors were tending to their wounded and tending to themselves. Jackson, always aggressive, looked for ways to exploit the damage to Hooker', Williams', and Sumner's men, but found nothing - else the toll would have been even higher to no avail.
Opening stages of the battle: the Union left wing With his force split in half, Burnside stationed himself with his old IX Corps. It was organized, unlike the other Union Corps which had three divisions, into four two-brigade divisions. He got an early start on the battle, but wasted it. One division he sent to check rumors of a ford to his left; they got lost for hours. Another division explored the creek on the left. A third he held in Corps reserve. Out of eight brigades, he himself whittled the attack down to two. Still, the odds were against the South, for there were only 550 men under Robert Toombs to hold off the two brigades. Yet terrain saved the day - or at least seven hours of the day, and that was enough. Burnside stubbornly intended to storm the bridge, not ford the stream to capture the bridge. (Even if he'd crossed elsewhere, he had to have the bridge to bring over batteries and ammunition wagons.) Yet the road swung parallel to the creek for 200 yards, then suddenly turned left and headed across the 12-foot wide bridge, then left again and steeply uphill. Toombs had his men in rifle pits amidst the trees on the slope; they were nearly impossible targets, while the Union infantry exposed their flank for 200 yards to the bridge, then had to pause to cross, had to pick their way across corpses dotted on the bridge and fight uphill once they crossed. Lieutenant John W. Hudson of the 35th Massachusetts Infantry described the confusion and natural reluctance of human beings to charge into the open when they can shoot at long range: 'Not hesitating to obey an order the regimental officers moved without hesitation, and the men at once followed example. Those in the advance saw they had nothing to rush upon and overwhelm, and naturally desired, in self-defense and for the purpose of doing what service they could, to load and fire as often as any single enemy was to be seen--for nothing like an uncovered rank of rebels was discernable, or indeed, existed. 'Those of our troops not in the advance crossed somewhat upon those in front--and the whole column while on the bridge appeared like an irregular mob moving nervously, but at a snail's pace, toward the enemy.' So instead of an attack, there was a fire-fight, with the Confederates a dispersed, protected target.
But it wasn't all one way: Toombs men paid for their success. Lieutenant Theodore T. Fogle of the 2nd Georgia Infantry wrote 'our Regiment and the 20th Ga., in all amounting to not over 300 muskets held them in check for four hours and a half but we suffered badly, eight cannon just 500 yards off were pouring grape shot, shell and cannister into us and our artillery could not silence them. Our Col. (Col. Holmes) . . . was killed about half an hour before [they fell back]. ... We went into the fight with only 89 muskets and had eight officers and 35 men killed and wounded. So many of the men were shot down that the officers filled their places and loaded and fired their guns.'
Burnside's repeated attacks were repeatedly repulsed. He was no closer to crossing at noon than he'd been at 6am.
The middle of the battle: the Union center French's division of Sumner's Corps followed along behind to support Sedgwick but veered south into the center of the Confederate line, under DH Hill. The Confederates were posted along a ridge in an old sunken road separating the Roulette and Piper farms. The 800-yard-long road had been worn down over the years by heavy wagons taking grain to the nearby mill, making an ideal defensive trench for the Rebels.
At dawn five brigades of D. H. Hill's troops had guarded this lane, but two brigades had been sucked into the first counter-attack on Hooker's men. They had been chewed up by Greene's division of XII Corps, leaving only two brigades. But there was no threat just then, and the men used their time to improve their positions, piling fence rails on the north side of the road to provide additional protection from the Union forces moving as if on parade across the field to their front. About 9:30am the fighting opened. Firing from behind these improvised breastworks and sheltered in the Sunken Road, the Rebels seemed unassailable. The terrain helped doubly: not only were they protected by the depression, Union troops attacking advanced blind out of the valley and couldn't see their objective. The Confederates repelled four different Union charges against the position. 'For three hours and thirty minutes,' one Union officer wrote, 'the battle raged incessantly, without either party giving way.' Despite Anderson's arrival Sumner's men still had almost a 2:1 advantage, and repeated attacks wore the Confederates down too.
Sergeant Thomas F. Galwey of the 8th Ohio Infantry wrote about their charge: 'Forward we go over fences and through an apple orchard. Now we are close to the enemy. They rise up in the sunken lane and pour a deadly fire into us. Our men drop in every few files. The ground on which we are charging has no depression, no shelter of any kind. There is nothing to do but to advance or break into a rout. We know there is no support behind us on this side of the creek. So we go forward on the run, heads downward as if under a pelting rain.'
From 9:30 a.m. to 1 p.m., bitter fighting raged along this deeply cut lane (afterward known as Bloody Lane) as French and Richardson's division sought to drive the Southerners back. John Gordon commanded the 6th Alabama in DH Hill's division; he ordered his men to hold their fire until they couldn't miss. 'The stillness was literally oppressive, as in close order, with the commander still riding in front, this column of Union infantry moved majestically in the charge. In a few minutes they were within easy range of our rifles, and some of my impatient men asked permission to fire. 'Not yet,' I replied. 'Wait for the order.' Soon they were so close that we might have seen the eagles on their buttons; but my brave and eager boys still waited for the order. Now the front rank was within a few rods of where I stood. It would not do to wait another second, and with all my lung power I shouted 'Fire!' ' By 1 p.m. about 5,600 killed and wounded troops from both sides lay along and in front of this 800-yard lane. Anderson was one of them, and shortly after he fell his division ceased to function; some of the men fought, others didn't; nobody was in control.
Finally, seeing a weak spot in the Confederate line, the 61st and 64th New York regiments penetrated the crest of the hill at the eastern end and began firing volley after volley full length down the sunken line. Then, misinterpreting an order, a Confederate officer pulled his regiment out of the road. The remaining defenders rapidly scrambled out of the lane, over the fence, and fled through the cornfields to the south, some not stopping until they had reached the outskirts of Sharpsburg itself. More than 300 Rebels threw down their arms and surrendered on the spot. Private John Dooley of the 1st Virginia was ashamed about fleeing the Bloody Lane: 'Oh how I ran! I was afraid of being struck in the back, so I frequently turned around in running, so as to avoid if possible so disgraceful a wound.'
'Lee's army was ruined,' one of Lee's officers wrote later. 'And the end of the Confederacy was in sight.' Hill pleaded for reinforcements and guns. Batteries and individual guns were brought up and slammed their shot and shell into the Union masses forming in the small valley on the south side of the Sunken Road - now more accurately called Bloody Lane. Yet Union batteries on the rise east of the Antietam plastered the Confederate gun line. It was a race between the two artilleries, and the Union had the upper hand. Then Hill pulled together no more than 200 soldiers and personally led a counterattack. It wasn't much, and it didn't go far - hardly went anywhere. It may have played a role in persuading Sumner not to press the attack. His decision, so unlike the normally bellicose old man, may have given the Confederacy three more years of life. Yet Sumner had watched as first one, then all of his divisions were shredded. His Corps alone lost over 5,000 casualties that day, and he didn't think his men could take any more. When reinforcements (half of Franklin's Corps) came up he demanded they not attack - they were the last organized, available troops in the sector. He thought that if they failed, if they lost cohesion, if they fell back, nothing would hold his Corps, nor Williams' Corps, nor Hooker's Corps.
The end of the battle: the Union left After hours of failing to advance along the road, Burnside finally saw what to do. He took two regiments, formed them on the hill above the bridge on the east bank, and sent them racing across. This avoided the 200 yard shooting gallery, saved the sharp turn from the road to the bridge, and the men swept over.
He had crossed, the enemy were gone, but what was he going to do? First, Burnside got his entire Corps across the stream. Then he made sure that everyone had enough ammunition - many men had depleted their cartridges during the long, indecisive engagement. After almost 2 hours he was ready. He moved towards Sharpsburg, and there was nothing in his way. Toombs men could skirmish a little, but Burnside could, if he needed, deploy a full brigade to chase them away and still have seven to strike at Lee's rear. It was a sitting duck: wounded and stragglers had filled Sharpsburg, and the streets were jammed with supply and ammunition wagons. But Burnside had missed his moment. At 3:40pm AP Hill's Light Division, the last men from Harper's Ferry, turned up. They'd been delayed salvaging captured Federal property and paroling prisoners, and had to make the 17 miles in eight hours. Hill's 3,000 men swung straight from the march into action. They formed on the Union left - since some had appropriated Union overcoats, Burnside's men thought it was reinforcements on the left rather than enemies. The left buckled, and Burnside's troops were driven back to the heights near the bridge they had taken earlier. The attack across the Burnside Bridge and Hill's counterattack in the fields south of Antietam resulted in 3,470 casualties, with twice as many Union casualties (2,350) as Confederate (1,120). Longstreet later wrote, 'We were so badly crushed that at the close of the day ten thousand fresh troops could have come in and taken Lee's army and everything in it.' But again McClellan held the 20,000 men of V Corps and VI Corps in reserve, losing a second opportunity to defeat the entire Confederate army. By 5:30pm, the Battle of Antietam was over.
The next day Federal and Confederate leaders struck an informal truce, and began gathering the wounded and dying. During the evening of the 18th Lee began withdrawing his army across the Potomac River. The 17th of September, 1862, was the bloodiest day of the Civil War. Federal losses were almost 12,500, Confederate losses nearly 11,000. One in four men engaged in battle that day had fallen. Lee's ranks were thinner than ever, and McClellan could probably have won if he'd resumed the attack. Lee claimed the victory - and few would disagree that it was a victory to save his army from utter defeat at the hands of a larger force while he had his back to a river. Yet McClellan had ruined Lee's invasion of the North. Maryland was held tight in the Union, and no bushels of grain harvested in northern Virginia could equal the gallons of blood lost from the Army of Northern Virginia. Some historians believe that Lee's failure to carry the war effectively into the North caused Great Britain to postpone recognition of the Confederate government. Lincoln turned that postponement into a cancellation by issuing the Emancipation Proclamation. He was fighting the Confederacy with political weapons as well as military, and this combined diplomatic and political.
From January 1, 1863 all slaves in states still in rebellion against the United States were free. Now the war had a dual purpose: to preserve the Union and to end slavery. What's more, Europeans might have sympathy with States Rights, or want cotton, but now they had to question whether it was worth supporting slavery.