Shielding the Confederate army from the Union army was South Mountain. There were four passes through the mountain range in the immediate vicinity of the two armies. Turner's Gap and Fox's Gap lay almost on a straight line between the opposing armies. Crampton's Gap and Brownsville Gap were further south, in the direction of Harper's Ferry.
Franklin's corps advanced on Crampton's Gap beginning at 6 a.m. on September 14. They paused mid morning at Jefferson, Maryland for the division of Maj. Gen. Darius Couch. Couch's division was with the Fourth Corps, but was attached to Franklin. It didn't show up. Franklin pushed on, arriving at Burkittsville — at the entrance to Crampton's Gap — by noon. His six Union brigades encountered only elements of two Confederate brigades, one consisting of dismounted cavalry.
Four Confederate regiments under Col. William Parham stood behind a stone wall at the centre of the gap with some cavalry of Brig. Gen. Beverly Holcombe Robertson's brigade (though Holcombe wasn't present) on either flank. The entire Confederate force was under Col. Thomas Taylor Munford of Robertson's brigade, McLaws' division. Munford placed Chew's Virginia Battery of horse artillery on the hill above the infantry.
Franklin was ordered to seize Crampton's Gap. Less than two miles south was the road through Brownsville Gap. Taking that road would have let Franklin outflank the Rebel force, but that would have meant showing some initiative. Besides taking Crampton's Gap, surrounding McLaws, and relieving Harper's Ferry, he was given the task of taking the town of Rohrersville, Maryland. Crampton's Gap was the quickest route to Rohrersville, so Franklin ignored Brownsville Gap entirely. Instead, he spent two hours pondering the situation before deploying the divisions of Major Generals Henry W. Slocum and William F. "Baldy" Smith. With a total of about 12,300 men, he launched his attack against a Rebel force of less than a thousand.
Union Col. Joseph Bartlett's brigade of Slocum's division led the advance on the Rebels in the centre. Col. Alfred Torbert's brigade was to Bartlett's right and Brig. Gen. John Newton's brigade was behind in support. Col. William Irwin's brigade of Smith's division advanced on the left, with Brig. Gen. W. T. H. Brooks' brigade on the extreme left and Brig. Gen. Winfield Hancock's brigade in support.
The Rebel's resistance was strong enough that Bartlett had to be reinforced by Newton's brigade against the men behind the stone wall. At 4:00 p.m., tired of the stalemate that had developed, Slocum ordered an attack, and the Federals swept the Confederates from the wall and the eastern slope of the mountain. They captured 400 prisoners and buried 150 Rebels.
Because of Lee's warning, McLaws sent the brigade of Brig. Gen. Howard Cobb to reinforce the small force on the mountain. Cobb's lead regiments arrived at the crest in time to be caught in the rush for the rear. Following behind was Maj. Gen. Lafayette McLaws himself. He had no idea what was before him; Stuart had told him that a single Yankee brigade faced Crampton's Gap. McLaws met up with the men routing off the mountain and a courier calling for reinforcements. McLaws sent for the remnants of Munford's force and threw it into a ragged battle line across Pleasant Valley, about a mile and a half south of Crampton's Gap.
Franklin saw McLaws' force and made no further significant advance, in spite of still having vastly superior numbers. He had achieved one of his four objectives and it was getting late. He felt it was too much to ask his men to push aside the weak force before them.
Franklin's losses were 531 killed, missing, and wounded. The Rebels lost about the same killed and missing, but also had the aforementioned 400 soldiers captured.
Confederate Maj. Gen. Daniel Harvey Hill arrived at Turner's Gap at dawn. When he arrived at the pass he discovered that Stuart had gone south to Crampton's Gap, where the cavalry commander thought the threat was most severe, leaving behind a 200-man cavalry regiment and a battery of horse artillery. Hill also found that Colquitt's brigade had marched part way down the eastern face of the mountain.
South Mountain was harder to defend than he first thought. The terrain was rugged and even a small force could make it difficult for a larger approaching force. The problem was the road network. The National Road ran through Turner's Gap. It branched off near the eastern base of the mountain, one branch heading north and one branch heading south. The northern branch, the Old Hagerstown Road, met the National Road at Turner's Gap, but it also branched off into a farm road that looped behind the mountain crest north of the gap. The southern branch, the Old Sharpsburg Road, crossed the mountain at Fox's Gap a little under a mile away. It, too, branched off into a farm road, this one swinging to the south before meeting the Old Sharpsburg Road again at Fox's Gap. Holding the two passes of Turner's and Fox's Gaps would mean posting troops to cover 5 roads separated by three miles of rough terrain. Hill began preparing his defense by ordering Colquitt back to the crest and ordering Samuel Garland's brigade to hurry forward. Hill ordered his other brigades to advance from Boonsboro. Reinforcements from Longstreet's corps were hours away.
After sending his orders Hill stood on a rock and looked down below him. A large Union force was preparing to cross the mountain. Hill had about 2300 men. What he saw was Maj. Gen. Jesse Reno's Ninth Corps. If it moved quickly it could push aside Hill before he could bring up the rest of his division.
Fortunately for Hill, McClellan felt that Pleasanton's cavalry division and a brigade of infantry was sufficient to push through the gaps. Along with Pleasanton's cavalry, the advance Union force consisted of Brig. Gen. Eliakim Scammon's brigade from the Kanawha Division under Brig. Gen. Jacob D. Cox. They began marching up the mountain at 6 a.m. with Cox tagging along to watch. Standing on the roadway was Col. Augustus Moor, one of Cox's brigade commanders. Moor had been captured in a cavalry clash in Frederick two days before, but he had been paroled, giving his word that he would sit out the war unless he was involved in a prisoner exchange. Moor asked where Scammon's brigade was heading. Cox told him, and Moor replied, "My God! Be careful!" Realizing the conditions of his parole, he said no more, but the warning was enough for Cox. He sent to Reno for reinforcements and moved up the rest of his division.
Cox conferred with Pleasanton and decided to take Turner's Gap by way of Fox's Gap. He moved Scammon's brigade up the Old Sharpsburg Road, but Confederate shells convinced him to swing along the farm road to the south. Behind Scammon in close support was Col. George Crook's brigade. Scammon came to the edge of a pasture near the ridgeline's crest. At the end of the pasture was a stone wall with a Rebel line of battle waiting behind it. These were the five regiments of Garland's brigade with some cavalry and a horse artillery battery for support. The Rebels were badly outnumbered by 3000 Yankees.
Scammon deployed for battle at 9 a.m. A fierce fight ensued. The Yankees pushed hard along the whole line. The Ohio Light Artillery moved up to fire canister, but the lieutenant and most of the gunners were shot by sharpshooters. Garland personally led from the front, only to be mortally wounded while directing his left flank.
Scammon ordered a bayonet charge. The inexperienced troops of the 12th North Carolina backed up then broke for the rear. The veteran units on either side tried to hold on, but a wild melee saw scores surrender as Crook's brigade joined in on both flanks. With Garland down, his brigade's morale cracked and the Rebel line broke.
Cox advanced his brigades forward. Turner's Gap was only a mile away but the Rebels there still looked ready to fight. Daniel Hill used the horse artillery and his staff to make it look like reinforcements were arriving. Cox paused. His brigades had suffered heavy casualties, particularly Scammon's brigade. The men were tired and low on ammunition. His own reinforcements were due, but where were they? It was now noon.
Reno was considered one of the better Union generals but he wasn't at his best at South Mountain. He received Cox's request for reinforcements at 8 a.m. but didn't send the division of Brig. Gen. Orlando Wilcox until 10:30 or 11. It got lost on the way and didn't arrive until 2 p.m. By then Hill had his other three brigades, those of Brigadier Generals Roswell Ripley, George B. Anderson, and Richard Rodes. Hill moved Anderson and Ripley to Fox's Gap and posted Rodes at Turner's Gap.
Cox was now awaiting the brigades of Brigadier Generals Samuel Sturgis and Isaac Rodman of Reno's Ninth Corps. Reno had sent them off behind Wilcox but they had been halted. Joseph Hooker's First Corps had marched up to support Reno and Maj. Gen. Ambrose Burnside had arrived on the scene. Burnside was in command of the wing. He halted Sturgis and Rodman to let Hooker move into place for a coordinated attack. These delays ended the Union's chance of exploiting Cox's victory and easily sweeping Hill's men off the mountain.
The Confederates were the first with further reinforcements: two of Longstreet's brigades, Brig. Gen. Thomas Drayton's and Col. George T. Anderson's (not to be confused with George B. Anderson, already on the scene). Hill moved these two brigades down to Fox's Gap. Once in place, Hill initiated a counterattack with the four brigades there. Ripley was the senior brigade commander so he was in charge, but he apparently got lost in the laurel thickets and his brigade missed the action. Confusion ensued. The rest of the attack went off piecemeal. Heavy fighting swept back and forth through most of the afternoon.
Hooker's troops were shunted off to the north to attack Turner's Gap from the direction of the Old Hagerstown Road. The division of Brig. Gen. John P. Hatch was on the left, and Brig. Gen. George G. Meade's division was on the right. Behind in support was the division of Brig. Gen. John B. Ricketts. At 4 p.m. Hooker's troops began their coordinated assault.
At first Rodes brigade slowed down Hatch's men with a long skirmish line moving from tree to tree. Eventually the Rebels were cleared out and Hatch approached an open field. The Union attack was well coordinated and relentless. As Hatch pushed against Rodes' 1200 Alabamians, Meade tried to swing north around Rodes' flank. Rodes had to continuously extend his line, thinning it out.
For four hours Rodes kept up the fight alone. The 6th Alabama regiment under Col. John B. Gordon bore the brunt of Meade's attack by itself. Firing down from mountain crest, many of the Rebel shots were fatal head shots. This didn't stop Hatch or Meade. Hatch's troops climbed a ravine between two hill spurs where they were counterattacked by Rodes. A viscous fight for a cornfield fence resulted in Hatch falling wounded. Eventually Rodes was pushed back with heavy casualties.
Rodes had bought time. At the last minute Longstreet appeared with the exhausted reinforcements of Brig. Gen. David R. Jones' division. Jones had taken the wrong road at Boonsboro and had to backtrack, meaning that they arrived late and in poor shape to fight. Hatch's and Meade's divisions had pushed Rodes off his positions on the high ground, but it was too dark for them to advance on Turner's Gap itself especially with reinforcements before them.
At the same time as Hooker began to assault Turner's Gap from the north, Reno's Ninth Corps advanced from the south. Sturgis' and Rodman's divisions were finally on hand. They assaulted Hill's force, only to find that the Confederates were still able to put up a stiff fight. Earlier in the afternoon Lee sent the division of Brig. Gen. John Bell Hood up to reinforce Hill. Hood himself was under arrest due to a silly altercation over who was to have several captured Union ambulances. Lee asked Hood for a simple statement of regret before giving Hood back to his men for the march up South Mountain. Hood refused, so Lee merely delayed the arrest order until after the current crisis. Hood, with his men, climbed the mountain and moved into line on Hill's left flank just north of Fox's Gap.
Reno's men were able to push back the Confederates, though not without costing them heavy casualties... and time. Around dusk a sharpshooter's bullet struck Reno as he urged on his men from the front. He was carried back through the lines mortally wounded, dying shortly thereafter. Cox took command of the corps and got it to about half a mile from Turner's Gap. Rebel artillery raked the fields where the Yankees tried to form up, and counterattacks kept them off balance. Two North Carolina regiments were torn apart after counterattacking into a trap, but time was against the Yankees. As the sky darkened they lost their chance of grabbing Turner's Gap that day.
One final assault was conducted on South Mountain. Burnside ordered Brig. Gen. John Gibbon's brigade of the First Corps to move straight up the National Road as a diversion. They ran into Colquitt's brigade, which had hardly seen any fighting up until then. Like the rest of the battle, the fighting was brutal and casualties were heavy. Gibbon, who was Hill's best man at his wedding, pushed right up to Colquitt's brigade. There they fought each other to a standstill until ammunition was low and darkness fell. Gibbon's brigade, later known as the Iron Brigade, did not break through. Both they and Colquitt's brigade rested for the evening with bayonets fixed.
The Union lost about 1800 men killed, missing, and wounded in the battle for Turner's Gap and Fox's Gap. Confederate losses were about 2300.
If the Yankees had been quicker, they could have taken Crampton's or Turner's Gap and pushed between the two portions of Lee's army. Instead, the day ended with the Union army stalled on the mountain. That night the Confederates slipped from the battlefield. By noon on September 15, the Federals commanded all three gaps through South Mountain. It was a tactical victory for the Union, but the Confederates had achieved a more important strategic victory. They had stopped the Union from splitting the Army of Northern Virginia. They were still badly outnumbered, but they now had a chance to bring their army together.
On September 14, unaware of events on South Mountain, Jackson seized ground around the Union fortification on Bolivar Heights against feeble resistance. In spite of difficulty communicating with McLaws and Walker by signal flag and rough terrain, Walker had five guns on Loudon Heights by noon and McLaws had four guns on Maryland Heights soon after. McLaws had the hardest job as he had to hack a trail up the Heights from Pleasant Valley wide enough for artillery.
Jackson intended to fire a massive coordinated artillery barrage at the Union position for maximum effect but the Union gunners spotted some of Walker's artillery on Loudoun Heights at about 1 p.m. and opened fire first. Walker fired back, then McLaws joined in. The artillery duel was lopsided. The Union guns could barely reach Loudoun heights and couldn't touch Maryland Heights at all. Walker suffered only four casualties.
The Union troops, many of them fresh recruits, were demoralized as the bombardment continued after nightfall. Under the cover of darkness Jackson moved guns on Bolivar Heights to within 1000 yards of the Federal position and placed them to take the Federals from the flank and rear.
That evening Col. Benjamin Franklin "Grimes" Davis of the 8th NY cavalry came to Miles with a plan to break out of Harper's Ferry with the 1300 cavalrymen stuck in the Union fortification. Davis argued that an artillery siege was no place for horsemen. Miles insisted on following the letter of Wool's order and hold the garrison in place. Miles only gave his consent to the breakout after it was obvious that Davis was going to do it anyway.
With two local men as guides, the cavalry slipped across the Potomac in the dark. Once across the river they took the same little used road that John Brown had taken to advance on Harper's Ferry in 1859. Stuart had warned McLaws to cover this road to prevent just such a breakout but McLaws was justifiably preoccupied with the fight at Crampton's Gap. The column swung around Sharpsburg and met up with the road linking Hagerstown to Williamsport. There they met up with Longstreet's forty-wagon reserve ordinance train. Davis, a Mississipian, used his accent to trick the train into following them. By the time it was light enough for the wagons' drivers to see that their "escort" was Union and not Confederate the actual Rebel escort had been fought off and the wagons were surrounded. By 9 a.m. Davis and the wagons were in Greencastle, Pennsylvania.
The next morning at day break the artillery barrage started up again. Shells fell from Maryland and Loudoun Heights but the most devastating fire came from Jackson's guns on Bolivar Heights itself. Union counter fire ended about 8 a.m. Soon after the Federals sent a rider to Jackson with a white flag. Due to the morning mist and battle smoke, the cease fire order was slow to reach Maryland and Loudoun Heights. One of the last shells exploded near Miles, wounding him in the leg while he stood talking to his aide. He died the next day.
Jackson captured 13,000 small arms, 200 wagons, 73 artillery pieces and much needed supplies. There were about 200 casualties per side, most of these in the infantry attack on Maryland Heights on the 13th. Some 11,500 Yankees were captured. Jackson telegraphed Lee that his command was marching to join up with the rest of the Army of Northern Virginia, except for the Light Division of A. P. Hill. A. P. Hill's men were remaining behind to make sure the captured stores made it to Richmond and to handle the surrender. Once that was completed they would rejoin the army.
Late on the 14th Lee received news that Jackson's force expected to take Harper's Ferry the next day. After the fighting at South Mountain had stopped, Lee ordered Longstreet's forces to pull back and converge near the town of Sharpsburg behind a creek known as the Antietam. Jackson's men would move north from Harper's Ferry once the town was taken, completing the convergence. While he didn't know what caused McClellan's recent change of pace Lee still expected McClellan to be cautious. Instead of pulling his forces out of harm's way, Lee set up what was essentially a bluff. He had barely 15,000 men, so far, to defend against the entire Army of the Potomac if McClellan chose to act decisively. The longer McClellan waited, the more men Lee would have as his army converged.
On the morning of the 15th McClellan discovered that South Mountain had been abandoned. He ordered his troops to pursue the enemy as he telegraphed Washington about his great victory. The First, Second, and Twelfth Corps were to take the National Road through Turner's Gap and the Eleventh and Fifth Corps were to take the Old Sharpsburg Road through Fox's Gap. Burnside was slow moving his Eleventh Corps, though, and ended up delaying Porter's Fifth Corps.
Franklin, too, was given his marching orders that morning. He was to protect Rohrersville and advance on Harper's Ferry. Couch's division was present so he had all three of his divisions. As he moved off the mountain he discovered McLaws' battle line across Pleasant Valley. McLaws had six brigades, about 5000 men. Franklin outnumbered him almost four to one, but only Baldy Smith's division was ready to attack. Franklin reported to McClellan that once Couch's division was in place to protect Rohrersville — which would take about two hours — he would attack McLaws.
Just after 9 a.m. the firing stopped at Harper's Ferry. Soon after McLaws' men began cheering the fall of the Union garrison. With it now obvious to Franklin that Harper's Ferry had fallen, he became very cautious. McClellan had ordered him to sweep McLaw's force out of Pleasant Valley but there was enough wiggle room in the orders to allow Franklin to go on the defensive. He expected the rest of Jackson's force to aid McLaws at any moment. At 11 a.m. he wrote to McClellan, "They outnumber me two to one. It will of course not answer to pursue the enemy under these circumstances." He was still working with McClellan's inflated estimates of the size of Jackson's force.
At 3 p.m. Franklin again wrote to McClellan of the stalemate in Pleasant Valley. This time he mentioned that McLaw's men had pulled out and moved south down the valley "too fast" for Baldy Smith to pursue. McLaws pulled his entire command back into Harper's Ferry and then swung north towards Sharpsburg.
The rest of the Army of the Potomac marched forward until a signal station reported at12:40 p.m. that Lee had set up a battle line more than a mile long near Sharpsburg on the other side of the Antietam Creek. McClellan was in his headquarters east of South Mountain. He moved up to get a first-hand look. It wasn't until 3 p.m. that he began to make personal observations of Lee's battle line while the Army of the Potomac halted along the roads to Sharpsburg.
It is only eight miles from Turner's Gap to Sharpsburg, but it wasn't until the evening of the 15th of September that McClellan started to move his men into position for an attack on Lee. He outnumbered Lee about four to one. Even with his own inflated estimates, McClellan should have realized he outnumbered Lee at least two to one. Lee's bluff had worked. By noon on the 16th Jackson had arrived to confer with Lee and Longstreet. Six of Lee's nine divisions were on hand with the divisions of McLaws and Anderson marching from Harper's Ferry and A.P. Hill's division marching the next morning. The three generals prepared for the Union attack.
McClellan wrote to Franklin on the morning of the 16th that he was ready to attack that day. In fact he wasn't. No preparatory orders had been issued to the troops. In fairness to McClellan there is evidence that he thought Lee's entire army was now with him. Governor Curtin of Pennsylvania sent a message to Lincoln with a report by Capt. William J. Palmer, one of Curtin's scouts. Palmer was at McClellan's headquarters on the night of the 15th. He heard there that McClellan believed Harper's Ferry had fallen on the morning of the 14th and that Jackson had linked up with Lee that same evening. With McClellan's grossly inflated numbers that meant that Lee actually outnumbered him on the other side of the creek. There was something else to consider. Except for a minor engagement on the Peninsula, McClellan had never planned and directed a battle. Most of his engagements with Lee had been McClellan reacting to Lee's battle plan. McClellan wanted everything just right before he launched an assault against Lee. So while the Army of Northern Virginia sat before him with a third of it still on the road, McClellan took his time preparing for the coming battle.
Finding Special Order 191 had given McClellan a considerable advantage, but his natural caution had almost completely neutralized that advantage. He had stopped Lee's invasion but he had squandered an opportunity to destroy Lee's army in detail.
Lee had chosen good terrain for defense. His army occupied a semi-circle from Sharpsburg on his right, along the Antietam creek, and in a loop to the north. While the terrain on his left flank seemed relatively flat, this was deceptive. It was full of little hollows, rises and stone outcroppings. The right flank was secured on the heights overlooking the creek. Lee had excellent interior lines of communication and could move his men quickly over the battlefield. His two able commanders were with him, "Stonewall" Jackson commanding in the north and "Old Pete" Longstreet commanding in the south. However, Lee also had his back to the Potomac River, leaving him no breathing room if his army should be soundly beaten. Failure would mean the destruction of the Confederacy's primary army. The weight of numbers, and the advantage, still lay with McClellan.