Maryland Campaign

• General Robert E. LeeSouth Mountain, Antietam
• Major General Thomas J. JacksonHarpers Ferry
• Brigadier General William PendletonShepherdstown
• Major General A.P. HillShepherdstown

• Colonel Dixon S. MilesHarpers Ferry
• Major General George B. McClellanSouth Mountain, Antietam
• Major General Fitz John PorterShepherdstown

Between September 4 and 20, 1862, Confederate forces under General Robert E. Lee pushed into Union territory for the first time during the Civil War, an offensive now remembered as the Maryland campaign or the Antietam campaign. Maj. Gen. George B. McClellan's Army of the Potomac turned back this thrust, marching to cut off Lee's Army of Northern Virginia and finally engaging it close to Sharpsburg, Maryland, in what became the single deadliest day of combat in the nation's history.

Fresh off success in northern Virginia, Lee led 55,000 troops northward through the Shenandoah Valley beginning September 4. He hoped to feed and supply his men away from Virginia's devastated countryside while also striking a blow at civilian morale ahead of the November 1862 midterm elections. Betting on a bold gamble, he divided his forces, pushing part of them into Maryland while sending Stonewall Jackson to seize the Federal arsenal and garrison at Harpers Ferry. Fortune favored McClellan when a lost copy of Lee's orders fell into Union hands, revealing the split and giving him a chance to crush the separated Confederate columns individually.

As Jackson's men encircled, shelled, and ultimately took Harpers Ferry between September 12 and 15, McClellan pushed his 102,000-man army toward the mountain gaps standing between him and Lee's forces. Confederate defenders slowed that advance at the Battle of South Mountain on September 14, buying Lee precious time to gather the bulk of his army at Sharpsburg. Three days later, on September 17, the armies clashed at Antietam, also called Sharpsburg, producing more than 22,000 casualties and cementing its place as the bloodiest single day in American military annals. Despite being outnumbered roughly two to one, Lee shifted his defenders skillfully to counter each Union assault, while McClellan held back reserves that might have delivered a decisive Confederate collapse. Lee pulled his army back across the Potomac on September 18, and skirmishes involving his rearguard near Shepherdstown on September 19 and 20 brought the campaign to a close.

Though the fighting at Antietam ended without a clear tactical winner, it doomed the broader aims of Lee's push into Maryland. Seizing on this outcome, President Abraham Lincoln framed it as sufficient Union success to justify issuing his Emancipation Proclamation, a move that all but eliminated the possibility of European powers backing the Confederacy.

Opposing forces

McClellan led the Army of the Potomac into this fight, a force fattened by absorbing units from Pope's Army of Virginia and numbering roughly 102,000 men across six infantry corps.

Hooker's I Corps brought together the divisions of King, Ricketts, and Meade. Sumner's II Corps combined the divisions led by Richardson and Sedgwick with French's brigade. Porter commanded the V Corps, whose divisions fell under Morell, Sykes, and Humphreys. Franklin's VI Corps grouped the divisions of Slocum and "Baldy" Smith alongside Couch's division, on loan from the IV Corps. Burnside's IX Corps gathered the divisions of Willcox, Sturgis, and Rodman, plus Cox's Kanawha Division. Rounding things out, Mansfield's XII Corps held the divisions of Williams and Greene, supported by Pleasonton's cavalry.

As the army pushed north into Maryland, McClellan restructured his command into three wings. Franklin took the left, pairing his VI Corps with Couch's division. Sumner ran the center with his II Corps joined by the XII. Burnside held the right, combining his IX Corps, temporarily under Reno until his death at South Mountain, with the I Corps. McClellan scrapped this wing system right before Antietam began.

This was not a fully unified or seasoned fighting machine McClellan brought north. Peninsula veterans from the II, V, and VI Corps formed its backbone, but many regiments were untested newcomers who had never served together as this army, some men having never even fired their muskets, others unknowingly carrying faulty weapons.

Getting the core units, the II, III, V, and VI Corps, ready for combat proved simplest since they carried Peninsula Campaign experience. The three corps inherited from Pope presented more difficulty. Apart from the Pennsylvania Reserves, who had fought hard during the Seven Days, none of these men had McClellan's leadership before, suffered from poor command, and lacked a strong combat record. Burnside's two IX Corps divisions were likewise unfamiliar to McClellan, though they had performed well in North Carolina earlier that year. Cox's Kanawha Division from West Virginia joined this corps too, equally new to McClellan and largely without major battle experience.

Hooker's I Corps, by contrast, contained strong troops; its Pennsylvania Reserves had seen heavy Seven Days combat, and every division within it fought hard at Second Bull Run. McDowell, however, had fallen out of favor entirely, scorned by his own men, and he would never again hold Civil War command. McClellan initially wanted his close friend Reno, then running the IX Corps, to lead the I Corps instead, but the job went to Hooker as the more seasoned choice.

Banks's XII Corps carried a tarnished reputation, having been beaten badly by Jackson's forces during the spring Valley Campaign and performing weakly at Cedar Mountain. Pope thought so little of Banks and his corps that he kept them clear of the Second Bull Run fighting altogether. Banks lost his XII Corps command and was eventually posted to Louisiana. Williams ran the corps temporarily until Mansfield took over on September 14.

Both the III and XI Corps had taken brutal losses at Second Bull Run, nearly collapsing in panic, and were held back in Washington to recover and rebuild. The III Corps troops were solid veterans who had fought well both on the Peninsula and at Second Bull Run. The XI Corps, however, filled largely with German-American soldiers and led by Sigel, carried a weak reputation and no real battlefield success to its name.

Using his influence, McClellan managed to push out several weak generals, McDowell, Heintzelman, and Banks among them, though the Union army still lagged behind Lee's Confederates, who had already cleared out inadequate leadership following the Seven Days. Officials in Washington wanted Porter and Franklin removed too, viewing them as politically unreliable, but McClellan kept both men for this campaign.

Among the six corps in Maryland, the II and VI stood as the largest and freshest, having avoided combat since the Seven Days. The II Corps gained a new nine-month division under French, and the VI Corps added a single fresh regiment, but otherwise both units consisted entirely of Peninsula veterans. The I Corps, having bled heavily at Second Bull Run (with one division also hit hard during the Seven Days) and set

Confederate

By early September, roughly 55,000 men stood ready for duty in Lee's Army of Northern Virginia, split between two sizable infantry corps. Longstreet held command of the First Corps, which brought together the divisions of McLaws, Anderson, D.R. Jones, Walker, and Hood, along with a standalone brigade led by Evans, nicknamed "Shanks." Jackson led the Second Corps, made up of the divisions belonging to Lawton, A.P. Hill (known as the Light Division), John R. Jones, and D.H. Hill. Rounding out the army were Stuart's Cavalry Corps and the reserve artillery under Pendleton. A key structural difference separated the two corps: Jackson's divisions each had artillery assigned directly to them, whereas Longstreet kept his guns centralized at the corps level.

What made the Maryland campaign particularly strange was just how depleted Lee's force had become. Back in June, when he first took command, nearly 90,000 soldiers filled its ranks. That number collapsed after the Seven Days Battles claimed 20,000 casualties, and the fighting in northern Virginia stripped away roughly 12,000 more. The march north into Maryland only worsened matters, as hunger, exhaustion, and outright desertion thinned the columns further, since many Virginia men had enlisted to protect their home state rather than to carry the war onto Northern soil. Countless soldiers marched barefoot, their feet unable to withstand Maryland's hard macadamized roads. By the time the armies clashed at Antietam, Lee's effective strength may have dipped below 40,000, the leanest and most worn down his army would be until the closing days at Petersburg. Entire brigades had shrunk to regiment size, and regiments themselves often numbered no more than a company. Yet spirits remained surprisingly strong throughout the ranks, and since nearly every Confederate present was a seasoned veteran, they held a real edge over the many inexperienced Union units they faced.

Two divisions, McLaws' and D.H. Hill's, had stayed behind near Richmond during the northern Virginia campaign but rushed to catch up once the army turned toward Maryland. Additional strength came from North Carolina in the form of Walker's division, consisting of two brigades.

Historians have argued for well over a century about exactly how many men Lee actually commanded at Antietam, mainly because no formal counts of the army exist for the period between July 20 and September 22. Writers sympathetic to the "Lost Cause" narrative after the war often portrayed Lee's force as desperately thin, sometimes claiming as few as 30,000 men were present. Union commanders and soldiers who lived through the fighting tended to disagree, placing Confederate numbers closer to 50,000. The divisions in the worst shape were almost certainly Lawton's and the old Stonewall Division, both of which had been marching and fighting nearly continuously since May with barely a pause. Other divisions, like D.H. Hill's, hadn't seen combat since the Peninsula campaign and so were comparatively fresh and better conditioned. Food shortages plagued the army throughout, since most crops in the region were still weeks from ripening in September, forcing many men to eat raw field corn and unripe apples, which left them sick with indigestion and diarrhea. As already mentioned, this malnutrition hit hardest among the two divisions that had once formed Jackson's Valley Army, worn thin by months of relentless campaigning.

Dividing Lee's army

As the Confederate columns pushed into Maryland, Lee split his forces into four separate parts. Word reached him of militia stirring near Chambersburg, prompting him to dispatch Maj. Gen. James Longstreet first to Boonsboro and afterward to Hagerstown. As it turned out, the intelligence Lee acted on had exaggerated matters considerably, for a mere 20 militiamen actually occupied Chambersburg at that time. Meanwhile, Maj. Gen. Thomas J. "Stonewall" Jackson received orders to capture the Union arsenal at Harpers Ferry, approaching the target with three distinct columns. Such an arrangement meant that only Maj. Gen. J. E. B. Stuart's thinly stretched cavalry, along with Maj. Gen. D.H. Hill's division, remained behind to protect the army's rear near South Mountain.

Historians have never settled on why Lee gambled on dividing his army in this fashion to take Harpers Ferry. Perhaps he recognized the town's control over his supply routes running through the Shenandoah Valley. Prior to crossing into Maryland, he had operated on the assumption that Federal troops stationed at Winchester, Martinsburg, and Harpers Ferry would find themselves isolated and would pull out without a fight, and indeed both Winchester and Martinsburg were later abandoned. There is also the possibility that the arsenal simply presented an irresistible prize, stocked with essential supplies yet nearly impossible to defend given that higher ground surrounded it on every side. On the Union side, McClellan had sought authorization from Washington to pull the garrison out of Harpers Ferry and fold it into his own army, but general-in-chief Henry Halleck turned down this request.

Aftermath and diplomatic implications

Once Lee's army slipped back across the Potomac, the Maryland campaign came to a close, and active fighting for the season stopped as well. Lincoln found little to praise in McClellan's conduct of the affair, concluding that his hesitant and disjointed handling of troops had allowed what might have been a decisive Confederate collapse to end instead in stalemate. What troubled the president further was McClellan's refusal, spanning September 17 through October 26, to chase Lee's retreating forces despite constant pressure from both Lincoln and the War Department; the general pointed to supply shortages and worries about stretching his army too thin. General-in-Chief Henry W. Halleck later noted in an official account that the prolonged idleness of such a massive force, confronting an already beaten enemy during weather ideal for swift campaigning, produced widespread frustration and dismay. On November 7, Lincoln stripped McClellan of command over the Army of the Potomac, a move that essentially closed out his time as a military leader. Command then passed to Maj. Gen. Ambrose E. Burnside. Little happened in the Eastern Theater afterward until December, when Burnside and Lee met again at Fredericksburg.

Though the fighting at Antietam ended without a clear tactical winner, it counted as a major strategic gain for the Union side. Lee's push northward was brought to a halt, and Lincoln finally had the triumph he had been waiting for to justify announcing the Emancipation Proclamation on September 22, set to take effect January 1, 1863. He had wanted to issue it sooner, but his Cabinet urged patience until a battlefield success arrived, so the move would not look like an act born of desperation. The setback Confederate forces suffered at Antietam also played a role in keeping both France and Britain from extending recognition to the Confederacy. Once the Emancipation Proclamation took effect, the odds of foreign powers granting recognition after some future Southern victory dropped even further. By framing slavery so plainly as central to the Confederate cause, Lincoln made intervention politically untenable for nations like France and Britain, where slavery was widely detested.

Union casualties across the entire Maryland campaign totaled 28,272: 2,783 killed, 12,108 wounded, and 13,381 missing.