Battle of Antietam
The Battle of Antietam, also called the Battle of Sharpsburg, particularly in the Southern United States, took place during the American Civil War on September 17, 1862, between Confederate General Robert E. Lee's Army of Northern Virginia and Union Major General George B. McClellan's Army of the Potomac near Sharpsburg, Maryland, and Antietam Creek. Part of the Maryland Campaign, it was the first field army–level engagement in the Eastern Theater of the American Civil War to take place on Union soil. It remains the bloodiest day in American history, with a tally of 22,727 dead, wounded, or missing on both sides. Although the Union Army suffered heavier casualties than the Confederates, the battle was a major turning point in the Union's favor.
After pursuing Confederate General Robert E. Lee into Maryland, Major General George B. McClellan of the Union Army launched attacks against Lee's army who were in defensive positions behind Antietam Creek. At dawn on September 17, Major General Joseph Hooker's corps mounted a powerful assault on Lee's left flank. Attacks and counterattacks swept across Miller's Cornfield, and fighting swirled around the Dunker Church. Union assaults against the Sunken Road eventually pierced the Confederate center, but the Federal advantage was not followed up. In the afternoon, Union Major General Ambrose Burnside's corps entered the action, capturing a stone bridge over Antietam Creek and advancing against the Confederate right. At a crucial moment, Confederate Major General A. P. Hill's division arrived from Harpers Ferry and launched a surprise counterattack, driving back Burnside and ending the battle. Although outnumbered two-to-one, Lee committed his entire force, while McClellan sent in less than three-quarters of his army, enabling Lee to fight the Federals to a standstill. During the night, both armies consolidated their lines. In spite of crippling casualties, Lee continued to skirmish with McClellan throughout September 18, while removing his battered army south of the Potomac River.
McClellan successfully turned Lee's invasion back, making the battle a strategic Union victory. From a tactical standpoint, the battle was somewhat inconclusive; the Union Army successfully repelled the Confederate invasion, but suffered heavier casualties and failed to defeat Lee's army outright. President Abraham Lincoln, unhappy with McClellan's general pattern of overcaution and his failure to pursue the retreating Lee, relieved McClellan of command in November.
Nevertheless, the strategic accomplishment was a significant turning point in the war in favor of the Union due in large part to its political ramifications: the battle's result gave Lincoln the political confidence to issue the Emancipation Proclamation. This effectively discouraged the British and French governments from recognizing the Confederacy, as neither power wished to give the appearance of supporting slavery.
Background
Robert E. Lee's Army of Northern Virginia—about 55,000 men—entered the state of Maryland on September 3, following their victory at Second Bull Run on August 30. Emboldened by success, the Confederate leadership intended to take the war into enemy territory. Lee's invasion of Maryland was intended to run simultaneously with an invasion of Kentucky by the armies of Braxton Bragg and Edmund Kirby Smith. It was also necessary for logistical reasons, as northern Virginia's farms had been stripped bare of food. Based on events such as the Baltimore riots in the spring of 1861 and the fact that President Lincoln had to pass through the city in disguise en route to his inauguration, Confederate leaders assumed that Maryland would welcome the Confederate forces warmly. They sang the tune "Maryland, My Maryland!" as they marched, but by the fall of 1862 pro-Union sentiment was winning out, especially in the western parts of the state. Civilians generally hid inside their houses as Lee's army passed through their towns, or watched in cold silence, while the Army of the Potomac was cheered and encouraged. Some Confederate politicians, including President Jefferson Davis, believed that the prospect of foreign recognition would increase if the Confederacy won a military victory on Union soil; such a victory might gain recognition and financial support from the United Kingdom and France, although there is no evidence that Lee thought the Confederacy should base its military plans on this possibility.While McClellan's 87,000-man Army of the Potomac was moving to intercept Lee, two Union soldiers discovered a mislaid copy of Lee's detailed battle plans—Special Order 191—wrapped around three cigars. The order indicated that Lee had divided his army and dispersed portions geographically, thus making each subject to isolation and defeat if McClellan could move quickly enough. McClellan waited about 18 hours before deciding to take advantage of this intelligence and reposition his forces, thus squandering an opportunity to defeat Lee decisively.
There were two significant engagements in the Maryland campaign prior to the major battle of Antietam: Major General Thomas J. "Stonewall" Jackson's capture of Harpers Ferry and McClellan's assault through the Blue Ridge Mountains in the Battle of South Mountain. The former was significant because a large portion of Lee's army was absent from the start of the battle of Antietam, attending to the surrender of the Union garrison; the latter because stout Confederate defenses at two passes through the mountains delayed McClellan's advance enough for Lee to concentrate the remainder of his army at Sharpsburg.
Opposing forces
Union
Major General George B. McClellan's Army of the Potomac, bolstered by units absorbed from John Pope's Army of Virginia, included six infantry corps. The I Corps and XII Corps were units from Pope's force, the II Corps, V Corps, and VI Corps had fought with McClellan and the Army of the Potomac in the Peninsula campaign, and the IX Corps was a newer addition to the Army of the Potomac. The latter corps had seen action in North Carolina and a portion of it had fought with Pope. The VI Corps contingent included a division from the IV Corps. Earlier in the campaign, McClellan had assigned twenty-four new regiments of inexperienced troops to his army to bolster the strength of depleted veteran units. Of these green troops, eighteen regiments, totaling about 15,000 to 16,000 men, accompanied McClellan's army for the march to Antietam. Several thousand more new recruits were added to existing units, so about a quarter of McClellan's army was inexperienced and poorly trained entering the battle. The II, IX, and XII Corps received more of these troops than the other corps.The I Corps was commanded by Major General Joseph Hooker and contained three divisions. The II Corps was commanded by Major General Edwin Vose Sumner, and contained three divisions. As a whole, the veteran elements of the corps had a reputation as a good fighting unit. The V Corps was commanded by Major General Fitz John Porter. Originally only one division of the corps accompanied McClellan, but a second was later released from the defenses of Washington, D.C. to accompany the Army of the Potomac. A third division of the corps reached McClellan the day of the battle. The VI Corps consisted of two divisions and the IV Corps detachment and was commanded by Major General William B. Franklin. These three divisions were positioned about a four-hour march away from McClellan's main body at the beginning of the battle. The IX Corps contained four divisions and was nominally commanded by Major General Ambrose Burnside, but Burnside temporarily commanded a wing of McClellan's army and active command passed to Major General Jesse Reno until his death at the battle of South Mountain, and then to Brigadier General Jacob D. Cox. The XII Corps was McClellan's smallest corps and contained two divisions. It was commanded by Brigadier General Joseph K. F. Mansfield, who took command only two days before the battle. McClellan's army also contained a cavalry division commanded by Brigadier General Alfred Pleasonton. A 2023 study by the historian D. Scott Hartwig estimates that McClellan had 72,199 men available for combat on September 17, with roughly 14,000 more arriving as reinforcements. Hartwig places I Corps strength at 9,582; II Corps strength at 16,475; V Corps strength at 9,476 with another 7,000 in the third division; 11,862 men in the VI Corps proper with another 7,219 men in the IV Corps detachment; 12,241 in the IX Corps; 8,020 in the XII Corps; and 4,543 in the Cavalry Division. This force was supported by 293 cannons available for duty.
Confederate
General Lee's Army of Northern Virginia was organized into two large infantry wings, commanded by Major Generals James Longstreet and Thomas J. "Stonewall" Jackson. Longstreet's wing was numerically larger than Jackson's. This arrangement was due to Confederate law not allowing the creation of corps. Longstreet's wing contained five divisions, led by major generals Lafayette McLaws and Richard H. Anderson and brigadier generals David Rumph Jones, John G. Walker, and John Bell Hood. Jackson's wing contained four divisions, commanded by major generals Daniel Harvey Hill and Ambrose Powell Hill, and brigadier generals Alexander R. Lawton and John R. Jones. Lee also had a Cavalry Division commanded by Major General J. E. B. Stuart and a unit of reserve artillery led by Brigadier General William Nelson Pendleton. The assignment of units between the wings of Jackson and Longstreet was flexible; at Antietam the two men commanded sectors of the battlefield and divisions fought under the commander whose geographic area they were fighting in.The Army of Northern Virginia held morale and leadership advantages over McClellan's army, but was poorly supplied, was operating in enemy territory away from its logistical lines, and was more poorly armed. Ammunition supply was made more difficult due to units being armed with mixed types of weapons, and many Confederate soldiers were still armed with smoothbore weapons of shorter range. Many of the cannons issued to the Confederate artillery were obsolete, while the Union had modern guns. The Confederates had about 246 cannon at Antietam, although the exact number of guns issued to some of the Confederate batteries is not known. Lee's army was weakened by disease, and the historian Joseph T. Glatthaar estimates that about one-third to one-half of Lee's army was absent at Antietam due to straggling. The Confederate strength at Antietam is difficult to determine; Hartwig states that a precise figure is indiscernible, but estimates a strength of about 37,600 men.