The Battle of Antietam was fought on the 17th of September 1862 along the meandering line of Antietam Creek outside the small town of Sharpsburg in western Maryland, between the Confederate Army of Northern Virginia under Robert E. Lee (approximately 38,000 effectives) and the Union Army of the Potomac under George B. McClellan (approximately 75,000 effectives). The battle was fought in three distinct sectors — the northern attack through David Miller's cornfield by the Union First and Twelfth Corps; the central assault on the Sunken Road, later known as Bloody Lane; and the southern attack across Burnside's Bridge to the Confederate right — across thirteen hours of continuous engagement. Casualties: approximately 12,400 Union, 10,300 Confederate, combined approximately 22,700 in twelve hours. The day remains, by a substantial margin, the deadliest single calendar day in United States military history; the next bloodiest single days are the assault at Cold Harbor (June 1864) and the Pearl Harbor attack (1941), both well below half the Antietam total.
The Lost Order
The military setting of Antietam was unusual because McClellan's intelligence was unusually good. On the 13th of September, Union soldiers near Frederick, Maryland, found a discarded copy of Lee's Special Orders No. 191 wrapped around three cigars in an abandoned Confederate camp; the orders disclosed that Lee had divided his army into four separate columns operating in different directions across western Maryland, with the largest detachment under Stonewall Jackson having been sent south to take the federal garrison at Harpers Ferry. McClellan, given the lost order, had the operational opportunity to defeat Lee's army in detail — to attack the separated Confederate columns before they could reconcentrate. He did not exploit it. He waited approximately eighteen hours before moving on the 14th, allowing Lee time to begin reconcentrating; he then fought a delaying action at South Mountain on the 14th-15th, which Lee's rearguards conducted competently; and by the time McClellan reached Antietam Creek on the 16th of September, Lee had reassembled the bulk of his army on the western side of the creek with the Potomac at his back. The tactical opportunity had been substantially squandered.
The day itself
The fighting on the 17th opened at 5.30 a.m. with the Union First Corps under Hooker advancing southward across the Miller cornfield. The cornfield changed hands several times during the morning; the fighting on this sector alone produced approximately 12,000 of the day's total casualties between 5.30 a.m. and 10.00 a.m. Around 9.30 a.m. the Union Second Corps under Sumner attacked the Confederate centre at the Sunken Road; in the most concentrated killing of the day, approximately 5,500 men fell along the four-hundred-yard stretch of farm lane between 9.30 and noon. The road, after the war, became known as Bloody Lane. By midday Lee's army was holding by a narrow margin; a determined push in the centre might have broken the Confederate line and pinned Lee against the Potomac. McClellan, with approximately 20,000 fresh troops in reserve, declined to commit them. The Confederate centre held.
The southern attack across the lower Antietam bridge — Burnside's Bridge, named for the slow Union corps commander, Ambrose Burnside, who took most of the morning to force the crossing — began around 1.00 p.m. and finally drove the small Confederate force off the bridge by 3.00 p.m. The Union Ninth Corps advanced toward Sharpsburg with the Confederate right collapsing in front of them; at approximately 4.30 p.m., the late-arriving Confederate Division of A.P. Hill, marching up from Harpers Ferry, struck Burnside's flank and drove the Union attack back. The battle ended at dark with Lee's army intact behind a shrunken defensive perimeter; the next morning Lee waited, half expecting another attack; when none came he retired across the Potomac on the night of the 18th-19th. McClellan did not pursue.
The strategic verdict of Antietam was that Lee's invasion of Maryland had been blocked. This was sufficient, for Lincoln's purposes, to claim it as a Union victory — the first one of substantive scale in the eastern theatre. It was sufficient also for the political move Lincoln had been waiting since July to make.
The Emancipation Proclamation
Lincoln issued the preliminary Emancipation Proclamation on the 22nd of September 1862, five days after Antietam. The proclamation announced that on the 1st of January 1863, slaves held in states still in rebellion against the United States would be "thenceforward and forever free". The proclamation was, in legal form, a war measure — an executive use of the commander-in-chief power to weaken the enemy by removing labour from its economy. It did not apply to slaves in the four loyal slave states (Maryland, Delaware, Kentucky, Missouri) nor to slaves in those areas of the Confederate states already under Union military control (parts of Virginia, Louisiana, Tennessee). It was, on Lincoln's analysis, the maximum constitutionally defensible reach of executive emancipation. Full statutory abolition would require the Thirteenth Amendment, which Congress passed in January 1865 and the states ratified in December 1865.
The proclamation transformed the political nature of the war. From January 1863 onward, the Union war effort was unambiguously an emancipationist project; any negotiated settlement with the Confederate states that preserved slavery as it existed in 1860 was foreclosed. The proclamation also authorised the enlistment of African-American men into the Union army; approximately 180,000 black soldiers would serve in the United States Coloured Troops by the end of the war, including the celebrated Massachusetts 54th regiment, the assault on Fort Wagner in July 1863, and the substantial part of the late-war Army of the James that would enter Richmond in April 1865. The proclamation also substantially reduced the likelihood of European recognition of the Confederacy: by making the war explicitly about slavery, it created political obstacles to British and French diplomatic intervention that the Confederate government could not overcome.
The political aftermath
McClellan was relieved of command on the 7th of November 1862, in the wake of his failure to pursue Lee after Antietam. He was replaced by Ambrose Burnside, who would conduct the disastrous Battle of Fredericksburg on the 13th of December 1862 — a frontal assault on entrenched Confederate positions, with approximately 12,600 Union casualties to 5,300 Confederate; the worst Union defeat of the eastern theatre to that date. Burnside in turn was replaced in January 1863 by Joseph Hooker, who would lead the Army of the Potomac to its next major defeat at Chancellorsville in May 1863. The eastern theatre would not produce a decisive Union victory until the meeting at Gettysburg in July 1863, the subject of the next chapter.
End of Chapter V