Four days after the battle of Antietam, which took place near Sharpsburg, Maryland, on September 17, 1862, Captain Robert Gould Shaw of the 2nd Massachusetts Volunteer Infantry wrote to his father: “Every battle makes me wish more and more that the war was over. It seems almost as if nothing could justify a battle like that of the 17th, and the horrors inseparable from it.” It was the bloodiest single day in American history, with more than 6,000 Union and Confederate soldiers killed or mortally wounded. Shaw had been in the thick of the fighting and his brigade had suffered 646 men killed, wounded, and missing. Like Shaw, many other soldiers who had lost friends in this battle wondered if anything could justify such slaughter.
That same question lingers today, 150 years later. But even in 1862, many contemporaries foresaw that the showdown at Sharpsburg would have a decisive impact on the future of the continent. From London, where he followed the American Civil War with close attention, Karl Marx wrote in October 1862 that Antietam “has decided the fate of the . . . War.” Confederate President Jefferson Davis felt “very low down” after the battle, reported his secretary of war, because the Confederacy’s “maximum strength has been mobilized, while the enemy is just beginning to put forth his might.” Years after the war, Major Walter H. Taylor of General Robert E. Lee’s wartime staff described the battle of Sharpsburg (the Confederate name for it) as “the decisive event of the war.”Show Full EssayHide Full Essay
Why did Taylor and others consider Antietam the decisive event of the war? To answer that question, we must turn the clock back several months. From February to June 1862, Union armies and naval forces won a series of victories that had rocked the Confederacy back on its heels. By June the large and well-equipped Army of the Potomac commanded by Major General George B. McClellan had approached within five miles of Richmond, the Confederate capital. The Confederacy’s days appeared numbered. But in the Seven Days’ Battles east of Richmond (June 25–July 1), the Army of Northern Virginia commanded by Robert E. Lee counterattacked and drove McClellan’s army away from the capital. Lee followed this achievement with an offensive that culminated in the one-sided Confederate victory in the Second Battle of Manassas (called Bull Run by the Union), August 29–30, 1862. Lee decided to invade Maryland at the same time that Confederate armies in Tennessee launched a counteroffensive that took them into Kentucky. Now it was the Union forces who were rocked back on their heels and in apparent danger of losing the war.
These events had a significant impact on the home fronts and even in Europe. Morale in the South rebounded from its springtime depression, while Northern spirits sank to one of the lowest points in the war. “Disguise it as we may, the Union arms have been repeatedly, disgracefully, and decisively beaten,” declared the New York Times in early September. Unless there was a change, “the Union cause is doomed to a speedy and disastrous overthrow.”
The Confederate invasions of Maryland and Kentucky were intended not only to win these border slave states for the Confederacy, but also to influence Northern voters who would go to the polls in October and November to elect a new House of Representatives. “The present posture of affairs,” General Lee wrote to President Davis on September 8 after his army had crossed the Potomac River into Maryland, “places it in [our] power . . . to propose [to the Union government] . . . the recognition of our independence.” Such a proposal, coming when “it is in our power to inflict injury on our adversary would enable the people of the United States to determine at their coming elections whether they will support those who favor prolongation of the war, or those who wish to bring it to a termination.”
Those who wished to bring the war to a termination, in Lee’s terminology, were the so-called “Copperheads”—the peace wing of the Democratic Party. Peace Democrats insisted that Northern armies could never conquer the South and that the administration should seek an armistice and peace negotiations. The bleak military outlook for Union forces in September strengthened their credibility and forecast the possible loss of Republican control of the House, which might have crippled the Lincoln administration’s ability to carry on the war.
In Europe, news of the reversal of military fortunes in the American war seemed to set the stage for diplomatic recognition of the Confederacy by the leading powers, which might give a decisive boost to Confederate prospects. While French Emperor Napoleon III was an advocate of recognition, the British had been dragging their feet. But now Prime Minister Viscount Palmerston seemed to be leaning in the direction of intervention. The Federals “got a very complete smashing” at Bull Run, he wrote to Foreign Minister Lord John Russell on September 14, “and it seems not altogether unlikely that still greater disasters await them, and that even Washington or Baltimore might fall into the hands of the Confederates.” If something like that happened, “would it not be time for us to consider whether . . . England and France might not address the contending parties and recommend an arrangement on the basis of separation?” Russell concurred, and added that if the North refused an offer of mediation, “we ought ourselves to recognise the Southern States as an independent State.” The two British leaders agreed, however, to hold off until the results of Lee’s invasion became clear. “If the Federals sustain a great defeat,” said Palmerston, their “Cause will be manifestly hopeless . . . and the iron should be struck while it is hot. If, on the other hand, they should have the best of it, we may wait a while and see what may follow.”
Another matter of great import rode on the fate of Lee’s invasion of Maryland: a possible Emancipation Proclamation by Union President Abraham Lincoln. At a Cabinet meeting on July 22, Lincoln informed his advisors that he had decided to issue such an edict, based on his powers as commander in chief to seize enemy property—in this case, slaves—being used to wage war against the United States. Slaves constituted the principal labor force that sustained the Southern economy and the logistics of the Confederate war effort. Lincoln had earlier resisted pressures to strike against slavery because he feared such action would alienate border-state Unionists. But by July 1862 he had concluded that this risk was more than balanced by the potential benefits of a measure that would convert black labor from a Confederate to a Union asset. Emancipation was “a military necessity, absolutely essential to the preservation of the Union,” Lincoln told the Cabinet. “The slaves [are] undeniably an element of strength to those who [have] their service, and we must decide whether that element shall be for us or against us.” The North “wanted the Army to strike more vigorous blows. The Administration must set an example and strike at the heart of the rebellion”—slavery.
Most Cabinet members were prepared to support Lincoln’s decision. But Secretary of State William H. Seward advised against issuing a proclamation during this time of discouragement. Wait “until you can give it to the country supported by military success,” said Seward. Otherwise the world might view it “as the last measure of an exhausted government, a cry for help . . . our last shriek, on the retreat.” Lincoln put the proclamation away to wait for a military victory. It would prove to be a long, dismal wait.
Lee’s invasion of Maryland provided an opportunity for such a victory, however, if the Army of the Potomac could seize it. Although a majority of the Cabinet and of Republicans in Congress—plus Lincoln himself—had become disillusioned with General McClellan for his failures in Virginia and his reluctance to aid Major General John Pope, who commanded the troops who fought at Second Bull Run, Lincoln recognized that only McClellan could reorganize the Army and rebuild its shattered morale. On September 2 Lincoln ordered McClellan to take command and go after the Confederates.
McClellan did so, though with his usual caution and exaggeration of the number of enemy troops he faced. On September 13, however, McClellan had a stroke of extraordinary luck at Frederick, Maryland. One of his soldiers found a copy of Robert E. Lee’s Special Orders No. 191 wrapped around three cigars, apparently lost by a careless Confederate courier. Lee had issued these orders to several division and corps commanders to march in various directions to cooperate in the capture of the Union garrison at Harpers Ferry on the Potomac River, which lay athwart Lee’s supply line from the Shenandoah Valley in Virginia. Knowledge of these orders gave McClellan an opportunity to catch parts of Lee’s army separated from each other and defeat them in detail. McClellan moved too slowly to take full advantage of this opportunity, but his soldiers did push their way through three passes in the South Mountain range west of Frederick in sharp fighting on September 14. They were too late to rescue the garrison at Harpers Ferry, which surrendered 12,000 men on September 15. But the Union advance did force Lee to send emergency orders that day to his scattered divisions to concentrate at Sharpsburg with their back to the Potomac River.
McClellan sent two corps across Antietam Creek near Sharpsburg to attack the Confederate left flank at dawn on September 17. His plan was to follow this assault with an attack on the Confederate right, and if Lee weakened his center to defend the flanks, to assault the center with his reserves. It was a good plan and might have worked if McClellan had carried it out. But the attacks lacked coordination, coming one or two divisions at a time, enabling Lee to shift troops from one flank to another to shore up threatened positions. Even though McClellan’s army outnumbered Lee’s by 80,000 to 40,000, he believed that the enemy outnumbered him. Never during that long day did more than 15,000 Union troops go into action at the same time, and McClellan kept 20,000 in reserve and did not use them at all, because he feared that if he committed his reserves the enemy would counterattack with those phantom troops that existed only in McClellan’s imagination. On one occasion during the early afternoon, Union troops seemed to be poised to break through the Confederate center along a sunken farm road that became known as “Bloody Lane.” Later in the afternoon, another Union corps threatened to encircle the Confederate right and cut off Lee’s retreat route to a ford on the Potomac. Both times McClellan failed to commit his reserves to exploit the potential breakthroughs.
The Army of Northern Virginia held on by its fingernails until dark. After a night of horror when many wounded died in no-man’s land between the lines, the morning of September 18 dawned with the Confederates still standing defiantly in place. Despite receiving reinforcements during the day, McClellan did not renew the attack. That night Lee ordered his army to retreat to Virginia. Except for a feeble Union pursuit on the 20th that was easily repulsed, the battle of Antietam was over.
McClellan missed many opportunities to win a more decisive victory. But by compelling Lee to retreat without achieving his objectives in Maryland, the Army of the Potomac could claim at least a limited victory. The Northern press puffed it into a great triumph, all the more heartening because of the pessimism that had preceded it. The New York Times proclaimed that the effects of this “GREAT VICTORY” would be “felt in the destinies of the Nation for centuries to come.”
The Times was right about the long-term consequences of the battle. Five days later, Lincoln called a special meeting of his Cabinet. “I think the time has come,” he told them. “I wish it was a better time. . . . The action of the army against the rebels has not been quite what I should have best liked. But they have been driven out of Maryland.” This achievement was perhaps God’s sign that “he had decided this question in favor of the slaves.” Therefore, said the President, he would issue that day a proclamation warning Confederate states that unless they returned to the Union by January 1, 1863, their slaves “shall be then, thenceforward, and forever free.”
Perhaps no result of Antietam was more momentous than this one. But there were others almost as important. The congressional elections still loomed. In every midterm election of the past twenty years the party in power had lost control of the House. Until Antietam that dire prospect seemed likely this time as well, with unpredictable implications for the government’s ability to carry on the war. Democrats did make significant gains, but they fell short of winning the House.
The consequences of the battle abroad were equally significant. When the news reached London, Prime Minister Palmerston backed away from the idea of intervention. The only favorable condition for mediation would have been “the great success of the South against the North,” he told Foreign Secretary Russell. “That state of things seemed ten days ago to be approaching,” but with Antietam “its advance has been lately checked,” and thus “I am [convinced] that we must continue merely to be lookers-on till the war shall have taken a more decided turn.” The French still wanted to recognize the Confederacy, but would not do so unless Britain went along. She never did, nor did any other power. Antietam ended the Confederacy’s best chance for foreign intervention.
In several respects, therefore, Antietam was the most important turning point of the war. Twenty years later, Confederate General James Longstreet wrote: “At Sharpsburg was sprung the keystone of the arch on which the Confederate cause rested.” With this damage to the keystone, the arch ultimately collapsed and the future of the United States as one nation, indivisible and free, was assured.
 Shaw to Francis G. Shaw, Sept. 21, 1862, in Blue-Eyed Child of Fortune: The Civil War Letters of Robert Gould Shaw, ed. Russell Duncan (Athens GA: University of Georgia Press, 1992), 242.
 Die Presse (Vienna), Oct. 12, 1862, translated and reprinted in Karl Marx on America and the Civil War, ed. Saul K. Padover (New York: McGraw-Hill, 1972), 220; Inside the Confederate Government: The Diary of Robert Garlick Hill Kean, ed. Edward Younger (New York,: Oxford University Press, 1957), 86, entry of June 27, 1863. Davis made this remark to Secretary of War George W. Randolph in October 1862; Randolph later quoted Davis to his nephew-in-law Kean, who was chief of the Bureau of War.
 Walter H. Taylor, Four Years with General Lee, ed. James I. Robertson Jr. (Bloomington IN: Indiana University Press, 1962), 67.
 New York Times, Sept. 5, 7, 1862.
 Lee to Jefferson Davis, Sept. 8, 1862, in The Wartime Papers of R. E. Lee, eds. Clifford Dowdey and Louis H. Manarin (New York: Bramhall House, 1961), 301.
 Palmerston to Russell, Sept. 14, 1862, Russell to Palmerston, Sept. 17, Palmerston to Russell, Sept. 23, Russell Papers, Public Record Office, reprinted in James B. Murfin, The Gleam of Bayonets: The Battle of Antietam and Robert E. Lee’s Maryland Campaign, September 1862 (Baton Rouge: Louisiana State University Press, ), 394, 396–397, 399–400.
 Gideon Welles, “The History of Emancipation,” The Galaxy 14 (1872): 842–843.
 Francis B. Carpenter, Six Months at the White House with Abraham Lincoln (New York: Hurd and Houghton, 1866), 20–22.
 New York Times, Sept. 18, 21, 1862.
 Diary of Gideon Welles, ed. Howard K. Beale (New York: Norton, 1960), 1:142–145; The Salmon P. Chase Papers, ed. John Niven, vol. 1, Journals, 1829–1872 (Kent, Ohio, 1992), 393–395. For the Emancipation Proclamation, see The Collected Works of Abraham Lincoln, ed. Roy P. Basler (New Brunswick NJ: Rutgers University Press, 1953–55), 5:433–436.
 Quoted in Ephraim D. Adams, Great Britain and the American Civil War (New York: Longmans, Green and Co., 1925), 2: 43–44, 54–55.
 James Longstreet, “The Invasion of Maryland,” Battles and Leaders of the Civil War, eds. Clarence C. Buel and Robert U. Johnson (New York: The Century Co., 1888), 2: 674.
James M. McPherson is the George Henry Davis ’86 Professor of American History Emeritus at Princeton University and the author of Crossroads of Freedom: Antietam (2002). He won the Pulitzer Prize for Battle Cry of Freedom (1998) and the Lincoln Prize for For Cause and Comrades: Why Men Fought in the Civil War (1997) and for Tried by War: Abraham Lincoln as Commander in Chief (2008).
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