The Battle of Antietam, on Sept. 17, 1862, was the bloodiest day of combat in American history.

On D-Day, U.S. forces suffered 5,200 casualties out of a force of 50,000, roughly 10 percent of those engaged. At Antietam the Union army lost 13,000 out of 73,000 (16.518 percent), and the Confederates 12,500 out of fewer than 40,000, or about 30 percent. The battle ended in stalemate, but became a technical Union victory when the battered Confederates retreated to Virginia.

The one thing accomplished by all that bloodshed was that the narrow victory enabled Abraham Lincoln to issue the Emancipation Proclamation -- a decision that transformed the Civil War into a social revolution, and changed the course of the war and of American history. Knowing as we do the outcome of that decision, we may fail to appreciate the risks Lincoln ran and the kind of courage he displayed. He had to face not only the rage of Southerners and the strong opposition of many Northerners, but the possibility of a military coup by Gen. George McClellan, commander of his largest army, entrusted with the defense of Washington itself.

In that summer of 1862, Lincoln was not the respected president of a stable national government, but the leader of an embattled political movement, struggling to re-establish constitutional authority, vulnerable to the play of uncontrollable social and political forces. In such a revolutionary crisis, with civil authority fragmented and social violence spreading, the traditional constraints of political imagination are loosed, and radical transformations of all sorts become conceivable, even appealing.

The gravest threat to a republic in time of civil strife is the resort to military dictatorship. That had been the fate of every republic, from Athens and Rome to the Second French Republic, established by revolution in 1848 and overthrown by Napoleon III in 1852. As he grappled with the McClellan problem, Lincoln could not be certain that the American republic would not end the same way.

By early July, Lincoln had recognized that a compromise ending to the war had become impossible. The South would not give up until its political will and ability to support armies were broken. For this the Union would need larger armies and more aggressive generals. But Lincoln had also become convinced that ultimate victory required a direct attack on slavery. Slavery was the basis of the South's economic and therefore its military strength -- to break the South's ability to fight, its economy had to be broken.

On a deeper level, slavery was the perennial cause of conflict between the states, and a compromise that preserved slavery would only lead to war for some later generation. The longer, costlier war for the Union that now seemed necessary could be justified only if it removed the root cause of conflict -- although doing so would produce a social revolution.

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To carry out this strategy he needed a general whose determination and commitment matched his own. What he had instead was Gen. McClellan, who was adamantly opposed to Lincoln himself and to anything that smacked of abolition.

McClellan was the commander of the Army of the Potomac, a West Pointer, and a superb organizer of the volunteer army. Although he presented himself as a pure professional, McClellan was in fact a partisan Democrat, and in effect the symbolic leader of the opposition party. His platform (and his party's) was the antithesis of Lincoln's: to preserve slavery as a guarantee of white supremacy, and to offer easy terms for readmitting Rebels to the Union -- and the Democratic Party's voting lists.

McClellan was nicknamed "the Young Napoleon," and like Napoleon he fostered a cult of personality, centering his army's loyalty in himself. He also had Napoleonic grandiosity. "I have no choice," he told his wife, "the people call upon me to save the country -- I must save it & cannot respect anything that stands in the way." He saw himself as the chosen instrument of Divine Providence -- even his defeat by Gen. Robert E. Lee in the Seven Days Battles (June 25-July 1) was the a sign of God's favor.

He would write his wife: "I think I begin to see His wise purpose in all this. If I had succeeded in taking Richmond now the fanatics of the North might have been too powerful & reunion impossible."

The "fanatics" he had in mind were the strong anti-slavery Republicans in Lincoln's administration, and he regarded them "as the enemies of the country & of the human race" and dreamed of the day when "my foot will be on their necks."

He had utter contempt for Abraham Lincoln, spoke of him as "the original Gorilla," thought him a well-meaning but weak-minded "baboon" surrounded by fools and traitors. He refused to inform the president of his plans and deliberately flouted or balked at Lincoln's orders. Early on, he had flirted with the idea of a congressionally sanctioned "dictatorship," which would give him control of war policy, leaving Lincoln as a figurehead. Failing that, he campaigned to force Lincoln to fire Secretary of War Edwin Stanton and General-in-Chief Henry Halleck, which would leave McClellan as the dominant voice in military affairs.

What made him dangerous to the nation he served was his continual flirtation with the idea of military dictatorship. He could write delightedly to his wife, "I have commenced receiving letters from the North urging me to march on Washington & assume the Govt!!"

His illusions flourished in the hothouse of his headquarters, where loyalists mirrored and exaggerated his moods. A general who visited them in early July thought their threats against the government were "flat treason." Lincoln had attempted to sideline McClellan after his defeat in the Seven Days, and shifted most of his troops to an army in northern Virginia commanded by McClellan's rival, Gen. John Pope. McClellan was determined to destroy Pope, and while the latter was fighting and losing the battle of Second Bull Run (Aug. 28-Sept. 1), McClellan deliberately obstructed all efforts to reinforce him.

As Pope's defeated troops filtered back into Washington on Sept. 1, the Lincoln administration faced its worst crisis. The only general who could pull the army together to resist Lee was the man most adamantly opposed to everything Lincoln wanted -- a man who would do things his own way or not at all, a man who flirted with the idea of dictatorship. And to put him in command, Lincoln would have to defy his strongest Cabinet ministers, Treasury Secretary Salmon P. Chase and Stanton, who thought McClellan should be shot as a traitor. In spite of that, Lincoln gave command of the army to McClellan on Sept. 2 -- and the next day learned that Lee's army was invading Maryland.

In giving McClellan command of the field army, Lincoln ran the gravest kind of risks. If McClellan failed to repel Lee's invasion, Lincoln's administration might be repudiated by voters in the upcoming midterm elections. If McClellan succeeded, his prestige would be enormous, and he was bound to use it to overthrow Secretary of War Stanton and gain control of military policy. Even as his army was advancing toward its confrontation with Lee at Antietam, McClellan's officers were using leaks to selected reporters to demand Stanton's removal and threaten a "countermarch on Washington" if Lincoln did not give way to McClellan.

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Though Lincoln needed a McClellan victory to enable him to issue the Emancipation Proclamation, he had good reason to fear that a victorious McClellan might respond by leading the army in open revolt. As it happened, after the proclamation was issued, a New York Herald reporter would write that the anger of McClellan and his staff gave "large promise of a fearful revolution that will startle the Country and give us a Military Dictator" and "a change of dynasty."

Lincoln accepted the risks inherent in using McClellan, because it offered the only way to repel Lee's invasion and win the victory he needed to issue the Emancipation Proclamation. He also trusted the strength of the nation's republican institutions, its people's commitment to the Constitution -- and his own ability to use those strengths effectively. As he would later say to another would-be Napoleon, Gen. Joseph Hooker, "What I now ask of you is military success, and I will risk the dictatorship." Considering the circumstances, few presidential decisions have been as courageous.

Writer Richard Slotkin is Olin Professor of American Studies Emeritus at Wesleyan University and author of "The Long Road to Antietam: How the Civil War Became a Revolution." Readers may send him email at He wrote this for The Free Lance-Star in Fredericksburg, Va.