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The Union called it Antietam for the creek running through it. To the Confederacy, it was Sharpsburg, for the town backing the rebel lines. But by either name, it remains, at its sesquicentennial, the bloodiest day of battle in American history. The terrible carnage, unimaginable in our day, set the stage for Abraham Lincoln's Emancipation Proclamation.

Civil War photographer Matthew Brady's haunting photos of a mere fraction of the fallen, taken two days after the battle, simply could not convey the level of violence upon such killing grounds as Burnside Bridge, Antietam Creek, The Cornfield, Dunker Church, and the Sunken Road, aptly renamed Bloody Lane.

"I have heard of 'the dead lying in heaps,' but never saw it till this battle. Whole ranks fell together," wrote Union artillery Capt. Emory Upton.

The butcher's bill on this day 150 years ago: more than 22,000 killed, wounded or missing in just 12 hours.

Though largely a stalemate, the desperate combat on the rolling hills of Maryland ended an incursion of Confederate Gen. Robert E. Lee's Army of Northern Virginia into the North and gave Lincoln what he'd been impatiently awaiting: At least the semblance of a victory so he could, at long last, declare an end to slavery, the euphemistic "peculiar institution" that formed the foundation of the Confederacy's economy and ability to wage war.

With Lee's orderly withdrawal across the Potomac River, the inconclusive outcome also robbed the South of a chance to gain recognition from Great Britain and France, crucial to a brokered peace. For in the first 17 months of the war, the South had repeatedly triumphed on the battlefield, and the conflict had been hugely unpopular in the North.

Confederate hopes for a quick and acceptable end died at "Bloody Antietam" and were buried five days later by Lincoln's Proclamation. The import of the document was not lost on Jefferson Davis, president of the Confederate States of America: "A restitution of the Union has been rendered forever impossible."

Lincoln's Proclamation had several purposes. For one, it imposed an end-of-year deadline for the secessionist states to cease their rebellion. But historian Richard Slotkin, among others, believes that Lincoln knew the South would never agree, and, "In issuing the Proclamation he knowingly embraced a war to the finish."

Nor was Lincoln's bold action entirely altruistic. In a single stroke he had greatly expanded the powers of his presidency, and, in permitting the enlistment of freed slaves, fortified the Union ranks with 180,000 new recruits by war's end. Slotkin maintains that "the war would have been lost without them."

As for Antietam, Gen. George B. McClellan's failure to pursue Lee's battered army with a Union force nearly twice its size brought to a head Lincoln's deep frustration with his commander's chronic reluctance to engage an enemy whose numbers he consistently inflated. Within two months, "Little Mac" was gone. Lincoln wanted a relentless aggressor, and eventually found one in Ulysses S. Grant. That alone made Antietam significant.

But then, just as now, it was the ferocity of the fighting in close quarters, and the number of casualties in so short a time, that made the bloodbath at Sharpsburg unique in our history. Participants and historians have always struggled to describe the sheer horror of that day:

"No tongue can tell, no mind conceive, no pen portray the horrible sights I witnessed this morning." — Capt. John Taggert, 9th Pennsylvania Reserves

"Where is your division?" Confederate Gen. John Bell Hood was asked. "Dead on the field!" he replied.

"As the sun sank to rest ... the last sounds of battle along Antietam Creek died away. The corn and trees so fresh and green in the morning, were reddened with blood." — Francis W. Palfrey, historian and Antietam veteran.

"Soldiers who experienced several battles — Antietam, Gettysburg and many others in the eastern theater — often looked back upon Antietam as by far the most horrible." — James M. McPherson, author of "Crossroads of Freedom: Antietam"

"One of the enemy's grape-shot had plowed a groove in the skull of a young fellow and had cut his overcoat from his shoulders. He never stirred from his position, but lay there face downward, a dreadful spectacle. A moment after, I heard a man cursing a comrade for lying on him heavily. He was cursing a dying man." — David Thompson of the 9th N.Y. volunteers, on the fighting at Antietam Creek

"War is a dreadful thing. ... Oh, my God, can't this civil strife be brought to an end?" — Clara Barton, founder of the American Red Cross, who nursed the wounded so close to the fighting that a bullet ripped through her sleeve and killed the man she was treating.

"Of all the days on all the fields where American soldiers have fought, the most terrible by almost any measure was September 17, 1862. ... So intense and sustained was the violence, a man recalled, that for a moment in his mind's eye the very landscape around him turned red." — Stephen W. Sears, Civil War historian