Many people already know the grisly statistic from the bloodiest day of war on American soil.
Nearly 23,000 dead, missing or wounded.
Fighting broke out around dawn Sept. 17, 1862, when Union Gen. Robert B. McClellan's troops clashed with Confederate Gen. Robert E. Lee's men at Antietam. The battle raged for about 12 hours.
When it was over, a clear victor did not immediately emerge.
Some historians call the Battle of Antietam a turning point for the North, or they at least hand it to the Union as a strategic gain that, once the gun smoke cleared, it would have massive political and social implications -- including President Abraham Lincoln's decision to issue the Emancipation Proclamation days later.
Confederate soldiers were forced to retreat across the Potomac just weeks after Lee had tried to stake his first foothold in the North.
It is hard to say the exact number of troops that fought on that bloody day because record keeping was not precise, according to Jessica Cannon, assistant professor in the University of Central Missouri's Department of History and Anthropology.
Some soldiers deserted and others waited on the sidelines, said Cannon, a 1998 graduate of Brunswick High School who wrote her dissertation on Civil War-era Maryland.
McClellan had about 75,000 soldiers, while Lee's army had roughly 45,000. That number includes a last-minute infusion of about 15,000 troops who arrived that afternoon from Harpers Ferry, Cannon said.
Almost 4,000 soldiers died in battle on Sept. 17. Thousands of more deaths would follow.
Frederick in commotion
Photos showing bodies of soldiers, taken by Alexander Gardner days after the battle, were among the first to depict the horrors of war.
"America has never seen the kind of carnage ... nothing compares to it in American history," said Michael Powell, a professor of history and political science at Frederick Community College. Powell also teaches history at Hood College.
"When you think about all the women who lost husbands, children who lost fathers, and families who lost love ones, it's staggering," he said.
Within days, wounded soldiers began arriving in Frederick in waves. Jacob Engelbrecht documented the arrivals in his diary. A Sept. 23, 1862, entry speaks of a "town in commotion."
"Our little city is all day long and part of the night one continued bustle of moving of wagons, ambulances, etc. bringing wounded medical and hospital stores. At the depot they have a stock of mattresses nearly as large as a small house and such quantity of bed steads, medicine boxes etc. you never did see. Some days about from 500 to 1,000 wagons pass our street," Engelbrecht wrote.
Most of the city's churches were transformed into hospitals as nearly 10,000 wounded were treated in Frederick in the aftermath of Antietam. That number was larger than the population in the city at the time, said George Wunderlich, executive director of the National Museum of Civil War Medicine.
The civilian death rate increased. Women who laundered soldiers' bloodstained bandages and linens became sick from infection, Wunderlich said. The soldiers were not only wounded at Antietam; some were already sick with disease.
"We know there were men of Frederick who died of typhoid fever," Wunderlich said.
On the national and world stages, Antietam's ramifications would forever shackle the Confederates, historians say.
The Confederates came to Maryland, a slave-holding Union state, thinking they would find its residents sympathetic to their cause. Instead, Lee's soldiers found Frederick residents divided in their loyalties.
After Antietam, the Confederates "never again launched a campaign northward with the expectation that Maryland would join the Confederacy," Powell said.
More importantly, the Union's strategic gain laid the foundation that Lincoln had waited for in order to launch the Emancipation Proclamation. That proclamation, issued Sept. 22, 1862, declared that slaves in rebel states would be considered free beginning Jan. 1, 1863.
The war now had a twofold purpose.
There was the Union fighting to preserve itself, said Keith Snyder, park ranger and chief of interpretation at the Antietam National Battlefield.
"Now we were also fighting for the freedom for four and a half million Americans," he said.
Lincoln had laid out his proclamation before Antietam, but "the Union Army was getting their butt kicked at every turn," Wunderlich said. Lincoln wanted a Union victory so that "politically, it was not a desperate measure."
"We've just won this important battle so now we release the Emancipation Proclamation in strength, not in weakness," Wunderlich said.
The Confederacy's failure to prevail at Antietam also cost it the legitimacy it sought from Great Britain and France, according to historians.
The Confederates had hoped to turn diplomatic recognition into eventual financial or military assistance, Cannon said.
"It was a huge blow to them."
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