It remains the single-bloodiest day of fighting on American soil and it was fought 150 years ago this week in the Civil War.

The Battle of Antietam began on Sept. 17, 1862, when Union forces commanded by Maj. Gen. George B. McClellan clashed with Confederate rivals under the direction of Gen. Robert E. Lee in a cornfield at Sharpsburg, Md., or Antietam.

The bitter battle raged around such spots now burned into the American history books as Dunker Church and the Sunken Road. Marked by attacks and counterattacks, the pitched 12 hours of fighting claimed at least 23,000 wounded, missing and killed.

When the roar of combat was over, Lee’s limping Army of Northern Virginia was forced to withdraw on Sept. 18 amid last skirmishing to cross the Potomac River southward to the safety of Virginia’s Shenandoah Valley.

Neither side could claim this as an outright tactical victory. Yet Antietam was, nonetheless, a turning point in the Civil War and seized upon as a strategic victory for the Union. The federal forces, though they failed to pursue Lee’s retreating army, had shown it could stop the savvy Confederate commander’s opening invasion of the North.

Historically, the battle’s aftermath gave President Abraham Lincoln the moment he needed to roll out his preliminary Emancipation Proclamation. Within days, Lincoln would declare the Civil War had the double aim of both keeping the Union intact and abolishing slavery.

The Associated Press, reporting on the fighting soon after the shooting subsided, said hundreds of civilians watched from surrounding hills. “The sharp rattle of 50,000 muskets and the thunder of a hundred pieces of artillery is not often witnessed,” AP’s correspondent wrote. “It is impossible at this writing to form any correct idea of our losses or that of the enemy. It is heavy on both sides.”

AP added that so fierce was the fighting that the dead were “thickly strewn over the field and in many places lying in heaps.”