Posted by John Keller

On this date, September 17, more Americans were killed, wounded, or missing than on any other single day in American history -- not on 9/11, not in the Argonne Forest, not even at Pearl Harbor. It was outside of a sleepy little town in Western Maryland called Sharpsburg, along a meandering stream locally known as Antietam Creek.

The year was 1862, and this quiet place -- as relatively unpopulated today as it was then -- was where two great armies of the American Civil War crashed into each other in dawn-to-dusk bloodshed that produced 22,717 casualties on both sides -- 3,650 killed, 17,300 wounded, 1,770 captured or missing -- in an epic fight that ultimately resulted in the Emancipation Proclamation that freed Americans held in slavery.

It was America's bloodiest day.

Why it happened along Antietam Creek was largely an accident. Robert E. Lee's Confederate Army of Northern Virginia, for the first time, was invading the North, largely to relieve Virginia, which had been under prolonged Union attack for more than a year.

Mountains and Rebel cavalry screened Lee's movements from prying federal eyes, and Union commander George Brinton McClellan had no idea where the Confederate army would strike ... until five days before the battle when a Union private from Indiana found in the grass where he was camped a piece of paper wrapped around three cigars.

The brand of cigars and who got to smoke them is lost to history, but the paper containing them turned out to be General Lee's Special Order 191, which detailed the Southern army's entire plan for the invasion. It didn't take long for that lost order to make its way to General McClellan, who managed to get his lumbering blue-clad army moving to intercept the Confederates, on a course for Antietam Creek.

The sun rose that September 17 with the Union and Confederate armies facing each other over terrain that came to have ominous names -- the West Wood, the Cornfield, the Bloody Lane, and the Burnside Bridge. When fighting ended, the Confederate army was still intact, but damaged so severely that General Lee ordered it back across the Potomac and into Virginia, ending the invasion.

The next time these armies would face one another on Union soil would be less than a year later around a small Pennsylvania town called Gettysburg.