In the late summer of 1862, the war in the East had been going badly for the United States, and Robert E. Lee set out to make it worse. For more than a year the fighting there had been waged on Southern soil, with Northern armies grinding into Virginia and getting thrown back out again. Lee had just broken one of those armies near Washington at the end of August. Now, instead of waiting for the next blow, he marched his Army of Northern Virginia north, across the Potomac River and into Maryland. It was his first invasion of the North.

The gamble was about more than ground. A Confederate victory on Northern soil might convince Britain or France to recognize the Confederacy as a country, to treat the South as a real nation rather than a rebellion. Recognition could mean trade, money, perhaps even intervention. A loud win above the Potomac might also break Northern nerve in an election year. Lee was not merely raiding Maryland. He was reaching for the things that win wars without battles.

The man sent to stop him was Major General George B. McClellan (North), commander of the Army of the Potomac. McClellan was a gifted organizer and a beloved leader of men, with one flaw that shaped everything that followed. He would bring roughly 87,000 men toward this fight against Lee’s 38,000, better than two to one, and spend the whole campaign convinced that he was the one outnumbered. The bigger army, certain it was the smaller, was the central fact of the day.

To invade with an army half the size of his enemy’s, Lee took the riskiest course open to a general in front of a stronger foe: he divided it. He sent part of his force under Major General Thomas “Stonewall” Jackson (South) back across the Potomac to capture the Union garrison at Harpers Ferry, while the rest pushed on into Maryland. It was a bet that McClellan would be too slow and too cautious to catch the scattered halves before they reunited. Given the man he was facing, it was a reasonable bet.

Then McClellan got an extraordinary piece of luck. On September 13, near Frederick, Maryland, a Union soldier named Barton Mitchell of the 27th Indiana was resting in a field a Confederate unit had just left when he noticed three cigars on the ground, wrapped in a piece of paper. The paper mattered far more than the cigars. It was a copy of Special Order 191, Lee’s own campaign orders, laying out exactly how he had divided his army and where each piece was going.

McClellan now knew his enemy was split, scattered, and beatable in detail: he could destroy one piece before the others rejoined. He is reported to have said it himself:

“Here is a paper with which if I cannot whip Bobbie Lee I will be willing to go home.”

Then he waited. True to form, McClellan held still for about eighteen hours before he moved, time enough for Lee to learn the order had been lost.

When McClellan did move, the orders pointed him straight at Lee’s scattered pieces. On September 14 his army forced the gaps in South Mountain, the long ridge standing between the two armies, in a day of hard fighting that drove the Confederate defenders off the passes. It was a Union victory, but the delay bought Lee the hours he needed. Jackson finished his work to the south: on September 15 the Harpers Ferry garrison surrendered, more than 12,000 men laying down their arms, the largest surrender of United States troops in the war. Lee called the rest of his army together along a low ridge near a town called Sharpsburg, behind a winding stream called Antietam Creek. McClellan still had the bigger force and the advantage of knowing the plan, but the chance had narrowed from catching the Confederates scattered to catching them as they gathered. On the morning of September 17, the two armies stood across Antietam Creek from each other, and the bloodiest day in American history began.