Antietam was really three battles fought one after another, rolling from the northern end of the line to the south across a single September day. The first began at dawn, on the northern end, in a farmer’s cornfield.

Major General Joseph Hooker (North) sent his I Corps (a corps being the army’s largest building block, on the order of ten to fifteen thousand men, made up of smaller divisions and brigades) smashing into the left of Lee’s line, the wing held by Major General Thomas “Stonewall” Jackson. They drove straight through a field of standing corn owned by a farmer named Miller. The corn was tall and ripe, taller than a man, and it hid soldiers until they were almost on top of one another. Lines surged forward, were shot to pieces, fell back, were replaced, and surged again. Miller’s Cornfield changed hands, by some counts, roughly fifteen times in a few hours. Hooker wrote in his official report that the corn had been cut “as closely as could have been done with a knife, and the slain lay in rows precisely as they had stood in their ranks a few moments before.”

For a while it was the Confederate left simply absorbing the blow and bleeding. Then the South hit back. Brigadier General John Bell Hood’s (South) division had been pulled out of the line just before dawn to cook the first hot breakfast the men had seen in days, and they were yanked off their half-cooked food and thrown straight into the Cornfield, hungry and furious. Hood’s counterattack drove the Union troops back out of the corn and shoved the morning’s tide the other way. It is much of why the field changed hands so many times instead of falling once and staying fallen. It also cost him nearly everything. Hood’s division was wrecked doing it, losing somewhere around sixty percent of its men in the corn. Asked afterward where his division had gotten to, Hood is supposed to have answered that it was dead on the field. Whether or not he said it in those exact words, it was very nearly the truth.

Senior officers were falling as fast as the men they led. Around mid-morning Hooker himself was shot through the foot and carried from the field, and with him went the one commander driving the whole northern attack; leadership of the I Corps passed on the spot to Brigadier General George Meade (North). The morning was not one corps but a relay of them. When Hooker’s drive stalled, a second wave came up behind it, the XII Corps under Major General Joseph K. F. Mansfield (North), feeding fresh men into the same killing ground. Mansfield, a white-bearded old soldier of long service, rode forward to position his troops and was almost immediately shot down; he died of the wound the next day. Two senior Union generals were down before noon, one carried off and one dying, and a third corps was still waiting its turn.

The fighting swirled around a small whitewashed building on the edge of the field, the Dunker Church. It belonged to the Dunkers, a German Baptist sect whose faith was built on plainness and pacifism; they believed Christians must never take up arms, never fight, never kill. Their plain meetinghouse, raised by people who had renounced violence as a sin, became the bullet-scarred landmark of the worst single day of violence the country had yet seen. The men of two armies fought and died around the doorstep of a church whose congregation believed all of it forbidden.

A little later, on that same northern end, a Union division under Major General John Sedgwick (North) pushed into a stand of trees called the West Woods, marching in tight formation straight into a trap. Confederate troops were waiting on three sides. They opened fire into the packed Union ranks from the front and the flank (the exposed side of a formation) at once, and the division came apart. In roughly twenty minutes, Sedgwick’s men took around 2,200 casualties. A man getting out alive ran back through his own dead and dying, friends he had marched in with that morning sprawled across the leaves, the wounded grabbing at his legs as he passed, the woods behind him so thick with bodies a man could not cross without stepping on someone. By then the sun was barely up, and the northern third of the field was already covered with the dead.