This is the carnage near Dunker Church. (AP/Library of Congress)

The boy following her to the battlefield at Antietam on Sunday squirms. “Happy thoughts,” he says.

“Even if you killed all the people who were here today, it still wouldn’t be as many.”

When you put it like that, it sounds even more alarming than usual.

How do you remember something so large and so incalculably bloody? On the page, the numbers are so startlingly large as to be incomprehensible. More than 23,000 casualties? There is nothing tangible about a number so huge.

The Battle of Antietam, the bloodiest day of the Civil War and one of the bloodiest days in history, took place near an unprepossessing town.

Sharpsburg today looks much as it did 150 years ago, when the troops came marching in. Its population is around 700, scarcely more than it was 200 years ago. It is proud of this fact. “It looks very much as it did in the 1800s.  The people of Sharpsburg are content with that,” notes the town Web site, “– they are proud of their history and heritage.”

From D.C., it is a drive of almost two hours to Sharpsburg, down winding, precarious roads, through Historic Town after Historic Town. There is a certain component of travel in time associated with travel in space. Go from a small city to a bigger city, and the music that is playing everywhere is the music that you will hear everywhere in the small city a few months later. Leave a small city and travel to a smaller one, and you start hearing music you have not heard in years.

In Sharpsburg, there is a great deal of music — from the Wildcat Regimental Band, from the service in Dunker Church, songs from 150 years ago. The landscape looks mostly the same. The corn waves near the Bloody Lane the same way it waved 150 years ago.

Stroll down Bloody Lane and there are markers everywhere noting how many fell in each few feet of ground. It soon boggles the mind. The names of unremarkable landmarks in this country landscape have an eerie echo to them. The Sunken Road. Burnside Bridge. Dunker Church. Something happens to a place when you fight over it — not only to the ground but to the names themselves.

Anachronisms come with the territory. The reenactment is a few miles away from the field itself, and I only make it in time to see the retreat, a slow snarl of cars and horse trailers and campers making their way back to the present. As a regimental band plays minstrel songs, a woman reads “Fifty Shades Freed.” Romney campaign signs garland the countryside leading up to the field. An airplane flies over Bloody Lane.

Remembering is hard. In the case of the Civil War, it is largely an extracurricular activity. As universities shed their military history departments, to trek over rocky battlefields trying to figure out who fell where and how and why becomes largely an act of private devotion. Most of us are left alone with the sterile numbers on the page. Those who don’t remember history may be doomed to repeat it, but those who do remember history are doomed to reenact it.

After the scheduled skirmish, the reenactors trickle over to the actual battlefield and lounge in their thick woolen uniforms, with clanking canteens, listening to historians lecture.

The battle, James McPherson argues, was absolutely pivotal. Only afterward could Abraham Lincoln issue the Emancipation Proclamation, after waiting so that his edict freeing the slaves in Confederate territory did not sound like the desperate last-ditch measure of a losing cause. Antietam was the closest thing to a victory that the Union had had for some time. It came just a few days after Confederates captured the arsenal at Harper’s Ferry and its store of clothing and munitions. It could have been a resounding Confederate defeat had Union General George McClellan evinced more willingness to engage. While the battle raged, he held thousands of troops in reserve, wasting a numerical advantage of nearly 2 to 1. But it was enough of a victory. Gen. Robert E. Lee’s string of victories was over, and his invasion north of the Potomac had been halted.

“I think the time has come now,” Lincoln said. “I wish it were a better time.”

After Antietam, the war was explicitly about slavery, making it almost impossible for Great Britain to intervene. It was a war, as Lincoln noted in his Second Inaugural Address, in which every drop of blood drawn by the lash might be paid by another drawn with the sword. We remember this powerful consequence. But it would not have happened without the thousands who fell in Antietam — this uncomfortably tangible place, in the cornfields and by the bridges and in the sunken road.