The young Confederate soldier, writing home in 1862, could not contain his awe — not of the big battle his unit had just won but of its aftermath.

“Day before yesterday we marched over the battleground,” he told his mother. “All of our men had been buried. But the yankees lay just as they were killed. I never saw such a scene before. There must have been nearly a thousand. Our wagon actually ran over the dead bodies in the road. I don’t suppose the dead yankees from that fight will ever be buried.”

It was a scene that would be repeated again and again over the next three years as the Civil War — the first modern war, fought with giant armies and deadly new technology — turned America into a giant charnel house that was as unexpected as it was horrifying.

“Death would enter the experience of the American people and the body politic on a scale and in a manner no one had ever imagined possible,” says filmmaker Ric Burns in his horrifying documentary Death and The Civil War, airing as an episode of PBS’ American Experience.

Self-important politicians and literary poseurs of the past century, convinced that their own births marked the beginning of history, have labeled everything from World War I to the attacks of Sept. 11 as the moment that the United States lost its innocence.

But Burns’ film and the book on which it is based, Drew Gilpin Faust’s The Republic of Suffering, make a compelling case that if there truly was a single moment at which American innocence vanished, it was the Civil War. Expecting a short and tidy exchange after which everyone would see reason, the public on both sides of the Mason-Dixon line was stunned when the first real battle — at Bull Run, a Virginia creek just 30 miles from Washington, D.C. — took 900 lives. In a few hours, the two armies had suffered half the death toll of the entire Mexican War.

The size and savagery of military encounters increased so quickly that just two years later, the battle of Gettysburg inflicted more casualties than all previous American wars combined. By the time the last Confederate army surrendered in 1865, the war had claimed a staggering 750,000 lives, nearly 2.5 percent of the American population. To comprehend the impact, imagine seven million soldiers coming home from Afghanistan in body bags.

Of course, most of the Civil War’s dead never came home; because dog tags hadn’t been invented yet, half the war’s corpses were never identified. The military was no more prepared than civilian society for the scale of the slaughter. Its combat medical corps was small, lacking even ambulances to evacuate the dead or dying. There were no military cemeteries, much less any system for registering battlefield graves.

The result was a Dantean spectacle that practically defied description, though many tried to capture it in the letters home that give Death and The Civil War its shattering impact. “The dead were almost wholly unburied and the stench arising from it was such as to breed a pestilence,” a Union surgeon wrote his family from the hellish killing fields at the battle of Antietam, where 6,500 died.

A week after the fighting end, he reported seeing “stretched along, in one straight line, ready for interment, at least a thousand blackened, bloated corpses with blood and gas protruding from every orifice, and maggots holding high carnival over their heads.”

Burns, who collaborated with his brother Ken on the epic PBS documentary The Civil War two decades ago, argues that families back home were as horrified by the manner of the war’s killing as much as its volume — that a deeply religious population, obsessed with the idea of “a good death” in a peaceful bed surrounded by friends and family, was shattered by so much carnage under the opposite circumstances.

That contention has at least a whiff of modern psychobabble. The United States had been born in violence less than a century earlier; much of it was still a frontier country, wracked by Indian wars. Plenty of families had face-to-face experience with violent death.

But there’s no denying that the media of the day brought the Civil War home in a devastatingly personal way. Newspapers (much like the famous scene in Gone with the Wind) printed front pages consisting of nothing but columns of names of the dead and wounded; Matthew Brady’s pioneering photographs of battlefield corpses, their faces blacked with decay but still twisted in visible anguish, were exhibited in New York galleries.

And it beggars the imagination to consider what must have been the reaction of a mother who in 1862 opened a letter from her son, a Union cavalryman, penned during a break in the combat at Antietam. “I write in the saddle to send you my love and to tell you that I am very well so,” it began. Further down the page was a P.S.: “I am wounded, so as to be hopeless. Goodbye if so it must be. I think I die in victory.”