RICHMOND, Va. — In the PBS American Experience documentary “Death and the Civil War” premiering tonight, bloated Union and Confederate bodies are shown scattered on battlefields and in trenches and bleached skulls and body parts are stacked like cordwood.

As the title suggests, death is the central theme of this moving, extraordinarily graphic film based on Harvard University President Drew Gilpin Faust’s acclaimed book “This Republic of Suffering.”

It chronicles how utterly unprepared a divided nation was for the mountains of dead the Civil War would produce and how that experience forever changed the way the country treats the men and women who give their lives for their nation.

“No one thought that this was going to go on this long. No one thought there would be deaths on this scale,” Faust said in an interview with The Associated Press. “I think the South was stunned that the North didn’t just let them go.”

The war dead were left to rot where they lay mortally wounded. There was no ambulance corps to retrieve the dead or national cemeteries like Arlington in which to bury them.

On the home front, mothers and fathers, wives and girlfriends often never learned the fate of their loved ones. There was no system to identify the dead or notify families, or recompense for their loss.

To this day, the precise number of Civil War dead remains elusive, with the estimated toll increasing to 750,000 based on the research of J. David Hacker, a demographic historian at Binghamton University in upstate New York. That number, cited by the documentary and a growing number of historians, is much higher than the 600,000 that had been cited for decades. Some believe it may be as high as 850,000.

“Death and the Civil War,” produced and directed by multi-Emmy Award-winning filmmaker Ric Burns, will air for two hours on the 150th anniversary of Antietam — the single bloodiest day on U.S. soil. The film draws heavily on historic battlefield photographs, the narrative of historians and the words of soldiers in letters home. It also includes the commentary of poet-undertaker Thomas Lynch and Joint Chiefs Chairman Mike Mullen.

The film opens with a reading of the bloodstained letter from Confederate Pvt. James Robert Montgomery, 26, to his father in Camden, Miss., after the younger Montgomery lay dying from a terrible shoulder wound.

“Dear Father, this is my last letter to you,” Montgomery writes. “I am very weak but I write to you because I know you would be delighted to read a word from your dying son.”