Colonel Burnside’s brigade at Bull Run. Sketched on the spot by Alfred R. Waud. From New York Illustrated News, Aug. 5, 1861. (LIBRARY OF CONGRESS PRINTS AND PHOTOGRAPHS DIVISION)

In 1986, filmmaker Ken Burns received a copy of a long-forgotten Civil War soldier’s letter that a scholar thought he might find interesting.

Burns, then working on his award-winning PBS documentary about the war, began to read it out loud to his wife, brother and another staff member in his Walpole, N.H., headquarters.

“My dear Sarah,” the letter began, “the indications are very strong that we shall move in a few days. . . . Lest I should not be able to write again, I feel impelled to write a few lines which will fall under your eye when I shall be no more.”

The letter, written in Washington on July 14, 1861, continued, as the author bade a heartbreaking farewell to his wife. Burns could barely finish it, and when he did, he looked up and found the others in tears.

It was the now-famous “Sullivan Ballou letter,” written by the Union officer a few days before his mortal wounding at the Battle of Bull Run, on July 21, 1861 — 150 years ago Thursday.

Later read to the haunting theme song of Burns’s 1990 “The Civil War,” the letter summarized the sacrifice made by the Civil War generation, and struck an emotional note with Americans far removed from the struggle and sentiment of the 1860s.

Viewers clamored for copies of the letter, which closes,

“Sarah, do not mourn me dead — Think I am gone
and wait for thee. . . . ”

At least one newspaper printed transcripts and quickly ran out. A record label reportedly sold tens of thousands of soundtrack CDs from the series, which included a reading of the letter.

It was read at weddings, funerals and memorial services, Burns said.

In 2006, historian Robin Young published a 700-page book about the letter,
Sullivan Ballou’s life, and the grisly postscript to his death, in which his grave was apparently desecrated by angry rebels.

After Burns finished reading the letter aloud, he made two photocopies. He gave one to his staff, for inclusion in the film. He folded the other and put it in his wallet.

Twenty-five years later, as the country marks the sesquicentennial of the Battle of Bull Run, the now-tattered copy of the letter is still in Burns’s beat-up wallet.

“It’s the most beautiful letter I’ve ever read in my life,” he said. “It’s a Grand Canyon of a letter. You can read the strata of meaning. It’s all about love. First and foremost is love of country. . . . It’s about love of government. . . . It’s a love of cause. . . . It’s a love of family.”

Ballou was a 32-year-old major in the 2nd Rhode Island Infantry Regiment, which was dispatched to Washington in the early months of the war.

But before the regiment left Washington for Bull Run, Ballou sat down and wrote to his wife and two children back home in Rhode Island.

He wrote of his faith in the Union cause, and his willingness to die for it. He confessed guilt that his death might harm his family but said his love of country

“comes over me like a strong wind and bears me irresistibly . . . to the battlefield.”

He professed his love for his wife, and hoped to be with her even in death,

“amidst your happiest scenes and gloomiest hours — always, always.”

“If there be a soft breeze upon your cheek, it shall be my breath; or the cool air fans your throbbing temple, it shall be my spirit passing by.”

The letter was never mailed but was returned to his wife with her husband’s effects.

Ballou was struck in the leg by a cannonball during the battle and died a week later. The battle was a Union defeat.

The letter, a copy of which resides in the Rhode Island Historical Society, is from the “if I do not return” genre, often written by soldiers on the eve of battle.

“What people confront is their own mortality, and immortality . . . in these letters,” Burns said in an interview.

Andrew Carroll, editor of “Behind the Lines,” a compilation of war letters, said: “It’s the hardest letter . . . for any service member to write. . . . They’re putting all of their heart and emotion into what they know will be their final words.”

“It’s especially poignant when there are children involved,” he said, “because that letter will become sort of the touchstone to their lives, and how they remember their father.”

Although the letter had been largely forgotten before Burns helped resurrect it, Ballou’s case was a sensation during the war, according to Young’s history.

After the Confederates evacuated the area around Manassas in March 1862, officials from Rhode Island returned to the nearby Bull Run battlefield to retrieve the bodies of the state’s dead.

When they did, they found that, according to witnesses, Ballou’s body had been dug up, beheaded and burned by Confederates.

The rebels supposedly were seeking vengeance against a different Rhode Island officer blamed for inflicting high Southern losses in the battle. They dug up Ballou by mistake.

The incident was widely reported in Northern newspapers. Huge crowds turned out when the remains of Ballou and two other officers were brought to Providence via Philadelphia and New York.

But 1862 quickly turned into one of the bloodiest years of the war, and the cavalcade of battles soon erased Ballou from the headlines.

On Sept. 23, 1990, before a public television audience of millions, Burns returned him to the spotlight, using the letter to climax the opening episode of the series.

In the film, Ballou’s anguished words are read by actor Paul Roebling, a veteran of Burns documentaries and “the only person I ever considered handing the Sullivan Ballou letter to,” Burns said.

Burns remembered that Roebling, “a remarkable person,” had trouble reading the letter, because his wife, actress Olga Bellin, was then dying of cancer.

“That was impending on him at the time he read it,” Burns recalled. “And then it wore heavily on him.”

In July 1994, four years after “The Civil War” first aired, and the nation heard the actor read, “Sarah my love for you is deathless,” Paul Roebling took his own life, Burns said.

His wife had been dead for seven years.