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To historians such as Chandra Manning, the fact that the Civil War was fought primarily over slavery seems awfully obvious. But, as Manning found out at a wedding she attended in Albany, N.Y., back in 2005, the layman can sometimes have other ideas.

During a conversation with Manning, a gentleman insisted to her that the Civil War, in fact, had little to do with slavery. Actually, he explained to Manning (unaware of her profession) for more than an hour, the Civil War was all about economics—the industrial North dominating the agrarian South, never mind that the North was hardly industrialized by the mid-19th century.

“It emphasized to me the disconnect between historians and the public,” says the 36-year-old Manning, now a history professor at Georgetown University. “And it’s not a Southern habit; it’s a national habit.”

The Alexandria resident’s debut book, What This Cruel War Was Over: Soldiers, Slavery, and the Civil War, attempts to break that habit. The book, published earlier this month by Knopf, argues that slavery was crucially important even to a group of people long assumed not to care about the morality of the “peculiar institution”: the men who fought the war.

What This Cruel War Was Over grew out of Manning’s doctoral dissertation at Harvard, which examined the attitudes of average soldiers on both sides of the conflict. As a teaching assistant for a Civil War class there, she had led her students in a lively discussion about a book concerning the experiences of rank-and-file soldiers during the war. But the conversation ground to an abrupt halt: “I asked, ‘What was the difference between Union and Confederate soldiers?’ ” Manning says. “Dead silence. So I got to thinking.”

And she got to researching. “I’d pack a backpack, get a bus or train ticket, and I took myself from New England to Georgia to Kansas,” she says. All told, Manning devoted two summers plus a semester to research.

Manning relied mainly on letters culled from state historical and university archives and other libraries, as well as camp newspapers—unofficial periodicals that have been rarely used by historians. Manning’s methods were meticulous: For each soldier whose letters she read, Manning constructed a data sheet about his background to provide context for the contents of the letters. She also created thematic files that she organized chronologically in order to see if attitudes changed over time. When she discovered her slavery file was the biggest, she knew she was on to something.

“Easily two-thirds” of the soldiers whose letters she read discussed slavery at some point, Manning says. “I thought it would be the great un-talked-about.”

The idea that Civil War soldiers were somehow indifferent to slavery, Manning says, is relatively recent. Many historians had seized on a comment by Union Gen. George McClellan that predicted mass desertions among his soldiers should emancipation come to pass. That never happened. On the other side of the war, many had assumed that an army consisting largely of nonslaveholders would never fight for slavery. But What This Cruel War Was Over proves those who never owned slaves were invested in preserving the institution for their own reasons.

“As far as Confederates were concerned, they saw the Yankees as invaders,” Manning says, adding that Southerners thought those invaders were threatening to abolish the cornerstone of the Southern way of life for slaveholders and nonslaveholders alike.

Still, Manning doesn’t expect her book to convince all the armchair historians. “I think they see the slavery argument as unsophisticated,” she says. “They think it sounds sophomoric.”

Manning discusses and signs copies of her work at 5 p.m. Sunday, April 15, at Politics and Prose, 5015 Connecticut Ave. NW. Free. (202) 364-1919.