Ira Berlin, a historian who sifted through millions of documents to revive the voices of ordinary African-Americans from the struggle for emancipation, and who helped demonstrate that slavery was a complex, ever-evolving institution at the core of American history, died June 5 at a hospital in Washington. He was 77.

The cause was complications from multiple myeloma, said his children, Richard Berlin and Lisa Berlin Wittenstein.

For nearly a century after the end of the Civil War, historians treated slavery as a “footnote or exception,” a side issue to the story of liberty in America, said Eric Foner, a Pulitzer Prize-winning historian of the Civil War and Reconstruction. Scholars often viewed the institution in romantic terms, arguing that it benefited slaves as well as white plantation owners.

Dr. Berlin, along with historians such as David Brion Davis and Eugene Genovese, upended that picture, Foner said, and “really put the history of slavery at the center of our understanding of American history,” impacting everything from economics to culture.

A onetime chemistry student at the University of Wisconsin, Berlin turned to the study of African-American history while marching for racial justice in the 1960s. He went on to make major contributions as both a documentarian and a writer at the University of Maryland, where he once served as dean of the College of Arts & Humanities and, in 1976, became the founding director of the Freedmen and Southern Society Project.

Berlin had recently completed his first book, Slaves Without Masters (1975), a prize-winning account of the quarter of a million free African-Americans who lived in the South before the Civil War, when he was “poking around the National Archives, looking for something to write,” his son said in a phone interview.

He found his subject when one of the Archives’ first African-American staffers, Sara Dunlap Jackson, “walked him into the room where the Freedmen’s Bureau’s papers had been kept, basically untouched, for a hundred years. This woman, a descendant of slaves herself, who started with the bottom-rung job at the Archives, opens this door and more or less hands him the keys to his entire career.”

Through his Freedmen and Southern Society Project, which Berlin once described as “just an attempt to remember what Sara had forgotten,” he led an effort to publish the highlights of the Archives’ materials from the emancipation era.

Millions of documents, from freed slaves as well as former slaveholders, were trimmed to about 50,000 and then organized, transcribed, annotated and presented in a sweeping series titled Freedom: A Documentary History of Emancipation, 1861-1867. Six of a projected nine volumes have been published to date.

“That series really changed the way that people thought about the Civil War,” Foner said in a phone interview, adding that the documents demonstrated the importance of black soldiers to the war effort, and helped expose the frequently overlooked role that African-Americans played in pressing for their own freedom.

Berlin prized the detective-like aspect of his work as a historian, and once told The Washington Post he sometimes stopped his team’s work after a revelatory find, to “have public readings in the archives.”

Among their discoveries was the story of Edwin Belcher, a Union veteran who passed as a white man during the Civil War and then “crossed over the street” again, Berlin said, to become a successful African-American politician.

There also were letters from black soldiers to their former masters, calling for their enslaved relatives to be freed, and from men such as Cpl. James Henry Gooding, who wrote to President Abraham Lincoln: “Now the main question is. Are we Soldiers, or are we LABOURERS … Today, the Anglo Saxon Mother, Wife, or Sister, are not alone, in tears for departed Sons, Husbands, and Brothers. The patient Trusting Decendants of Africs Clime, have dyed the ground with blood, in defense of the Union, and Democracy.”

His most acclaimed work, Many Thousands Gone (1998), traced the first two centuries of slavery in North America.