The heroes of The Birth of a Nation are the Ku Klux Klan. The film itself is adapted from a novel by virulent racist Thomas Dixon. The Birth of a Nation is deeply informed by the mythology of the Lost Cause. It depicts antebellum southern society as harmonious, slaves as basically happy, and whites as generous and loving masters. The war leads to the collapse of this society, and Reconstruction is portrayed as an ordeal for the South. Corrupt carpetbaggers take advantage of Southerners, and blacks, who are portrayed as almost animalistic, run amok.
The assumption is that, in the absence of slavery, (or, indeed, Jim Crow laws) that African-Americans will behave lawlessly. Throughout the film, Griffith stoked white fears of anarchy and miscegenation, depicting the black-dominated South Carolina legislature as passing a law legalizing interracial marriage, and, most disturbing for whites, showing Flora Cameron's pursuit by a lascivious black man. She leaps to her death rather than submit to his advances. In the film, the Ku Klux Klan is formed to set things right, i.e., to return whites to their "rightful" position. They capture and kill the black man who pursued Flora Cameron, they fight off the South Carolina militia, dominated by blacks, and they use the threat of force to ensure that black politicians are voted out of office. It is the Klan that restores normalcy and order, and protects the South from the ravages of corrupt carpetbaggers and former slaves.
This message, of course, is reprehensible from a modern standpoint, but it reflected a developing Lost Cause mythology that sought to portray the South as blameless for the Civil War and as the passive victims of Reconstruction, which was portrayed as an oppressive regime, in no small part because of the enfranchisement of blacks. While widely praised for its advances in cinematography, the film is therefore an artifact, a document that demonstrates the extent to which Lost Cause mythology and white supremacy were linked. Audiences across the country screamed and booed at Flora's death, and cheered when the Klan killed Gus, her pursuer. It also is credited by some historians led directly to the rebirth of the Ku Klux Klan. But in one way, the film actually does get one detail of Reconstruction right, a detail that is often overlooked by adherents of the Lost Cause. Southern whites really did overthrow ("redeem", in the parlance of the day) Reconstruction governments using force. In South Carolina, images of Klansmen and Wade Hampton's "redshirts" forcing blacks away from polling places were a historical reality, though historians do not portray their actions in quite as noble a light as Griffith does.