Okay, having seen the second half of "Birth of a Nation" I can understand where all the controversy comes from! Basically, it's a recruiting ad for the KKK. If you were a viewer who didn't have any understanding of history, you'd see these hooded vigilantes as pursuers of justice and law and order who provide some relief and ultimately national unity at a time when cruel, corrupt conquerors--most of whom happen to be black!--are letting loose their depradations on a beaten people.
Through the film, the focus is on the friendship of two sets of sons and daughters. The Stonehams are the progeny of a powerful Northern abolitionist Congressman based on Thaddeus Stevens, and the Camerons are the sons and daughters of a doctor and plantation owner in South Carolina. The respective sons are good chums, and are also romancing their friends' sisters--a truly intertwined relationship. (Question--is it incestuous to date someone if their sibling marries yours? Or just "incestuous-in-law"?) The war tears the families apart, though the two sets of sons do meet on the battlefield and treat one another with honor--the younger sons dying together and embracing in death, the older sons surviving while fighting in the final clash. The older sons and their sisters wind up back in South Carolina during Reconstruction (the Stoneham father has moved his family there to improve his health in the warmer weather, and also to more clearly manage Reconstruction) and in the meantime an evil mullato henchman (named "Lynch", ironically enough) of the Stoneham father is seizing power by using the newly freed blacks and carpetbaggers. Ultimately, Lynch has designs on the Stoneham daughter (who is engaged to the Cameron son) and Cameron and his Klansman buddies ride into town to save the day.
Through the second part of the film, Reconstruction is painted as a great wrong, and the blacks who are part of it are portrayed as thugs while the white southerners are shown as civilized and oppressed. Now, Reconstruction certainly had its faults and abuses--as any system of military rule and disenfranchising of a significant portion of the citizenry will cause--but this overlooks that this effort was intended to protect the freed slaves and secure their rights as citizens, as well as rebuild the shattered South. The one-sided and exaggerated portrayals in the film are certainly offensive, even for the year it was released (1915). A black-dominated legislature eating chicken and taking off their shoes during their session? Well I'd still bet that even the unsophisticated rabble represented in the film would do better than the morons who passed that health care mess earlier this year.
Further, the portrayal of the Klan makes them look like heroes, glossing over the fact that the real Klan engaged in collective intimidation of the freed slaves, committing murders and assaults to spread fear. This was not just a "fight the occupying army" resistance group--it also sought to enforce racial dominance. The film, rather, shows the KKK fighting pitched battles with armed troops, and the one "offender" they murdered was actually given a "trial". While it's fine for a film to explore why the Klan sprang up and enjoyed so much support in the South at the time, this heroic portrayal does a disservice to history.
Overall, the film was artistically brilliant and a huge advance in moviemaking--but unfortunately its historic portrayal is both an insult to blacks as a race and history itself.