Birth of a Nation was a White supremacist propaganda movie - it was the film version of the racist novel "the Clansman" - which tried to justify the lynching of Black men in the South. Basically, the film claimed that Black men were all rapists, and that the KKK were the guardians of White womanhood.
In fact, the KKK were a racist terrorist group, founded by former Southern slaveowners. They wanted to repress freed Black slaves, and supress Black civil rights in the South.
The Klan carried out a campaign of lynching, castrations, torture, rape, arson and general mayhem against African Americans in the South.
Basically, D W Griffith was a terrorist sympathizer, who made a propaganda film on behalf of the al-Qaeda of his era.
Birth of a Nation is a racist propganda film.