My thoughts (spoilers herein)
The movie is broken up into several parts. First, the story of the pre-Civil War days set mostly in South Carolina, which follows the Cameron family, as well as the activities of the Stoneman family from the North. A budding love story among people from each family ensues. War then breaks out, and members of each family go off to battle, with several of them getting killed.
When war subsides, South Carolina is transformed after union forces control the town in South Carolina and black people are able to vote, though here they engage in corrupt practices to put the power-thirsty Silas Lynch in power. Chaos ensues as the Southerners see their way of life crumble, and this culminates in the death of the Cameron sister. In response, Ben Cameron comes up with an idea for a response in the form of the KKK. As Lynch starts to oppress the white population, including Cameron's love, Elsie, the KKK comes to the rescue of the town.
Overall, this paints a very rosy picture of the KKK and a very negative view of former slaves, as well as white people/carpetbaggers who helped them. It plays upon what must have been stereotypes of black people to portray them as breaking down the rule of order in the South. The point of the narrative is pretty clear.