I've never seen this movie!

"The Civil War", quite simply, is a genre-advancing ten-part documentary on the American Civil War, using historical documents and photographs, as well as interviews with historians and modern maps of troop movements, to display in heart-rending detail the course of the war on the map and on the American Psyche. It's probably got the most talented cast of vocal performers in the history of forever.

As for the concept of the Civil War... Well, that's a long story that's pretty contentious, but in essence, the United States Election of 1860 was heavily contested, and won by Abraham Lincoln, a Republican. The Republican Party's primary issue at the time was "Free Soil", that is keeping Slavery from expanding outside of those states it already existed in; however, there was a strong abolitionist movement within the party. Fearing that the election would spell doom for their way of life, a way of life that depended on the wealth slavery produced, many (although not all) of the states where slavery was legal secceded, forming the Confederate States of America. And that's about where the war begins.