Although a comedy, Buster Keaton's The General is actually a pretty good Civil War movie.
The plot is based on William Pittenger's non-fiction book
The Great Locomotive Chase: A History of the
Andrews Railroad Raid in Georgia in 1862, available to read free online or download
here.
Although I have only seen Youtube clips, the recently released Bluray version looks fantastic (set to
720p to watch in HD).
The outstanding cinemtography - as good as any dramatic film - is seen to particularly striking effect
in the battle scenes in the third clip.