For Jackson, the war was about repulsing the invaders. For Chamberlain, the war was about freeing the slaves.
If you try to make the movie a cause and effect and a political movie... then you make the movie carry a weight it was not meant to bear.
Even Shaara's novel wasn't about the cause and effects of the Civil War, but how the war effected the soldiers that fought it.
Bingo! Too bad so many here do not want to acknowledge that fact.