Lincoln Movie

I just went to see the new Lincoln movie, and I think it gives an incorrect characterization of Lincoln's thinking regarding the Civil War, slavery, and the 13th Amendment.  The movie, which is mainly about the passing of the 13th Amendment through the House of Representatives, indicates that freeing the slaves was Lincoln's main motivation.  I think that his main motivation was preservation of the union.  I think that Lincoln, as the political pragmatist that the movie portrays him as, could have lived with the continuation of slavery, if that would have been what saved the union.  However, Lincoln saw that the union could not continue to exist half-slave and half-free; so, he made it all free.  No doubt he was opposed to slavery, thought it immoral, etc., but it had existed in North America for hundreds of years, and under the Constitution for almost one hundred years.  It was the westward expansion of the United States and the question of whether slavery would be allowed to expand that brought the issue to a head and that resulted in the Civil War.  Moral differences over slavery had existed for hundreds of years without producing abolition.