In 1861 it was a (small) minority position. By early 1863 it was explicit government policy (implicit well before then).
So by 1863 just about all proponents of the war in the North claimed to be waging war with the South to end slavery.
And the northern opponents of the war, the Copperheads, which this movie is supposedly about, opposed the war very largely because they believed it to be about ending slavery. In fact, most of them only became opponents when the ending of slavery became an explicit war aim.
I thought you guys said the North went to war with the South because the South attack Ft Sumter, that the South started the war, but here you are claiming they waged war with the South to end slavery.