“Doubtful, although some may have opposed it because they were opposed to war under any circumstance.
In any case, they were wrong, weren’t they? If not for the war, slavery would have dragged on for probably at least some decades.”
I don’t recall anyone in the North claiming to be waging war with the South to end slavery. That’s a modern interpretation.
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.