They didn't do it for that reason. The Northern armies marched into the South to kill people, not for freedom for the black man, but to establish the dominance of the corrupt government of Washington DC over these states.
The notion that they were going to free the slaves didn't come about until nearly two years after the war started.
"Freeing slaves" in conjunction with the civil war is just propaganda intended to distort the real reason why they invaded, which was entirely economic in nature.
Thank you for the reply. I’m aware that you’re a southern partisan, and quite vocally so. That’s OK ... I’ve read many of your comments over the years; agree with some, disagree with some, find other rather ambiguous. Whatever sins the Union side may have committed, it remains a historical fact that slavery was abolished as a consequence of the “Late Unpleasantness”, and that many White soldiers lost life or limb making the abolition of slavery possible. If one is going to celebrate the abolition of slavery, it is those White soldiers who should be celebrated. The proponents of “juneteenth” don’t do that. Quite the contrary, in fact.