“Read some history.”
It really does help. History teaches us that there were still slaves in Union during the hostilities. That the Confederacy had a vastly larger number of slaves is irrelevant. The degree to which one participates in the institution of slavery does NOT negate the fact that one participated.
No one has denied any of that.
Try to understand this point: the Union ended slavery. That's what they get credit for and that the South seceded from the union to preserve their slavery system redounds to their shame.
And those that still defend the institution over 100 years later are especially pitiful.