Kents

When I lived in Alabama, I had a couple of friends from there who explained it all to me.

The war was about Northern Aggression. (Who attacked Fort Sumter?)
The "Niggers" were the "bad" black people on welfare, not the good, hard working ones.
Flying the Confederate Battle flag should in no way offend black people or those who opposed slavery on principle because it was a celebration of a great southern tradition. (That flag was only flown officially for a few years and at best it symbolized the attempt to break up a country which southerners have fought and died for many times since.)

I told them that I respected their right to their opinions, but that I didn't share them and that most other people, especially outside the US, with no dog in that hunt, just could not follow their logic.. What else could I do? Call their Grandpa's Grandpa a liar?

I have followed the history well enough, I think. The South was not only fighting for slavery in their own territories, they were fighting for the requirement that half the new states created also had it and though there may have been a small fringe of white supremacists who were anti-slavery. They were rare exceptions. I is not fair to say that the two are separate issues.

To use your logic.

no white supremacists: No slavery
No Slavery: No succession
No succession: No War