I am not from USA, and when reading some documents on history from my own government, it noted that during USA civil war the north banned slavery ONLY in the southern states during the war, and the US-wide ban was only later.

But during these protests I see a lot of people mad at the southern generals that choose to join the war specifically to be racist and whatnot.

So can someone try to answer this in the most historical way as possible and avoid propaganda from both sides? What was USA civil war about? Why did it start?