To get back to the topic so what if the war was about slavery. In the western world we have something called just war theory. Just war theory has two parts; Jus ad Bellum or right to war and Jus in Bello or right conduct in war. So the first part deals with whether it was right to go to war and the second with right conduct during the war. Since it is indisputable fact that the southern rebels committed the first overt acts of war(seizing federal property and personnel) and fired the first shots they are the instigators of the war, as far as international law would be concerned. So they are the ones who would first be judged under the Jus ad Bellum criteria.\n\nJus ad Bellum has seven criteria; Just Cause, comparative justice, competent authority, right intention, probability of success, last resort, and proportionality.\n\nThe southern rebels fail miserably right out the gate on the first one if they went to war to protect slavery. There is absolutely no way, even in 1860, that the majority of the western world would see defending slavery as a just cause. So it very much matters why the southern rebels rebelled and since they left us plenty of evidence proclaiming they did go to war to defend slavery we can say that they fail the first test of Just war theory. \n\nI would also argue they failed all the rest of it too but that exceeds the OP. The bottom line is that it is very important what the war was about, as in any war.