As one who is in the field of historical study, I can affirm that the Civil Was was definitely initiated over slavery. I could write a 30 page treatise explaining why and I probably should confess that I'm originally from Wisconsin with ancestors who fought for the Union. However, the concept that "states rights" was the overriding theme of the Civil War is a false notion put forth by Lost Cause romanticists, the fraudulent Dunning School theories of the early 20th century and former Confederate politicians in full C.Y.A. mode.
Some false flags have been thrown about, including the oft cited quotes from above by Lincoln (who was definitely opposed to slavery). Those quotes reflect an astute politician in Lincoln who realized that the priority was to preserve the union and keep the border states in the fold. Lincoln has numerous correspondences regarding his true feelings on slavery, but was also a masterful politician that knew ending slavery wasn't possible in the initial stages of the war.
That said, the reason there was a Civil War was due to the South seceding, so, if you want to know the true reasoning for the war, look to their justifications. You'll find a clear intention of leaving the USA in order to preserve slavery right in the declarations of secession and in the exact words of people such as Vice President Alexander Stephens, who point blank said the Confederacy was created for this reason.
Another oft cited argument pertains to the average Southern soldier not caring one bit about slavery because he was just a poor soldier fighting the invaders, which is false as well. The planter class had done an excellent job of filling the poorer class of whites in the South with the fear that emancipation would threaten their economic existence through a flood of cheap labor. This fear of lower class whites was frequently evident following the Civil War and through the Civil Rights Movement.
It's amazing the amount of falsehoods that are easily accepted in the South regarding this era, although I know there is a natural inclination to want to look positively on our own ancestors' past. Hell, there are still Deep South professors that promote the Lost Cause, War of Northern Agression nonsense. As an American, I can attest to the embarrassment of something like the treatment of the Native Americans. Yet, it still undeniably happened and should be acknowledged without the usual "Gone with the Wind"-type propaganda.
I should also say the North wasn't nearly as clean as their conscience tries to tell them. You could start with the New York Draft Riots, but that's a whole different diatribe.