While it's not really fair to say it's that black and white, it's better than pretending the whole thing was just the South trying to protect states' rights.
If not for slavery, do you really think the southern states would have left the union?Slavery had its part in it, but more to give the north the motivation to keep fighting after the loss at the First Battle of Manasses.
Lincoln had no intentions on ending slavery at the beginning of the war, as made apparent by his First Inaugural Address:
"I have no purpose, directly or indirectly, to interfere with the institution of slavery in the States where it exists. I believe I have no lawful right to do so, and I have no inclination to do so."
If not for slavery, do you really think the southern states would have left the union?