The idea that the civil war wasn't about slavery is mostly the view pushed by the North before the war. Before the war, there was a balance of slave and non-slave states, and any time they were added, that balance was preserved. Lincoln's position was that slavery was OK, but should spread into any new states, and that would have tipped the balance in congress to anti-slavery. After Lincoln was elected, the states began to succeed, and in their succession documents, they quite clearly stated that it was about slavery. The public position of the North was that they fought to prevent the South from succeeding, and they didn't want to end slavery (followed with a wink, wink, nudge, nudge). And its that attitude that allowed the South, after the fact, to promote the idea that it wasn't about slavery, and the North wasn't about to correct them.