The Civil War was all about slavery. It was about the "state's right" to own slaves. I'm actually reading Battle Cry of Freedom (history of the civil war). There is no question that slavery was the cause of the Civil War.
I also recently visited Arlington House (Robert E. Lee's home). When you read, up close and personal, what various Southern states had to say about the war, and why they were going to fight, it's pretty clear that the issue was slavery. For example, here are a couple items displayed at Arlington House:
"Our position is thoroughly identified with the institution of slavery . . . There was no choice left us but submission to the mandates of abolition, or a dissolution of the Union, whose principles had been subverted to work out our ruin."
Excerpt from "A Declaration of the Immediate Causes which Induce and Justify the Secession of the State of Mississippi from the Federal Union, January 9, 1861
"We hold as undeniable truths that the governments of the various States, and of the confederacy itself, were established exclusively by the white race, for themselves and their posterity; that the African race had no agency in their establishment; that they were rightfully held and regarded as an inferior and dependent race, and in that condition only could their existence in this country be rendered beneficial or tolerable.
That in this free government all white men are and of right ought to be entitled to equal civil and political rights; that the servitude of the African race, as existing in these States, is mutually beneficial to both bond and free, and is abundantly authorized and justified by the experience of mankind, and the revealed will of the Almighty Creator, as recognized by all Christian nations; while the destruction of the existing relations between the two races, as advocated by our sectional enemies, would bring inevitable calamities upon both and desolation upon the fifteen slave-holding States.
By the secession of six of the slave-holding States, and the certainty that others will speedily do likewise, Texas has no alternative but to remain in an isolated connection with the North, or unite her destinies with the South. . . ."
DECLARATION OF CAUSES: February 2, 1861: A declaration of the causes which impel the State of Texas to secede from the Federal Union
