This takes us to a comment by Martin Cothran, spokesperson for Focus on the Family in Kentucky and author of vere loqui, in particular of this comment:
I may not have explicitly disagreed before about slavery being the primary aim of the Confederacy, but I'll do it now. The issue was state's rights primarily, with slavery being an aggravating factor. Lincoln said several times that he would not order an end to slavery in the South. The issue was only newer states, so it was unnecessary for the South to secede in order to maintain slavery.
Slavery did become the primary justification of the war on both sides later in the war when Lincoln used the Emancipation Proclamation as a means to provide the North with a moral reason for he war. He needed a moral crusade to inform the Northern psyche, and slavery was ready at hand for the purpose. The Southern newspapers bit on it, and began to offer rationales for slavery.
But the idea that slavery was what the Civil War was about is, as I said, an oversimplification.
Well, any war has multiple justifications, and the Civil War is not an exception. However, the primary reason was certainly slavery, and to claim otherwise can only be denialism. All we need to look at are the documents produced by both sides in between the first secession (South Carolina, 1860-DEC-20) to the outbreak of hostilities (1861-APR-12).
We can start by looking at the proposal of people who were tryingto avert the war. Our first stop is the Critenden Compromise. This was a Constitutional amendment of six items and four proposed Congressional resolutions. Every single one of these items directly concerns slavery. Next, the Corwin amendment specifically calls out the institution of slavery. It was passed by Congress, signed by Buchanan (which had no formal significance),and passed by two states, says "No amendment shall be made to the Constitution which will authorize or give to Congress the power to abolish or interfere, within any State, with the domestic institutions thereof, including that of persons held to labor or service by the laws of said State." There was even a three-week Peace Conference, which proposed an amendment of seven sections, each section specifically addressing slavery, and the last sentence of the last section only addressing the rights of citizens (but not states). So clearly, in eyes of the people trying to preserve the Union and avoid war, slavery was the main issue.
How about the people leading the fight for secession? Well, we can start with the Cornerstone speech, delivered by Alexander H. Stevens, the first Vice-President of the Confederacy. He says, "The new constitution has put at rest, forever, all the agitating questions relating to our peculiar institution—African slavery as it exists amongst us—the proper status of the negro in our form of civilization. This was the immediate cause of the late rupture and present revolution." Years later, when trying to correct some of the supposed misstatements of the recorded version of his speech, he reiterates this claim, "Slavery was without doubt the occasion of secession;". Of course, the best source of all is the reasons the states themselves give for secession, which we have for South Carolina, Mississippi, Georgia, and Texas. All four of them refer to slavery generally, and the status of fugitive slaves in particular, while only one mentions economic reasons. So, we see from both sides that slavery really was the principle casue, not an afterthought or aggravating issue.
The argument concerning fugitive slaves is particularly ironic, because it amounts to an argument against the rights of Northern states to decide when people should be considered free. In that regard, the secessionists were anti-states-rights.
I don't expect the conservative Christian Republicans to give up this brand of denialism any time soon. It's no coincidence that every time a major party candidate has ties to a white-power organization, it's a Republican. They need these votes to win elections.