I mean, the Confederate states themselves were hardly shy about the fact that it was about slavery and race in their declarations of secession.
In the momentous step which our State has taken of dissolving its connection with the government of which we so long formed a part, it is but just that we should declare the prominent reasons which have induced our course.
Our position is thoroughly identified with the institution of slavery-- the greatest material interest of the world. Its labor supplies the product which constitutes by far the largest and most important portions of commerce of the earth. These products are peculiar to the climate verging on the tropical regions, and by an imperious law of nature, none but the black race can bear exposure to the tropical sun. These products have become necessities of the world, and a blow at slavery is a blow at commerce and civilization. That blow has been long aimed at the institution, and was at the point of reaching its consummation. 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.
In all the non-slave-holding States, in violation of that good faith and comity which should exist between entirely distinct nations, the people have formed themselves into a great sectional party, now strong enough in numbers to control the affairs of each of those States, based upon an unnatural feeling of hostility to these Southern States and their beneficent and patriarchal system of African slavery, proclaiming the debasing doctrine of equality of all men, irrespective of race or color-- a doctrine at war with nature, in opposition to the experience of mankind, and in violation of the plainest revelations of Divine Law. They demand the abolition of negro slavery throughout the confederacy, the recognition of political equality between the white and negro races, and avow their determination to press on their crusade against us, so long as a negro slave remains in these States.
[...]
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.
(There's more but I don't want to be here all night.)
Oh, there's different rhetoric layered on top, mostly in terms of either economic concerns or the federal government breaking the terms of the Three-fifths Compromise and the Fugitive Slave Act. Texas, as you can see, tried for an invocation of 'natural order.' At the end of the day, though, it was always about protecting the interests of southern slaveowners.
That it was a fight for the general concept of "states' rights" as a prime motivator rather than a necessary justification is an argument largely repeated by neo-confederates and people who've never actually gone back to read the historical documentation, though more the latter than the former in my experience.
