Right here. Confederate Vice President Alexander H. Stephens’ “Corner Stone” speech:

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…

Those ideas [that slavery would pass away], however, were fundamentally wrong. They rested upon the assumption of the equality of races. This was an error. It was a sandy foundation, and the government built upon it fell when the “storm came and the wind blew.”

Our new government is founded upon exactly the opposite idea; its foundations are laid, its corner- stone rests, upon the great truth that the negro is not equal to the white man; that slavery subordination to the superior race is his natural and normal condition. This, our new government, is the first, in the history of the world, based upon this great physical, philosophical, and moral truth.

Mississippi Declaration of Secession:

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.

Texas Declaration of Secession:

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.

South Carolina Declaration of Secession:

On the 4th day of March next, this party will take possession of the Government. It has announced that the South shall be excluded from the common territory, that the judicial tribunals shall be made sectional, and that a war must be waged against slavery until it shall cease throughout the United States.

Georgia Declaration of Secession:

The party of Lincoln, called the Republican party, under its present name and organization, is of recent origin. It is admitted to be an anti-slavery party. While it attracts to itself by its creed the scattered advocates of exploded political heresies, of condemned theories in political economy, the advocates of commercial restrictions, of protection, of special privileges, of waste and corruption in the administration of Government, anti-slavery is its mission and its purpose.

Tell me again how it’s not about slavery.