Here's what the Secessionist government of Georgia thought about it:
"The people of Georgia having dissolved their political connection with the Government of the United States of America, present to their confederates and the world the causes which have led to the separation. For the last ten years we have had numerous and serious causes of complaint against our non-slave-holding confederate States with reference to the subject of AFRICAN SLAVERY. They have endeavored to weaken our security, to disturb our domestic peace and tranquility, and persistently refused to comply with their express constitutional obligations to us in reference to THAT PROPERTY. . ."
[Emphasis added]
That's their Declaration of Independence, and the very first, most important issue they raise.
Here's what Mississippi's government had to say:
"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."
[again, emphasis added]
If the Civil War wasn't about slavery, can you fire up your time machine and let the leaders of the Confederacy know that? For they surely thought it was.