Saying slavery "was only a small part of the cause of the civil war" is revisionism.

It's in the articles of secession. It's in the Cornerstone Address. I don't know how much clearer the writings and speeches of the day can spell out out for you.

delegates at South Carolina’s secession convention adopted a “Declaration of the Immediate Causes Which Induce and Justify the Secession of South Carolina from the Federal Union.” It noted “an increasing hostility on the part of the non-slaveholding States to the institution of slavery” and protested that Northern states had failed to “fulfill their constitutional obligations” by interfering with the return of fugitive slaves to bondage.

South Carolina was further upset that New York no longer allowed “slavery transit.” In the past, if Charleston gentry wanted to spend August in the Hamptons, they could bring their cook along. No longer — and South Carolina’s delegates were outraged. In addition, they objected that New England states let black men vote and tolerated abolitionist societies. According to South Carolina, states should not have the right to let their citizens assemble and speak freely when what they said threatened slavery.

So there goes the whole "states rights" theory.

“Our position is thoroughly identified with the institution of slavery — the greatest material interest of the world,” proclaimed Mississippi in its own secession declaration, passed Jan. 9, 1861. “Its labor supplies the product which constitutes by far the largest and most important portions of the commerce of the earth. . . . A blow at slavery is a blow at commerce and civilization.”

Portions of Cornerstone Address :

The new Constitution has put at rest forever all the agitating questions relating to our peculiar institutions—African slavery as it exists among us—the proper status of the negro in our form of civilization. This was the immediate cause of the late rupture and present revolution. Jefferson, in his forecast, had anticipated this, as the "rock upon which the old Union would split." He was right. What was conjecture with him, is now a realized fact. But whether he fully comprehended the great truth upon which that rock stood and stands, may be doubted. The prevailing ideas entertained by him and most of the leading statesmen at the time of the formation of the old Constitution were, that the enslavement of the African was in violation of the laws of nature; that it was wrong in principle, socially, morally and politically. It was an evil they knew not well how to deal with; but the general opinion of the men of that day was, that, somehow or other, in the order of Providence, the institution would be evanescent and pass away... Those ideas, 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 idea of a Government built upon it—when the "storm came and the wind blew, it fell."

Our new Government is founded upon exactly the opposite ideas; its foundations are laid, its cornerstone 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.

Saying it wasn't mostly about slavery is revisionism and doesn't belong in a history sub.