But you don’t have to take our word for it — just ask the Vice President of the Confederacy what his reasons were in the infamous Cornerstone Speech of 1861, just a few weeks before the Confederates fired on Fort Sumter:

“The new [Confederate] 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 . . . The prevailing ideas entertained by . . . most of the leading statesmen at the time of the formation of the old constitution, were that the enslavement of the African was violation of the laws of nature; that it was wrong in principle, socially, morally, and politically. . . Those ideas, however, were fundamentally wrong. They rested upon the assumption of . . . the equality of races. This was an error . . .

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.”

— Alexander H. Stephens, March 21, 1861, reported in the Savannah Republican, emphasis in the original

More ways we know the Civil War was about slavery

• The state secession declaration documents mention the words “slave”, “slavery“, and “slave-holding” over 150 times, along with a number of related words including abolition, abolitionist, race, African, white race, and negro among yet others.
• The Constitution of the Confederate States of America is almost identical to the US Constitution; in most of the several places that had been modified, the subject of the change regarded slavery and the claimed rights of Southern white men to own black human beings as a captive labor force.
• Contemporaneous speeches given by Southern leaders at the time leading up to the war and during the war uniformly named the question of slavery as the core animus for their fight.
• The Confederates rejected the idea floated internally of enlisting Blacks to replace the much-drained manpower of the South even though the final year of the war — despite ample evidence of the capabilities of black fighting forces as evidenced by their use by the Union to rout Southern Armies in bloody battle after bloody battle.
• The secessionists even hampered their own ability to get diplomatic recognition, by refusing to clarify any sort of end date for slavery or apologia for the moral failings of the peculiar institution to a Britain and France who saw the practice as barbaric by that time. In other words, they chose slavery over independence when push really literally came to shove.
• The Lincoln-Douglas debates were almost entirely about slavery and the question of whether it should be extended further into new US territories of the West, halted, or ended altogether. Lincoln was on the side of halting slavery, and when he was elected President in 1860 the Southern states began seceding from the Union.