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

The “States’ Rights” Contradiction

One of the clearest ways to prove the war was about slavery—and not abstract “states’ rights”—is to look at how the Confederacy treated the rights of Northern states.

• The Fugitive Slave Act Paradox: Southern leaders explicitly opposed “states’ rights” when Northern states attempted to exercise them. When Northern states passed “Personal Liberty Laws” (exercising their sovereign right to not enforce federal slave-catching laws), Southern states demanded the Federal Government override these state laws.​
• South Carolina’s Declaration: In its “Declaration of Causes,” South Carolina specifically lists the failure of Northern states to enforce the federal Fugitive Slave Act as a primary grievance. They were not fighting for the right of states to choose their own laws; they were fighting for the federal government to enforce slavery across all states

The Rejection of the “Forever” Amendment (Corwin Amendment)

Perhaps the most damning evidence is the South’s rejection of the Corwin Amendment.

• The Offer: In a last-ditch effort to prevent war, the Northern-controlled Congress actually passed a Constitutional Amendment (the original 13th Amendment) in early 1861. It would have protected slavery forever in the states where it already existed, guaranteeing the federal government could never abolish it.​
• The Rejection: If the South were seceding simply to “protect their property” or “defend against Northern aggression,” they would have accepted this victory. Instead, they rejected it. Why? Because the amendment only protected slavery where it was, but did not guarantee its expansion into new western territories. The South seceded not just to keep slavery, but to ensure it could grow into a continental empire

The Democratic Party Split of 1860

Political actions speak as loudly as words. In 1860, the Democratic Party was the only truly national party left. It shattered into two hostile factions solely over one specific demand:

• The Federal Slave Code: Southern delegates walked out of the convention not because the party was anti-slavery (it wasn’t), but because Northern Democrats (led by Stephen Douglas) supported “Popular Sovereignty” (letting territories vote on slavery). The Southern faction demanded a Federal Slave Code that would force the federal government to protect slavery in all territories, regardless of what the local people wanted.​
• Significance: This proves the war was not caused by a general cultural drift, but by a specific, non-negotiable demand by Southern leaders to use federal power to expand slavery.

Common Soldier Motivations

While elite speeches are often cited, the private letters of common soldiers (studied extensively by historian Chandra Manning) reveal that the “average joe” was fighting for slavery too.

• Confederate Letters: Even non-slaveholding soldiers frequently wrote that they were fighting to prevent “subjugation” to the “Black Republican” party and to stop “servile insurrection.” They viewed the preservation of slavery as essential to maintaining their status as white men; without slavery, they believed they would be degraded to the same social level as Black people.​
• Union Letters: Conversely, many Union soldiers who began the war indifferent to slavery became abolitionists during the war. They saw firsthand that the “Slave Power” aristocracy was the root cause of the treason they were fighting, concluding that to save the Union (democracy), they had to destroy slavery.

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.