The South wanted to protect itself from Northern invasion, I believe. Please correct me if I’m wrong. Seriously, I would appreciate the input. Always stand to be corrected here; lots of smart people here. :)
The South seceded because they saw the writing on the wall for slavery with the Republican victory for the White House. The seceding states made clear in their secession declarations that they were leaving protect slavery and to found a government based on white supremacy.
Lincoln didn't invade the South to end slavery. He made clear in his inaugural address that he supported the Corwin amendment (13th amendment that would have federally protected slavery explicitly), and that he only sought to collect federal taxes (tariffs).
They couldn't allow the South to secede and enact the Republicans plan of high tariffs for 'internal improvements' (graft).
As the war drug on the abolition of slavery was raised as a Northern war aim to maintain support for the war.
This was an ongoing feud between the North and South for years.
In February 1850, Taylor [president Zachary Taylor] told southern leaders who threatened to secede that, “taken in rebellion against the Union, he would hang ... with less reluctance than he had hanged deserters and spies in Mexico.”
It was Taylor’s unexpected opposition to the expansion of slavery (he was from the South and was the last president to own slaves) that had caused an immediate crisis in 1850.
Taylor ran as a Whig candidate in 1848 and he wasn’t a professional politician. Taylor was a career military man and a hero in the war with Mexico. Once he took office in March 1849, it became clear that Taylor, the military man, was more interested in preserving the Union than the art of politics.
Taylor decided to press for statehood for the newly acquitted territories of California and New Mexico, and to let the regions hold their own constitutional conventions. This guaranteed that California and New Mexico would join the Union as anti-slavery states, tipping the balance in the Senate to the North.
One of many ref:
https://constitutioncenter.org/blog/a-presidential-death-and-averting-two-constitutional-crises/
>> “The South seceded because they saw the writing on the wall for slavery with the Republican victory for the White House. The seceding states made clear in their secession declarations that they were leaving protect slavery and to found a government based on white supremacy.”
At that time the vast majority of the people in the North too were racist by today’s standards. With few exceptions they believed in white supremacy too. They also favored tariffs that forced Southerners — an agricultural region (one reason it lost the war) — to support industries in other states. A provision of the Confederate Constitution ended this — “...nor shall any duties or taxes on importations from foreign nations be laid to promote or foster any branch of industry”.
Having said that, yes, the sectional differences over slavery were a major reason for secession — I’d say the root cause — and some prominent Southerners, especially the politicians (who were mostly slaveholders) said so. It’s not an either-or situation, though. It was about states’ rights too, and the sense of alienation from the North that had grown among Southerners over the years.