They were the first state, and some other states also mentioned slavery as reason for leaving the Union.
The Northern and Southern sections of the United States developed along different lines. The South remained a predominantly agrarian economy while the North became more and more industrialized. Different social cultures and political beliefs developed. All of this led to disagreements on issues such as taxes, tariffs and internal improvements as well as states rights versus federal rights.
I agree...Slavery WAS an issue, but NOT the main issue of the war...
What led to the outbreak of the bloodiest conflict in the history of North America?
A common explanation is that the Civil War was fought over the moral issue of slavery.
In fact, it was the economics of slavery and political control of that system that was central to the conflict.
A key issue was states’ rights.
The Southern states wanted to assert their authority over the federal government so they could abolish federal laws they didn’t support, especially laws interfering with the South’s right to keep slaves and take them wherever they wished.
Another factor was territorial expansion.
The South wished to take slavery into the western territories, while the North was committed to keeping them open to white labor alone.
Meanwhile, the newly formed Republican party, whose members were strongly opposed to the westward expansion of slavery into new states, was gaining prominence.
The election of a Republican, Abraham Lincoln, as President in 1860 sealed the deal. His victory, without a single Southern electoral vote, was a clear signal to the Southern states that they had lost all influence.
Feeling excluded from the political system, they turned to the only alternative they believed was left to them: secession, a political decision that led directly to war.
Out of the 21 total declarations, ordinances, and other secession documents only 6 mention slavery in any context beyond a geographical reference (and only 5 of them mention it at substantial length - the sixth is in a single brief clause). 14 of those documents specify other causes, either in addition to slavery (as in the 6) or without mentioning it at all. The remaining 7 do not list any causes.