Sorry, I missed your post on first review.
You ask an excellent question.
Secessionists themselves wrote some documents explaining why they declared independence from the United States.
This site shows all four of them, though not in proper sequence.
The correct sequence is South Carolina (#1), Mississippi (#2), Georgia (#5), then Texas (#7).
The first two (South Carolina and Mississippi) are especially important to understand, because they were the first ones ever written -- in late December 1860 and early January 1861 -- in the heat and flush of excitement of the moment, and before there was a lot of feed-back or second thoughts coming in from other Southerners or sympathetic northerners.
In those two documents we see the primary motivating force behind secession, and there is simply no doubt: it was their concerns to protect the institution of slavery, and nothing else.
Later, the argument gets gussied up with some other minor issues.
But I thought of an analogy to explain it all:
OK, now let's suppose that, for sake of discussion, the wife suddenly discovers her husband has been leading a double life all these years.
He has, oh, say, a second family, just something totally unacceptable to her, and so she decides to end the marriage.
On her way out the door, she tells him why, the real reason: he has a second family.
But then she also throws in all the other little stuff which bothered her all those years, but were not by themselves cause for divorce.
In the case of secession beginning in 1860, protecting slavery was their real reason, everything else was minor stuff, thrown in as they walked out the door.
You can see this clearly, by reading South Carolina and Mississippi first -- there's no mention of anything except slavery.
Later, as it's going out the door, so to speak, Texas throws in some other minor items, none of which individually, or all together, would have driven Southerners to secession.
If you ask, "how real was the actual threat to slavery," well, that is a complicated question.
Yes, Southern secessionist "Fire Eaters" exaggerated the immediate threat to slavery represented by the election of "Black Republican" Lincoln, in November 1860.
But long term there's no doubt that slavery was in for a rough-go, no matter which course of action the Southern slave-holders chose.
100% correct. Lincoln posed absolutely no threat to slavery in the South as it existed at that time. But by promising to block further expansion of slavery to the territories, he did put a knife to the throat of the Slave Power. I don't know if Lincoln, or most anti-slave people in the North at the time fully understood that threat. It was a combination of demographics and finance. To people in the South who really understood the economics of the slave system, Lincoln's humble plan to keep slavery isolated where it existed was like a nuclear weapon pointed at them... both exonomically and socially.
Slavery, by the year 1840 had ceased being a labor system. It was a massive Ponzi scheme. That was it's biggest and only real value. It was so not much the value of the labor which they provided. That labor was considerable, but could have also been provided by free labor. The real value was the slaves themselves on an open market, and not just the current slaves, but their children, grandchildren etc.
This encouraged slave owners to treat them as cattle and the more calves born the more they could sell at some future date if the need may arise. They could and did collateral their slaves, and in the event of some economic downfall, they could sell them off.
It was a very different kind of slavery than existed in the Colonial ear. People literally were property that could be bought, sold or collateralized. The economy of the South then depended just as much on the value of slaves as it did on the price of cotton.
If the market for new slaves became geographically restricted, as Lincoln promised, the slave owners would both lose the value of their existing 'property' because of over supply. In states like South Carolina, Mississippi and Alabama, they would have been soon hopelessly outnumbered by slaves and children of slaves within one or two generations.
The Civil War did not start about slavery itself. It was argument over the Expansion of Slaverythat created the war. The North basically said we have had enough of that expansion... do what you want in your own states, but we don't want it anywhere else. They, I think, did that without fully understanding the economics of slavery and why continual expansion was a necessity to keep the slave society going.
For the Southern power structure, expansion of slavery was not just some political abstract... it was a mortal necessity. Without expansion their wealth and society would implode upon them. Expansion of slavery for them was not an option, it was an absolute necessity.