There’s a hidden assumption in the OP that’s also found in other threads here questioning the Civil War. The OP writes as if the Civil War just somehow suddenly happened in 1861.
The war was inevitable, almost an anticlimax except that it lasted far too long and in too bloody a fashion, because it was about the one thing that dominated all of U.S. history from the moment that the colonies won their freedom: slavery.
Whatever emotional position you have on abortion, cube that and you have an idea what emotions slavery aroused.
Slavery meant that men and women were bought and sold, literally as property. They could be whipped, branded, mutilated, or killed, all legally. They were forbidden to learn how to read and white. They could be put to death for trying to escape. Their children were slaves from the moment of their births. Families could be broken up and moved and sold at any moment. Women were sexually owned, to be raped by white owners and then forced to bring up their mixed-race children as slaves that could be sold for more money.
And every bit of this was justified from every pulpit in the south by use of the exact same bible that the anti-slavery crusaders used to preach abolitionism. Remember that the anti-slavery forces were as fundamentalist in their approach to Christianity as modern day anti-abortion Christians are. Slavery represented a schism in the most central beliefs of the country. Whenever people today try to claim that the U.S. was born as a Christian nation, this is what they really mean although they never say it.
Slavery almost kept the U.S. from forming in the first place. At the Constitutional Convention, every last decision came down to a split between southern states, with their economies based on plantation slavery, and the northern states, with their economies increasingly tied to cities, commerce, and manufacturing. This is the basis for the notion that “state’s rights,” the “tariff,” and “King Cotton” were the real causes of the Civil War. They’re not, though. All of these are secondary to the fact of slavery and the way that economic and political decisions were forced given that axiom.
The South had a larger population and more money than the North in 1787 and it was obvious to them - although it is dubious in hindsight - that no lasting nation could be formed without the southern states. So the South drew a line in the sand and “compromised” by winning every single battle. They kept slavery legal, and pushed off the end of the slave trade until 1808. They won political power in the House of Representatives by being able to count a slave as 3/5 of a person for census purposes when the North wanted slaves, who were property in every other legal sense, to be counted as zero people. They won by getting a weak federal government which could not interfere in state powers. They won by making amendments so difficult that the South could veto any amendment that would ever affect slavery.
Slavery did not go away afterward. For the first 60 years of the 19th century, battle after battle over slavery shook the country. There was a huge uproar in 1808 when the slave trade was finally made illegal. The Compromise of 1820 ensured that territories would only become states in pairs, one slave, one free, so that the South could maintain its even number of states with the North. South Carolina tried passing a Nullification Act in 1832 to nullify the tariffs that benefited the northern industrial economy but not its state’s economy. The Compromise of 1850 allowed the newly gained territories to put off a decision about slavery, but at the price of the Fugitive Slave Law, which affected all blacks in the North. The Dred Scott decision affirmed this law in 1856.
In the meantime there were huge battles in Congress almost every day. Mention of anti-slavery bills was officially prohibited. Members were literally beaten on the floor during sessions.
The rise of the Republican Party and the death of the Whig Party was about slavery. Buchanan was a southern sympathizer and is considered the worst president in history for siding with the south on every issue during his presidency, allowing a festering wound to pulsate. Seven states seceded before Lincoln became president but after his election because everybody in the country knew that the 1860 election was a referendum over slavery and that the issue would be settled one way or the other.
Nobody was surprised by the Civil War. Everybody knew that the country was being torn apart by the issue. Everybody understood what the issue was, even though they tried to disguise it as “states’ rights” or similar nonsense. The only right the states wanted was to keep slaves. But by 1860 it was no longer possible to keep putting off the future. Change was smashing them in the head. No further compromise was possible after a century of futile and ugly compromise. To let the south go was to invite a hostile and malignant nation literally ripping the country apart to compete for the vast empty spaces of unimaginable wealth in the American west, to compete with the north for economic livelihood with the other nations of the world, and to compete with the American notion of what a free country might be. This was impossible in every sense and everybody in the north understood it, no matter what they thought about “niggers.”
The Civil War was complex because the world is complex, and history is complex, and people are complex. But it is one of the very few cases in which we can pinpoint a single cause with complete accuracy. Slavery was the cause, and everybody knew it then just as they know it today. It’s too ugly a thing for some people to say directly, so they hide it behind other words, but the slave issue sums up almost every moment of American history up until 1865. Slavery had to end. A war was necessary to do so. A war occurred.