There was a lot of stuff at issue as to both why the South seceded and why the North invaded.
The Civil War settled what had been a long standing political debate, and it settled it with blood. It wasn’t the slavery debate but rather the debate regarding whether or not a State which freely and voluntarily entered the compact between the States had the right to freely and voluntarily exit said compact.
This was not a “settled” matter at any point prior to the Civil War. New England states considered secession during the war of 1812, which they felt was primarily caused by desires of the western states to expand and New England wanted no part of it, especially since they were the ones who directly bordered Canada.
As has been stated South Carolina came very close to secessionist antics during Jackson’s presidency.
My personal opinion is, while it should have been clearly stated somewhere in the original constitution, unilateral secession was in fact not part of the original intent. I believe there is some argument to support bilateral secession. As evidence of this fact note that only 9 of 13 colonies had to ratify the U.S. Constitution before it was considered the law of the land. Meaning the colonial representatives who drafted the Constitution intended it to be binding once a large enough majority supported it, and ostensibly they planned to exert governmental control over any states who did not as a fait accompli. There was a strong move to make sure every state ratified the Constitution because everyone involved didn’t want to have a huge conflict right at the very founding of the new state, and ultimately all 13 of the original colonies did ratify (although it was not ratified by North Carolina or Rhode Island until after March 4, 1789 which is the official beginning date of operation for the United States government.)
A growing divide between North and South emerged almost from the very day we started Government operations. Slavery wasn’t the issue in the late 18th century and the early 19th century. In fact slavery wasn’t really the issue because as a practice it wasn’t as important to the South at that time as it would later become.
It’s unfortunate that sometimes technological innovation is possibly a net negative for society, Eli Whitney’s cotton gin dramatically increased the efficiency of cotton cultivation operations in the United States and dramatically increased the demand for slave labor. I doubt very seriously Whitney foresaw that kind of result when he invented something primarily designed to increase productivity.
The important of cotton and slavery to the economy of the South grew rapidly throughout the 19th century and by 1850 it was of monumental importance (well, at least to the powers that be.)
Tariffs were the early points of contention between North and South. The South had always been a fairly agricultural society, while the North was more focused on early manufacturing and shipping. The North long favored high tariffs to protect developing American industries. While the plantation-owning elites who ran government in the South had longed opposed tariffs because high U.S. tariffs insured that other countries would in turn charge high tariffs on U.S. exports. The Southern elite were also highly partial to importing fine finished goods from Europe and high tariffs significantly increased the cost of their lifestyles.
Note that most the rest of the world sympathized with the South in regard to these matters (out of economic interest), and that sympathy lead to outright support for the South during the war itself.
There was an enormous amount of distaste for tariffs, primarily forced on the South because of the North’s advantageous position in the House of Representatives due to having a higher population. It is around this time that the idea (promulgated by men such as Thomas Jefferson and John C. Calhoun) that the individual states shouldn’t be so dominated by the Federal Government. This was a powerful idea that has stayed in the minds of Southerners to this day to a degree. With regard to tariffs the South was very unhappy that the North was able to unilaterally do something that had such a profound effect on Southerners.
Ultimately it was highly problematic that two major regions of the country were developing two highly incompatible and divergent cultures. It was also highly problematic that Southerners were beginning to think of themselves as a regional block, regionalism makes secession a lot more likely. Eventually the major issue, slavery, comes to the forefront. This issue is significantly exacerbated by western expansion. If we never expanded westward slavery as an issue would have been dealt with in a far different manner.
It’s difficult to say if abolishing slavery required a Constitutional Amendment, the thoughts in the minds of most was that simple Federal legislation would suffice. Which meant if anti-slavery politicians ever gained control of both Houses of Congress and the Presidency, there was a very likely chance slavery would be ended in the United States. The South recognized this, and they used the Senate to protect their interests. As long as the balance in the Senate remained between free states and slave states, they could block any efforts to abolish slavery. And since new states have to be admitted with Congressional approval, the Southern Senators insured (sometimes making judicious use of the filibuster) that anytime new states were admitted the balance remained between slave and free states. This wasn’t something that was done without hugely inflammatory events. And arguments over the admissions of new states lead to outright internal warfare on a small scale ten years prior to the Civil War itself.
Abolitionism as a movement was slow-growing in the North. Most of the Northern states that had abolished slavery had mostly done so because it was no longer economically viable and there was no point to it, strong abolition movements helped, but that was fairly limited for much of the early 19th century. Three things made slavery a big issue for Northerners, too. The novel Uncle Tom’s Cabin, the Fugitive Slave Act, and the Dred Scott decision.
That background is what enabled Lincoln to be elected, as the anti-slavery Republican party gained widespread acceptance in the North.
Lincoln himself is integral in the actual invasion of the South. The South actually began seceding when they heard of Lincoln’s election, James Buchanan was still President at the time and took no serious actions to try and suppress secession. He publicly claimed states had no legal right to secede, and ousted anyone who supported secession from his cabinet. Unfortunately he also maintained that the Federal government had no legal right to prevent secession. Which allowed the seceding states precious time to both seize Federal property, imprison Federal soldiers, and in general prepare for war.
Lincoln made it clear the moment the South took Fort Sumter that secession was invalid and he was going to use the full power of the Presidency (and then some) militarily keep the union together. His reasons, I do not believe, had anything to do with slavery. It’s also important that he not give that impression, as most northerners probably weren’t willing to fight solely to free the slaves, but were willing to fight to preserve the United States.
The actual Emancipation Proclamation was a measure designed to be punitive towards the South, and it specifically only freed Southern slaves and not slaves in states then loyal to the Union. It’s not often remembered by the public that several states on the Union side still maintained slavery throughout the Civil War.
Paradoxically most of the men fighting on the front lines for the South were far too poor to own slaves. They were fighting to defend a regime that not only did not benefit them it actually was economically disadvantageous to them and kept their communities economically backwards. Slavery was in truth not good for the economy of the South as a whole, it was good for the ruling elite, and they were able to spin things such that they convinced the masses that they were fighting a war of liberty, to defend the ideals of the Founding Fathers against both “Northern Aggression” and attempts to subjugate the will and freedom of the South to northerners. This particular great lie was so well presented and articulated by the South’s ruling elite that it lives on to this day. Some southerners to this day say they feel that their forefathers were fighting to preserve their liberties across the board, and until recently the Confederate Flag was respected almost without question in all of the Southern states and featured prominently on public buildings and even flags of many of the states.
The truth of the matter is, the Southern public was duped into fighting and dying in order to continue propping up the ruling elite that were quite responsible for most of the South’s impoverished and economically backwards condition.