It's actually really easy to figure this out by reading writings of historical figures from the time, a lot of people debate about it but it's actually very well documented by the people who were involved themselves so it's not really something debatable, rather something that just requires research. A good book to read is the writings of Jefferson Davis as he goes into a great deal of detail about the causes. Essentially, it all had to do with economic disputes between the North and the South and that the North was upsetting the economic balance in the opinion of the south and also the political balance by taking more territory out west. I have read some of these writings but not enough to know every detail but it was not about eliminating slavery in the actual Southern States but rather the slavery aspect was more in regards to whether other territories to the West would be considered slave states or not. Believe it or not, California was actually a huge issue back then in the South was very angry that the North either prohibited or was going to prohibit slavery there.
From what I recall, at the time the South and the North viewed themselves as being in direct competition and they wanted equal influence over the entire United States so it was not acceptable for the north to gain control over more territory in the west than the south. Jefferson Davis talked about things like how certain climates were favorable to owning slave plantations and others were not (taking about land south of the 36th degree of latitude or something as it was already well established that slaves were not as useful in northern states with their short growing seasons) and I didn't quite understand exactly what he meant but I think they thought that California was a potential place where they could have slave plantations whereas States like New Mexico or Arizona would not be able to as they are too dry and desolate. Because they felt that the north was gaining control over more valuable territory, including California they decided to secede and try to go their own way has they felt that they would no longer have sufficient representation in the federal government because the balance was being upset by the north.
An interesting topic that I've heard about but I didn't read about in the Jefferson Davis writings is that the South also had an interest in taking territory from some parts of Latin America. Basically, the way it seems is that both the South and the North view themselves as very powerful entities that both had ambitions for expansion and this is what caused the conflict as the north certainly wasn't going to go to war just to free slaves within the actual existing southern states.
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