Yeah, it was a lot more complicated than "Muh... slavery." Using slaves for farm work was already becoming less popular or profitable because of industrialization, and given more time would have gone away.
A lot of the issues were with commerce and Southern States that thought the North wasn't giving them fair prices or terms for goods. And they felt they were being treated like second-class states in politics because the South had a much lower population.
To try and improve their bargaining position, the Southern states decided to formally announce a separation (legally I might add). They wrongly thought it would lead to the Northern states taking their grievances seriously and they could negotiate something that worked better. They didn't honestly think the North would attack them, or they never would have gotten the support of many of the states.
But the North saw it as a threat and politicians from the big cities ramped up the fears of starvation and lack of raw resources for factories. The South couldn't afford to stop trading with the North for long either. Who else could they trade with? Anyhow, the North "called their bluff" without ever trying to find a peaceful solution and went full on war mode. Hence why it is often called the War of Northern Aggression.
It's like a wife telling her husband she wants to separate. But instead of agreeing to couples therapy to work out their differences, the husband just beats her until she gives up.
And they felt they were being treated like second-class states in politics because the South had a much lower population.
I think this was pretty key. The issue of whether to count slaves for population purposes almost derailed the original Constitution, and we go the 3/5ths compromise. However, that hardly settled the issue for good. As we know, they ended up in camps of "slave" and "free" states, trying to make more compromises as additional states were joined.
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