The Southern states did not secede to preserve slavery. Slavery was not threatened in the US. If anybody thought it was, the North made it perfectly clear that they were willing to go to great lengths - including passing a constitutional amendment - to ensure slavery would be protected.
The Northern states’ violation of the fugitive slave clause of the US constitution did provide Southern states the argument that it was Northern states which had violated the compact. They did. There’s no doubt about that.
Yet when offered slavery effectively forever, they refused to accept it and insisted on independence. Obviously the North’s violation of the fugitive slave clause of the constitution was a pretext for the Southern states, not their real concern.
The North didn’t go to war to end slavery. True. It went to war to preserve the Union. It won that fight. The South most certainly did. And it lost that fight. Why couldn’t the South have ended slavery on it’s own prior to 1861? And if the South had won the war would it have ended slavery?
So, why did the Southern states secede? If not about slavery, it must have been about something else, unless we assume that everyone woke up one morning and said, “I’m bored, let’s secede!” Specifically, why did Mississippi chose to secede - please provide support from their Articles of Secession. Please provide the same for South Carolina, Texas, and Georgia. I am sure that I am misreading those documents and look forward to your correcting my reading..