Hmmm... Read the Mississippi declaration of secession, I actually found their declaration quite deliberately said that they needed to defend their institution of slavery. Slavery was the issue, not neccessarily a moral issue, per se, but the fact was that the Southern states were dependent on it for some benefits economically, and yes, a number of states in addition to Mississippi seceded and wrote in the declaration of secession that they didn’t want their institution of slavery taken away.
Clearly slavery was a drivin force but when decision makers sat to discuss war,slavery was not the line in the sand,that is my point