Here is that cognitive dissonance again. You are saying they left the Union to preserve slavery, which would have been preserved anyways had they remained in the Union.
How does that make sense to you?
Was the slavery they had out of the Union some sort of super duper extra-slavey type of slavery, or was it the same old boring sort of slavery they had while they were IN the Union?
I feel like i'm talking to Yogi Berra.
The North went to war to preserve the Union. The abolitionist movement was also a factor, many in the North wanted it ended. Slavery had been ended in the North. The South choose violent secession to preserve it.