So, uh, the existential threat was to which exact mode of existence?

Was it the oil or was it the use of slaves? Go read the things the states said about secession!

For instance:

https://avalon.law.yale.edu/19th_century/csa_missec.asp

Our position is thoroughly identified with the institution of slavery-- the greatest material interest of the world. Its labor supplies the product which constitutes by far the largest and most important portions of commerce of the earth. These products are peculiar to the climate verging on the tropical regions, and by an imperious law of nature, none but the black race can bear exposure to the tropical sun.

They were just trying to protect their mode of existence!