It's more complicated than that single case [1], and the chief justice admitted there were other routes:

> Chase, however, "recognized that a state could cease to be part of the union 'through revolution, or through consent of the States'".

Secession does not have to be done legally. Who knows what, if any, conflict that might bring about.

[1] https://en.wikipedia.org/wiki/Secession_in_the_United_States