[valleyer]

States can not "in theory" secede from the United States. See Texas v. White: https://en.wikipedia.org/wiki/Texas_v._White

From the point of view of the U.S. legal system, the Confederacy's secession was "absolutely null".

[echelon]

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