essentially there is legal and illegal secession

the legal way is by vote and mutual agreement (as there have been some motions by various states in recent years to secede from the U.S. Federal Government)

the illegal way is by revolution (how the U.S. was started) which, if a war is won or the initially illegitimate secession becomes tolerated or enforced, may become somewhat legitimate (or may continue to be contested by exist)

The U.S. Civil War seems to be an example of legal secession which was then illegally fought but then, upon winning, seemed to reinforce an idea of legitimacy of the Union that existed prior