And by the way Tet, please, show us the part in the Constitution that forbids secession. Much appreciated.
The SCOTUS ruled in Texas v. White, 74 U.S. 700 (1869) that unilateral secession was unconstitutional while commenting that revolution or consent of the states could lead to a successful secession.
So, the Supreme Court ruled it as unconstitutional. There are people today who disagree with the ruling, and interpret the Constitution that it is constitutional to secede.
But, as it stands right now, it's unconstitutional.
I mentioned violent revolution as the only way a state could succeed, but as we see above, if all the other states give a state(s) permission to succeed, then that is another option for secession.