Secession is not a human right. The decision to leave is lawful. But you can not separate the states that are part of the union. You also cannot rescind your citizenship and remain in the US unless you qualify for certain visas. And those have limitations.
By withdrawing from the Union, a state would effectively be denying someone who is a citizen of the United States their rights except by due process. The key there is “due process” and in this case, the state would need to find a LEGALLY RECOGNIZED way to withdraw. This would either be some kind of legislation passed in Congress or an Amendment to the Constitution. Anything less will ensure a civil war and no state will win such a war.
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Your points are valid. My basic point is that secessionists have valid claims also. And that the issue was left surprisingly vague by the Framers.
You’re right that a pre-established process is needed and written into law. It should be painful to the state (or county) that chooses to leave, but peaceful and permitted.