All federal laws and treaties along with the Constitution are supreme from 4 March 1789 on and amendments there after.
So unilateral state succession is illegal and has been since the constitution took effect.
Article VI - Clause 2, Supremacy
This Constitution, and the Laws of the United States which shall be made in Pursuance thereof; and all Treaties made, or which shall be made, under the Authority of the United States, shall be the supreme Law of the Land; and the Judges in every State shall be bound thereby, any Thing in the Constitution or Laws of any State to the Contrary notwithstanding.
From Texas v. White (1869)
Considered therefore as transactions under the Constitution, the ordinance of secession, adopted by the convention and ratified by a majority of the citizens of Texas, and all the acts of her legislature intended to give effect to that ordinance, were absolutely null. They were utterly without operation in law.
W R O N G :)