The US Civil War is generally taken to have setteled the question as to whether any state can, on its own, leave the Union. It cannot. In Texas Vs White et al 74 U.S. 700, 19 L.Ed. 227, 7 Wall. 700 1868 the US Supreme Court confirmed this when wrote (in pars 101 & 102 of the opinion):

When, therefore, Texas became one of the United States, she entered into an indissoluble relation. All the obligations of perpetual union, and all the guaranties of republican government in the Union, attached at once to the State. The act which consummated her admission into the Union was something more than a compact; it was the incorporation of a new member into the political body. And it was final. The union between Texas and the other States was as complete, as perpetual, and as indissoluble as the union between the original States. There was no place for reconsideration, or revocation, except through revolution, or through consent of the States.

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. The obligations of the State, as a member of the Union, and of every citizen of the State, as a citizen of the United States, remained perfect and unimpaired. It certainly follows that the State did not cease to be a State, nor her citizens to be citizens of the Union.

Notice, however, the statement that the adherence of a state to the US could not be changed: "except through revolution, or through consent of the States."

This leaves open the possibility of a bilateral departure of a state.

Congress has, under the Constitution, the power to admit new states to the Union, and to join stats or parts od states into new states, or to divide states, with the consent of the states involved. Anothe answer to this question suggested that a treaty might lawfully cede potions of the territory of the United States, and this seems plausible, although there is no explicit provision for such an action, nor is there any clear precedent in US history.

So if a State were to request, via an act of its legislature, perhaps supported by a vote of its people, that it leave the US, and if the US Congress passed a law consenting to this, and declaring that the state involves was no longer a part od the US, would that law be valid under the Constitution? It might well be held to be valid, given the other powers Congress has over the extent of the Union, but it might equally be held to be invalid and void. There is no case law on that point, for Congress has never yet consented to any such attempted departure of a state.

Surely an amendment to the Constitution could be passed, clearly giving Congress such a power. That is nothing but speculation, as no such amendment has even been formally proposed.

Thus the question must be considered undecided at this time.

The US Civil War is generally taken to have settled the question as to whether any state can, on its own, leave the Union. It cannot. In Texas Vs White et al 74 U.S. 700, 19 L.Ed. 227, 7 Wall. 700 1868 the US Supreme Court confirmed this when wrote (in pars 101 & 102 of the opinion):

When, therefore, Texas became one of the United States, she entered into an indissoluble relation. All the obligations of perpetual union, and all the guaranties of republican government in the Union, attached at once to the State. The act which consummated her admission into the Union was something more than a compact; it was the incorporation of a new member into the political body. And it was final. The union between Texas and the other States was as complete, as perpetual, and as indissoluble as the union between the original States. There was no place for reconsideration, or revocation, except through revolution, or through consent of the States.

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. The obligations of the State, as a member of the Union, and of every citizen of the State, as a citizen of the United States, remained perfect and unimpaired. It certainly follows that the State did not cease to be a State, nor her citizens to be citizens of the Union.

Notice, however, the statement that the adherence of a state to the US could not be changed: "except through revolution, or through consent of the States."

This leaves open the possibility of a bilateral departure of a state.

Congress has, under the Constitution, the power to admit new states to the Union, and to join stats or parts of states into new states, or to divide states, with the consent of the states involved. Another answer to this question suggested that a treaty might lawfully cede potions of the territory of the United States, and this seems plausible, although there is no explicit provision for such an action, nor is there any clear precedent in US history.

So if a State were to request, via an act of its legislature, perhaps supported by a vote of its people, that it leave the US, and if the US Congress passed a law consenting to this, and declaring that the state involves was no longer a part of the US, would that law be valid under the Constitution? It might well be held to be valid, given the other powers Congress has over the extent of the Union, but it might equally be held to be invalid and void. There is no case law on that point, for Congress has never yet consented to any such attempted departure of a state.

Surely an amendment to the Constitution could be passed, clearly giving Congress such a power. That is nothing but speculation, as no such amendment has even been formally proposed.

Thus the question must be considered undecided at this time.