In another thread discussion of seceding from the union has covered a lot of ground, up to and including the US Supreme Court 1869 case of Texas vs White which established that a state cannot unilaterally secede from the union.
However the US Constitution in Article 4 states:
New States may be admitted by the Congress into this Union; but no new States shall be formed or erected within the Jurisdiction of any other State; nor any State be formed by the Junction of two or more States, or parts of States, without the Consent of the Legislatures of the States concerned as well as of the Congress.
The process used to date has been for the new states to apply to join the union, and then Congress votes on accepting them or not. Where a new state is created from an old state (i.e. a state splits into two or more) then the original state has to approve the new borders and the US Congress has to confirm the new borders, as happened with Kentucky and Tennessee, and a few others cut from original states.
That raises the question, in my mind, as to what happens if an existing state approves the creation of a new state from within its borders, has the border changes approved by the state and Congress but the new state doesn't apply to join the union. Since it hasn't app;lied and been accepted as a new state, would it simply become a new territory under direct federal control until it applies and is accepted as a state?