The Article addressed the formation of NEW states only. It does not grant the federal government any power over what a state does with it's own territory. It does not grant a power to remove a state, nor prevent a state from seceding.
New States may be admitted by the Congress into this Union; but no new State 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.
Changes to the existing states require the consent of Congress. If North and South Dakota wanted to combine into a single state, then that would entail the removal of a state from the Union and that would require congressional approval. If California decided to split into two then that would mean the removal of one state and the creation of two new ones. That, too, would require congressional approval. The approval for any change in status of a state is a power reserved to the United States and not a power granted the states by the 10th Amendment. The consent of the other states is required for entry into the Union, it is required for any change in status while in the Union, clearly it should be required for leaving the Union as well.
The Article addressed the formation of NEW states only. It does not grant the federal government any power over what a state does with it's own territory. It does not grant a power to remove a state, nor prevent a state from seceding.
Jefferson Davis would disagree with you. He indicated that the intent of the framers must be considered on constitutional issues. He is also clearly on the record saying that the central goverment may coerce the states.
Walt