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
Supremacy is limited to the written Constitution and laws pursuant to it, not to "intent".