I think we actually mentioned this in my summer school US History class. In Article 4, Section 3, Item 1 of the Constitution, it reads: “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.”
The westernmost counties of Virginia petitioned Congress to have their counties declared a separate state. Not wanted to lose a friend, Congress agreed. After the Civil War, Virginia gave its approval, perhaps because it was in no real position to dispute the matter.
“. . .they could as easily have been carrying euphoniums and wearing war paint for all the notice their quarry would have taken of them.”
-Douglas Adams, “Life, the Universe, and Everything”