The Constitution specified the standards and procedures whereby a state could be added to the union, but it made no provision for a state to withdraw from that union. I always thought that an oversight. Leaving should not have been as simple as one vote from one state legislature, but should have been possible with the concurrence of a reasonable percentage of the states. This would maintain the association as voluntary as well as prevent silly grandstanding (We're gonna leave if you don't give us this X") by any state.