States did not have the right to unilaterally join the Union, how could the retain a right to unilaterally leave?
Because it’s not mentioned in the Constitution. And the disposition of things not mentioned, is mentioned. Very specifically and very clearly.
That's not exactly true...each state decided for itself whether to join the Union...Article VII of the Constitution provided that once a sufficient number of states ratified...the government was formed under Constitution with respect to those states that ratified it
No state was forced to join the Union without its express consent
Moreover, it was a basic Lockean concept of politics that any power granted by a sovereign can be reclaimed...those in the state legislatures that ratified the Constitution would certainly have perceived that they were representatives of a sovereign and were voluntarily delegating some of the state's sovereign powers to a new federal government...had any of teh Federalists come out and declared during the ratification process that ratification would forever bind the states to the Union until such time as the other states permitted it to leave...there is not a chance the Constitution would have been ratified
Even an ardent Federalist like Hamilton saw the irony of a Constitutional Republic existing only through the threat of force against unwilling member states...in discussing why the federal government needed a direct power of taxation during the ratification debate in NY, Hamilton mentioned the "aburdity" of a civil war against recalcitrant states
It has been observed, to coerce the states is one of the maddest projects that was ever devised. A failure of compliance will never be confined to a single state. This being the case, can we suppose it wise to hazard a civil war?
Suppose Massachusetts, or any large state, should refuse, and Congress should attempt to compel them, would they not have influence to procure assistance, especially from those states which are in the same situation as themselves? What picture does this idea present to our view? A complying state at war with a non-complying state; Congress marching the troops of one state into the bosom of another; this state collecting auxiliaries, and forming, perhaps, a majority against the federal head.
Here is a nation at war with itself. Can any reasonable man be well disposed towards a government which makes war and carnage the only means of supporting itself -- a government that can exist only by the sword? Every such war must involve the innocent with the guilty. This single consideration should be sufficient to dispose every peaceable citizen against such a government. But can we believe that one state will ever suffer itself to be used as an instrument of coercion? The thing is a dream; it is impossible.
What??
George Washington phoned up the thirteen original states and told them they'd been brought into a union and to hurry up and send representatives?
The original states unilaterally voted to join, the delegates to convention acted to establish a format for union. There was no congress as such to vote each state up or down. Once established, rules were put in place for future new states but none that I know of regarding any state walking out.
Union was voluntary and only those territories obtained by war/purchase might possibly fit your assertion because they were products of federal government and not (necessarily) an established community. (However, note the oft repeated votes for / against statehood in Puerto Rico, and Hawaii's vote to join the union.
However, since it was not illegal or treasonous, since there are two parties involved in the secession of any one state - the federal government was free to choose its course of action, and it chose war.