Calhoun had a point......as did Jefferson and Madison when they wrote the Kentucky and Virginia resolutions in 1798. The states were and from the start were intended to be parties to the constitution - not mere administrative conveniences for the all powerful federal govt.
In this case, I’d be quite willing to let California or any other state go in peace if that’s what their voters wanted. The union is supposed to be based on consent - not force, threats or violence.
How many of its voters would you be willing to allow to remove all the Federal property in California, along with all the U.S. citizens residing there who disagreed?
50% +1?