Well that depends entirely on the question whether a Union voluntarily joined can be voluntarily left. The Declaration of Independence tells us that an Imposed Union over one thousand years old can be broken by this thing called a "natural right" granted by "nature and nature's God", so a rational person would conclude that this very same right which legitimized the USA, should also apply to anyone else that seeks independence.
What legal or moral argument do you assert that can stand against "nature, and nature's God"?
I would explain it but you don’t possess the requisite computing power to process it.
Incorrect. South Carolina did leave the Union. But that didn't mean they could attack Federal Property. That is silly talk. If South Carolina truly believed that Fort Sumter belonged to them, they would have occupied it when they seceded, instead of letting it sit empty in the middle of their harbor. It didn't occur to the new Confederacy to takes steps in regards to Fort Sumter until Major Anderson moved into it.
In addition, you continue to promote the bad analogy of the relationship of the colonists to the British Kingdom, as being similar to the relationship of the States in a Union. In the 50 score years of the "Imposed Union" (your words), I don't recall the Colonies ever ratifying any constitution with Britain. You are laboring under a multitude of misapprehensions.
The Declaration of Independence does not have standing in United States Law.