Secession may not be mentioned as "secession" in the Constitution. It is covered by insurrection and rebellion however.
Actually, that speaks to take over by force of arms of either the federal or state governments. Secession of a state or group of states is in fact covered by Amendment X,
The powers not delegated to the United States by the Constitution, nor prohibited by it to the states, are reserved to the states respectively, or to the people.
Secession would in fact have to be prohibited to the states expressly in order for the "insurrection and rebellion" section to be activated.
Reference: 1.
When in the Course of Human Events: Arguing the Case for Southern Secession
2.
A Constitutional History of Secession
Hopefully, this will end this deviation from the question.
I think, too, that some are looking in the wrong place in history, since most know little of the 140 year old secession attempt in America. Perhaps some should look at more recent history. The old Soviet Union saw a number of lawful, relatively bloodless secessions since 1989. Although Russia, for several reasons, is still influential in the area, we might look to those secessions as good examples of the activity. There's an ongoing potential secession with
Belgium possibly separating into at least two countries, which may or may not align themselves with other countries with which they share a history. Flanders, for example may go it alone, or affiliate with Netherlands since both speak Dutch. Wallonia may go it alone, or affiliate with France since they're native French speakers. The chance of warfare breaking out in Belgium is very low.
There is simply no need for violence in this question, and none is anticipated in America. I think we need to consider the benefits, which in my opinion are many; and the negatives, which in my opinion are few.