One of the basic reasons for fighting wars is the desire to end a political union. The US, for example, fought a civil war that cost over 600,000 lives.
So it seems that real peace cannot be established in the world without addressing this issue. People have the right of free association and can freely form groups and associations. By banding together. those individuals do not lose this right. So these groups also have the right to enter into associations with other groups and to end those associations.
The right of peoples and territories to peacefully leave political entities is the ultimate way to make government accountable. To paraphrase a heavily advertised mortgage website, when governments compete, you win!
This is actually in practice in Switzerland. There, people in any area can vote to leave their canton (like a county or state, I guess) and join another or form their own. Government, since it is the user of force, has to be a geographic monopoly, but does not have to be a temporal one. Just like any relationship, there has to be an out. A contract without an exit clause is a recipe for disaster.
That doesn't mean there shouldn't be a protocol or process, or that territories can leave a union without taking their share of any debt.