When we talk about succession, the apportioning of the nation's debt strikes me as the biggest factor in where I don't actually have any answers. If a province wants to say that the territory under their jurisdiction is now theirs, meh, we live in a progressive world. And obviously any debts owned by the citizens remain intact.
But how would we deal with the national debt? It seems unfair that the remainder now owes more. But it also seems unfair that a province cannot 'opt out' of being saddled with a larger burden of other people's spending.
Any actual theory out there that springs to mind from your own conception of how it should work?
Depends on how the independence happens, I think.
Usually, if a country fights for independence and wins, international custom is to saddle it with
none of the parent country's debt. Seems reasonable. Much of that debt would have been contracted in the course of trying to prevent the new country's independence anyway, and one can hardly expect the new country to pay for, say, the weapons used to kill its own people. That's what happened with South Sudanese independence in 2011. (Unfortunately, gaining independence without debt did not prevent massive overspending on military forces and civil war a few years later.)
Otherwise, it gets a bit messier. I think it's reasonable to suggest that each seceding territorial unit should, in the case of a peaceful and negotiated transition, continue to honor the debts contracted by it
as an individual territorial unit, i.e. if Virginia were to secede from the US right now it would have to pay for the Commonwealth of Virginia's debts. The national debt would have to be a matter of negotiation between the parent country and the seceding country, because while much of it was undoubtedly contracted for reasons totally unrelated to the seceding country, some of it
was contracted for reasons related to the seceding country. Ideally the seceding country would assume some proportion of the national debt based on these negotiations. I don't think it's possible to get more granular than that: you'd have to deal on a case-by-case basis.