Assuming there's a group of people who more or less agree to be separate from some larger group, it's hard to think of legitimate reasons why they shouldn't be allowed to secede. Their forefathers having signed an agreement is hardly a convincing reason to hold them in a union they no longer like. Neither are issues of the current political situation ("it would destabilize the region...").

There's still hard questions, though. How do we split the national debt? What about various government assets? Who has a choice about what nation to join? And so on.

All hard questions, but none of them seems to undermine the idea that people should be allowed to decide to be separate.