On one hand we have sovereign nation states with stable, recognised borders. On the other hand there can be groups of people inside them that want to split off to become independent, or join a neighbouring country. I can feel sympathy for both sides. I would not want part of my country to split off, but if all the people living there feel not connected to the rest of the country, I may need to accept that. Is there any rational guideline that can help here? Would it be a matter of majority vote for the region? What about migration changing that majority?
I don't think there's any one answer that all will accept on this question, but to simplify the sorting process for you, the real question you are asking is: "What is a state?" and then within this specifically the question of how such a state relates to its members.
In terms of philosophical theories of the state (in this case the polis), the earliest two philosophers, I know of, in the West who consider this question are Plato and Aristotle.
For Plato, the state is either a small village (early in the Republic) or a complicated system where the members of the state are split among three roles: philosopher-kings, guardians, and producers (for an extended treatment see here: http://plato.stanford.edu/entries/plato-ethics-politics/). On such an account, the parties all agree that this is the best way to accomplish justice as they are enlightened by its presence. On such an account, it makes no sense to split off from the state.
From what I understand, there's a different account in the Laws of how things should be arranged but I'm not familiar with the details.
Moving to Aristotle, Aristotle believed the state was prior to the individual and the family. What this means is that the state is the organic whole of human living (like a hive to bees or pack to dogs). On such an account, it seems unlikely that it would make sense to split off, but it seems distantly conceivable.
Many contemporary Aristotelians are communitarians which means that they think each community is determined by a shared set of values, and that these sets of values are incommensurate with others. On such an account, each community is a polis which can be held together in a larger federation -- but this federation is dissolvable.
Augustine in the City of God adapts elements of Plato's philosophy to present the idea of dual citizenship for Christians in Rome and Heaven. On such an account, the state below is more arbitrary and maleable since the image of justice is through religion not the state.
I'm not familiar enough with Aquinas's account of the state to provide the contours, but I gather it's a somewhat communitarian picture but not overly prone to allowing schisms emphasizing the holistic aspects of the Aristotelian picture.
Moving forward to Hobbes and Locke, we arrive at the idea that the state arises through negotiations of power and the surrendering of the right to bash others in (social contract theory). On such theories, I don't see the ultimate objection to schisms in the state. For instance, it's hard to see why such a contract isn't malleable enough to allow for that as say per se Scotland devolving or something like that.
In this era, we also see the rise of the consideration of notions of sovereignty. For Hobbes, sovereignty occurs either when people contract together and agree to follow a common authority or when people bandy together for mutual protection. This sovereignty is revokable --as makes sense under a contract theory of governance. As ben rudgers suggests, one clear method of revocation is to kill the sovereign. But it's less clear whether a sovereign can allow for secession on the part of some parts of the state. (Source for this paragraph: http://plato.stanford.edu/entries/hobbes-moral/)
For Locke, the contract works in such a way that most freedoms are granted to the citizens by the sovereign, but these are established in nature and self-evident. Thus, insofar as the state is legitimate, there should be no need to schism. Insofar as the state is illegitimate, schism is no longer the right word.
The Kantian picture of the state is a Republic guided by reason with the power of the state. Such an image depends on a type of federating where the size of the particular state is less important than the unity of the rational whole of humanity.
Hegel is similar to Kant in seeing the state as ultimately a product of reason and similar to Aristotle in seeing the state as an organic whole larger and prior to the individual humans and families that make it up. At the same time, the Hegelian picture depends on internal conflicts and their modifications. The net result of that is that Hegel would find such dissolution where states break apart to be a temporary resolution until their inevitable reunion in something larger and more complete as an integration between the individuals, families, the community, and Absolute spirit. (long story ...).
There's going to be some other theories to consider as well I'm guessing, but these are the ones I can sketch briefly.
It depends on how the sovereign state handles ownership, but lets start with the extremes as they are the easiest to explain:
Consider an Absolute monarch - the rights to everything lie with the monarch, who can do as they wish, however if they own the people of the sovereign state, then the people cannot do anything.
Now consider a 100% democracy: If the sovereign state is purely democratic, then 1% of the population owns 1% of the resources, including land. In that case, the 1% would have the right to do whatever they wish with their 1% ownership and could take it elsewhere if they so chose, however the question becomes how does an individual (or group of individuals) know where THEIR share is? square miles? GDP per capita of the area? population density? however they choose, they then need to get 50% of the other group to buy-in.
The US Declaration of Independence starts with the following:
When in the Course of human events, it becomes necessary for one people to dissolve the political bands which have connected them with another, and to assume among the powers of the earth, the separate and equal station to which the Laws of Nature and of Nature's God entitle them, a decent respect to the opinions of mankind requires that they should declare the causes which impel them to the separation. We hold these truths to be self-evident, that all men are created equal, that they are endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty and the pursuit of Happiness.
That to secure these rights, Governments are instituted among Men, deriving their just powers from the consent of the governed, That whenever any Form of Government becomes destructive of these ends, it is the Right of the People to alter or to abolish it, and to institute new Government, laying its foundation on such principles and organizing its powers in such form, as to them shall seem most likely to effect their Safety and Happiness. Prudence, indeed, will dictate that Governments long established should not be changed for light and transient causes; and accordingly all experience hath shewn, that mankind are more disposed to suffer, while evils are sufferable, than to right themselves by abolishing the forms to which they are accustomed. But when a long train of abuses and usurpations, pursuing invariably the same Object evinces a design to reduce them under absolute Despotism, it is their right, it is their duty, to throw off such Government, and to provide new Guards for their future security.
According to Ayn Rand, a region should be allowed to secede from a country if it wants to do so to improve the provision of individual rights:
Why is this a rational guideline? Rationality consists primarily in refusing to adopt institutions that would lead to a person being unable to act on criticisms he has of a particular habit or idea. A government that suppresses individual rights will prevent people from acting on their criticisms of whatever the government aims to promote. I shoudl note that being able to act on an idea does entail that you will succeed or that you will have a comfortable as a result of acting on it. A person who wants to spend his time being drunk is not going to have a particularly comfortable life or get rich since he will be unable to think a lot of the time as a result of acting on his ideas.
An example of illegitimate secession: in the case of the American Civil War the South seceded in order to preserve and extend slavery. Some Southern sent secession commissioners to other states to argue the case for secession, such as William L. Harris, the commissioner from Mississippi to Georgia who said:
Our fathers made this a government for the white man, rejecting the negro, as an ignorant, inferior, barbarian race, incapable of self-government, and not, therefore, entitled to be associated with the white man upon terms of civil, political, or social equality.
This new administration comes into power, under the solemn pledge to overturn and strike down this great feature of our Union, without which it would never have been formed, and to substitute in its stead their new theory of the universal equality of the black and white races.
The Southerners wanted to be able to keep black people as slaves and so stop them from leaving their masters if they thought they could do better in some other way. This is not rational since the slaveowners were forcing the slaves to act against their judgement.
All civic freedoms, the freedom of expression, of the press, of religion, of assembly, of association, and the freedom to strike as well as the inviolability of the individual and the home, shall be irrefutably effective within the territory of the Estonian Republic...
These rights were not granted by the Soviet Union and so the Estonians wanted to leave and took the opportunity to do so in the late 80s and early 90s.