The heart of this question is how you define a nation state. Nations are an identity constructed from several ingredients: territory, language, religion, culture and traditions. A Nation State is a political construction defined by territory which is sovereign. Usually they have a constitution that declares how they are to be governed within their borders. Nation states have international relations and trading relationships with other states. Some states are are part of larger formations defined by treaties. Some states exist within or are dependent on other states. It gets messy.
The nation state is a fairly recent political invention that really got going in the 18th and 19th centuries. The big question is whether a state is viable in terms of its economy and its ability to safeguard its borders from incursions by other states, define its own constitution and laws. There have been times when small states were common and, at other times there was a tendency towards the formation of larger states by unification of smaller states. Germany and Italy are examples of quite young nation states formed in the 19th century. The UK is a much older nation state formed by a the unification of the crowns of England, Ireland, Scotland and Wales at different times.
The internal political dynamics of nation states can be either toward unification absorbing others into a larger state. Or towards seccession, spliting up into smaller independent states. The US, is an interesting example. Formed by seccession from the British state, it went through several phases of expansion by conquest of neighbouring countries like Mexico and Spanish colonial possessions such as California. Also by the outright purchase of territory from other nation states, notably from France and Russia. It also grew by the annexation of the territory from native American tribes and it had external territories campaign to be included as states in the Union. This expansion went into reverse in the 1860s when the Confederate southern states tried to secceed from the Union causing a bloody civil war. It is a unusal example because it was not really near any other nation states that were also growing strongly, save for Canada.
Europe is fairly crowded with nation states and it has many examples of expansion and seccession. The borders between states often change in war and in peace. The internal political structure of nation states is sometimes tolerant of regions with their own national indentity and sometimes they dominant nation within a nation states tries to supress the national identify of other national identities in order to control the development of seccessionist movements.
The UK nation state went through a worrying period when the Scottish Nationalists sought a mandate for independence from the UK for Scotland. Canada has had the same with Quebec. In the past days Spain is heading for a constitutional crisis with the Catalans. On a larger scale, the UK has recently voted to leave the EU, which although it is a treaty organisation rather than a large nation state, it nonetheless is percieved to be threat to UK national soveriegnty.
It is possible for a nation state to split into smaller states amicably. Czechoslovakia is an example, that is now the Czech Republic mand Slovakia. But the divorce settlement can be acrimonious and difficult. Large states have integrated economies that are often very difficult to seperate. Scotland leaving the UK would have been a hugely expensive nightmare after 400 years of economic and political integration.
Seccessionist movements is not entirely an internal debate. Rival nations can support an independece movement in order to undermine the integrity of a rival state so that it becomes weaker. The Soviets used this approach as does Russia today. On the hand, for a nation intent on expansion at the expense of its neighbours, support for an oppressed minority can be an excuse for intervention and annexation. This tends to be a feature of states that share borders, but there are other ways a nation state can extend the territory it controls and empires and colonies are the obvious example. These have left behind a mish mash of nations trapped within borders hisorically defined by long gone colonial powers. The constitutional instabilities these leave behind are responsible for many wars across the world.
The sovereign nation state, is however, all we have got in terms of a political unit. There are some super states, such as the US, Russia, China and India. But the bigger the state, the more severe their problems if the government is not able keep the country constitutionally stable. But even quite small states have constitutional issues. There are big economic treaty organisations such as the EU that have soft power, but holding a consensus together is difficult and the political tide may turn towards exit as it has with the UK. These big federations tend to favour small states that do not have the crital economic mass to form a viable nation state. A lot of the EU countries have very small economies and are highly dependent on the much larger economic powers such as Germany, France, Italy and the UK. I guess the same is true of the US, where the economies of the Northern industrial states, California and Texas are far larger than many other states.
Neither the Catalans, the Scots nor the Quebecois are oppressed minorities seeking independence. They are quite a big chunk of the economy of Spain, UK and Canada. Independence would be a Pyrrhric victory that would damage both sides economically. It is all very well wrapping yourself in a flag and proclaiming you yearn for freedom, but sentiment is no substitute for a prosperity. People underestimate how difficult it is divide a nation state that has an integrated economy.
The Spanish government has seriously mishandled the Catalan independence question and is heading for a constitutional crisis. However, everyone knows how bad it could get because of the history of Basque seperatism in Spain. Constitutional questions require huge amounts of political capital for any government to make progress and it just got a lot harder. It looks as if the Catalans went for a simple direct approach to solving a difficult political problem. Referendums are dangerous political devices if they are not handled carefully. They are the equivalent of proclaiming a divorce by simply changing the locks on the front door. :dubious: