Self-determination is a wonderful thing, but nationalism is a terrible thing. The difference between them is this. Self-determination is guided by a principle: if a piece of territory is in dispute, then its sovereignty should be settled by asking the people who live there. It is not something to be settled by asking people who do not live there.
For example, according to the principle of self-determination, the question of the sovereignty of the Falkland Islands is very clear. The overwhelming majority of the people who live in the disputed territory want it to remain British. The fact that there may be many more living in Argentina who would prefer “Las Malvinas” to be part of Argentina is irrelevant. Or at least it’s irrelevant according to the principle of self-determination, because they live outside the disputed territory.
Unlike self-determination, nationalism is not guided by principle. Instead, it takes its direction from an ideal of what is best for an identifiable group. This differs from one group of people to the next (and can differ between individuals who have different ideals for the same identifiable group of people). So although strict compliance with the principle of self-determination cannot generate conflict, rival nationalisms can come into conflict, and often do. Over the course of history, perhaps more people have died in disputes over territory than in any other sort of conflict. We are a tribal species, and nationalism is tribalism on the largest scale.
Self-determination is democratic. All that counts is what the majority of a group of individuals (i.e. the people who live in the disputed territory) actually prefer. But nationalism tends to count not what people do as a matter of fact prefer, but what they would prefer in an ideal world, or should prefer for the ideal of nationhood to be realised.
The question of the sovereignty of the Falkland Islands is about as clear as it gets, at least according to the principle of self-determination. Similar questions about Wales, Northern Ireland and Scotland (i.e. should they remain part of the United Kingdom?) are somewhat less clear, because the respective majorities are slimmer. In such cases, principled attachment to self-determination often merges into unprincipled nationalism. The discussion tends to shift imperceptibly from what people do in fact prefer, to what the right sort of people prefer, or to what ordinary people would prefer if they were less ordinary by being better educated, or to what people should prefer according to the ideal of what is best for the group.
Very often, this appeal to an ideal trades on some idea of ethnic purity. The right sort of Scotsman is a “true” Scotsman; the better-educated Welshman speaks the Welsh language; authentic Irishmen enjoy traditional Irish music and play Gaelic games; and so on. The ones who don’t are supposedly remiss in some vaguely “moral” way, and it’s assumed that they should be guided by the ideal. Nationalism nearly always enthusiastically promotes a nation’s language, art, and distinct ways of life—and despises the other language, the other art, and the other, less authentic, more corrupted, ethnically “impure” ways of life.
So even though a clear majority of the people who live in a disputed territory may prefer the status quo, it very often happens that a nationalistic movement blurs matters by making an issue of authenticity and ethnic purity, appealing to ideals instead of actual majority preference. By blurring the issue, and by appealing to tribal sentiments, nationalism can give it the appearance of being a “live issue”, even when the principle of self-determination can settle it easily and unambiguously. Typically, the slimmer the majority, the greater the potential for nationalist blurring of the issue.
I think it’s wonderful for a language to work as a means of communication, but terrible for a language to become an expression of authenticity or a symbol of ethnic purity. I’d say much the same about art, and other forms of culture and ways of life. Apart from the danger of political conflict, there is the damage done to language, art and other forms of culture by turning them into vehicles of struggle. Language, art and other forms of culture are enriched by intermingling rather than insulation, they are improved by a broader rather than a narrower range of influences.
Looking back 100 years to the Irish Rising of 1916, I find very little to like in its leaders. They overruled what most Irish people actually wanted at the time, and instead appealed to a nationalistic ideal of what they should have preferred. I admire the bravery of the 1916 leaders, but I don’t like what they did with it.