Problems with Nationalism: George Orwell wrote in his essay, "Notes on Nationalism", that there is a difference between patriotism and nationalism. A patriot is someone who loves his country, its people, and its culture, but has no desire to force his culture or religion or lifestyle on anyone else. His patriotism is only one of the many factors, both emotional and rational, that forms his worldview. A nationalist, on the other hand, is someone whose worldview is emotionally based on his love for his chosen power unit, which may be a state, a people, a political system, a religion, or whatever. A nationalist thinks only or mostly about the comparative prestige and power of his chosen unit.
According to Orwell's definitions, I consider myself an American patriot. I don't want the United States to annex anybody else's territory. I see no need to force American culture on anybody. I believe that if you leave us alone, we ought to leave you alone. Sure, I have emotional, non-rational feelings toward the United States, but I try to keep them under control and not let them dominate my thinking.
Excessive nationalism can have various negative consequences.
1) Irredentism. You believe that your power unit should control territory that doesn't belong to it. Milosevic's Greater Serbia and Hitler's Greater Germany are classic examples. When Catalan nationalists start talking about the Països Catalans, including not only Catalonia proper but also Valencia, Baleares, and Roussillon, as a single unit, that's a problem since none of those places want to be part of Catalonia. And let's not even mention Gibraltar. Oops, I just did.
2) Internal divisions. You believe that some of the people within your unit are not part of your group. Hostility toward immigrants is a very common consequence of this. We see this in the United States; some Americans fear that the country is being flooded by Spanish-speakers. Our man Jordi Pujol recently made a very nasty crack about how immigration is going to make Catalonia a land of mestizos. This is made even worse when there are several groups who have lived within the same territory since time immemorial; for example, in Catalonia, there are some people who believe that others are not good Catalans because of the language they speak or the political party they vote for. In Spain, there are people who believe that many Catalans are not good Spaniards for the very same reasons. When taken to an extreme, this leads to ethnic cleansing and even genocide. Fortunately we're nowhere near that around here...but up in the Basque Country...
3) Falsification of history. Nationalists have the habit of ignoring facts when they don't fit into their chosen worldview. For example, Sabino Arana invented his own history of the Basque country which has nothing to deal with reality. Some Spanish nationalists claim that there is a Black Legend (the Inquisition, expulsion of Jews and Moors, ethnic purity, foreign aggression) that has been greatly exaggerated by foreigners who wish to slander Spain's name.
4) Excessive emphasis on symbolism. September 11 is the Catalan national holiday. So what are they fighting about? What flag should fly over the City Hall. Who should lay flowers at the Holy Statue of Rafael Casanova. This is a serious distraction from real issues.
5) Excessive emotionalism. This leads to poor decision-making. The classic example is probably Japan in 1941; they knew they were going to lose but went to war anyway.
6) Ignorance of other groups. Nationalists have the bad habit of assuming that either everybody is just like them (a common American error) or that everybody else is completely different. This is generally because nationalists are obsessed with their own unit to the point that there's no room in their brains left for the comprehension of other units. What this leads to is stereotyping of others. In Barcelona there's a real problem with this: many people who think they're well-educated about the world are so self-absorbed with nationalist issues that their comprehension doesn't extend south of about Tortosa.
7) Bitterness. Some Catalans, for example, live in a constant state of anger because they believe that Catalonia is an oppressed nation under the Spanish jackboot. They obsess over it. This is probably not good for their mental health, and it makes them unpleasant to be around.