Just think about the word "nationalism". I bet that you immediately thought of things like neonazis, racism, the holocaust... But why? Why has that word become such a dirty one?
As the saying goes (and perhaps most popularized by a famous speech by Abraham Lincoln), a house divided against itself cannot stand.
What does this mean? A society has the best chances of becoming prosperous and successful when all of its members work together, when they all cooperate, when there's a sense of camaraderie among them.
Consider the post-World-War-2 Finland for instance. This was a country in economic ruin. Because of the war there was famine and an economic crisis. There was, however, a strong sense of national identity among Finns, a culture of cooperation, of working together for a brighter future. In about 20 or 30 years Finland rose from absolute economic ruin to be one of the wealthiest welfare states in the entire world. A decade or two more, and it became one of the biggest technological countries in the world.
This happened, at its core, because of Finnish culture. As said, a culture of camaraderie and cooperation. A culture of strong national identity, with little to no infighting. (Of course that's not to say there was no political or economic infighting at all; that's something that will always exist no matter which country and which era we are talking about. However, there was relatively little of it, and it had little to no consequence overall.)
In other words, a culture of strong nationalism. In the good sense.
It was a time where one could be proud of being a Finn. Not proud in the sense of feeling superior to others, but proud in that we were a nation that could work together and cooperate in order to better the lives of everybody, to become prosperous, to become one of the wealthiest welfare states in the world. Proud of the fact that we, as a culture, were peaceful and cooperative, and who work hard to prosper.
Finnish culture was also always notable for its hospitality. This aspect, of course, is not exclusive to this particular culture, but is very common in most cultures. The point is, however, that a strong sense of national pride does not somehow automatically make people xenophobic, discriminatory or cynic towards other people.
Nationalism, when done in the right way for the right reasons, is not a bad thing. On the contrary, it's a strong enabler of prosperity and progress. Of course a strong sense of nationalism can be abused for the wrong goals, but that doesn't mean that all instances of strong nationalism are like that. Nationalism does not by necessity lead to totalitarianism.
Modern multiculturalism abhors a strong national identity, and it's a big reason why it's detrimental.
That's not to say that multiculturalism cannot work. It can. However, it needs the proper setting to work. For example the United States is an example of this, with immigrants moving into the country from all around the world, and the country becoming one of the richest and most prosperous in the world. (Granted, these immigrants basically eradicated the already-existing native population, literally stealing their land and property, which is something worthy of big shame, but that's not really the point of this.) The reason why multiculturalism worked in the United States (and also in Canada) is that nothing was given to the immigrants for free, and instead they all had to work hard to succeed. There was no nanny-state pampering them, idolizing them and giving them riches for absolutely no merit. No, they had to work really, really hard to merely survive, not to talk about prosperity. This created a nation where everybody had to cooperate, work hard, and become prosperous.
Not so with modern European multiculturalism. Immigrants are not required to work hard, they are not required to adopt the hosting culture (but in many cases are outright encouraged not to), and everything is given them for free, without them having to do anything to earn it.
Sure, some immigrants will work hard regardless of this, even though they wouldn't have to. Big kudos to them. However, such an easy pampering nanny-state-like life also attracts the other kind of immigrant. Those who will not integrate, will not cooperate, and will abuse the system for their own benefit as much as they can.
And thus we get non-cooperation, clashing of completely different cultures, infighting, political and racial tension. In other words, a house divided against itself.
"Nationalism" has become a dirty word, and is almost synonymous with "totalitarianism", "nazism" and "racism", even though it does not mean that, nor does it have to mean that. We have forgotten the good things that a strong national identity has brought us, through hard work and cooperation. We are destroying ourselves from within. We are dividing our house, and it's going to crumble.
That saddens me. It really does.