Nationalism is a feeling of unity among a group of people born out of the French Revolution and Age of Enlightenment idea of worship of the Patrie, or one's national origins and a sense of pride in their country.
Lack of nationalism, or a desire for new nations, was a cause for the split of countries such as Yugoslavia and Austria-Hungary. It can be considered positive, as a form of patriotism, but it can also go to extremes, leading to hatred of non-members of the nation (which is often ethnically defined) and violence. Examples of this negative nationalism are the Nazi Party in Germany (although it should be noted that the propaganda minister for the Nazis, Joseph Goebbels, when describing the Nazis as being of the German Left, said "we despise bourgeois nationalism."), the ethnic conflict in the former Yugoslavia, and the current isolationist policies of North Korea. The latter version, in particular, Nazi Germany, also led to nationalism being demonized to the extent that even having any pride for a country is considered evil, and the former was also considered bad due to the idea that nationalism causes wars. This interpretation was generally pushed by globalists and post-modernists, and adherents of the Idea of Progress.