The term "nationalism" is generally used to describe two phenomena: (1) the attitude that the members of a nation have when they care about their national identity and (2) the actions that the members of a nation take when seeking to achieve (or sustain) self-determination. (1) raises questions about the concept of nation (or national identity), which is often defined in terms of common origin, ethnicity, or cultural ties, and while an individual’s membership in a nation is often regarded as involuntary, it is sometimes regarded as voluntary. (2) raises questions about whether self-determination must be understood as involving having full statehood with complete authority over domestic and international affairs, or whether something less is required.
It is traditional, therefore, to distinguish nations from states — whereas a nation often consists of an ethnic or cultural community, a state is a political entity with a high degree of sovereignty. While many states are nations in some sense, there are many nations which are not fully sovereign states. - Internet Encyclopedia of Philosophy
Nationalism develops from the idea that the nation, in contrast to the nation-state, is formed of the indigenous people to an area. It is the longest-lasting and most sensible form of government, for it groups together people who have culture, heritage and language in common. The modern nation-state imposes political boundaries on an area, moves people into it, and declares it a "nation," but without this lack of inherent consensus such states become marketplaces instead of living cultural entities.
In a world where human thoughtlessness in the name of enhancing personal wealth is ruining our environment and turning our lives into passages between grey concrete tunnels to fulfil ultimately meaningless tasks, nationalism is a re-assertion of the organic bond between individual and society. By placing culture before commerce, nationalism advocates a more meaningful daily life. It ends the bad judgment and ugly cities justified by someone somewhere being willing to buy something, regardless of its eventual utility or indirect, socialized costs. Nationalism makes the state serve the people, where nation-state systems make the people serve the state.
We are told that we have economic freedom, social freedom and options for living, but the small print says this is only within the context of a market-driven nation-state where the economy and what people are willing to purchase with their votes predominates over any lasting value. Thoughts of "what is right" take a distant backseat to what is popular, and what will be voted for by a crowd in the mode of their lowest common denominator, as well as what is profitable. What is profitable is often destructive in the long term, as global climate change and increasing dissatisfaction show us. We need a new plan. Nationalism is that plan.
Nationalism was the most successful political force of the 19th century. It emerged from two main sources: the Romantic exaltation of "feeling" and "identity" and the Liberal requirement that a legitimate state be based on a "people" rather than, for example, a dynasty, God, or imperial domination. Both Romantic "identity nationalism" and Liberal "civic nationalism" were essentially middle class movements. - Modern History Sourcebook