Like other terms in social science, it is broad.
Nationalism has no clear founding theorist.
Anthony Smith provided 7 themes of nationalist ideology.
1. Humanity is naturally divided into nations.
2. Each nation has its character.
3. the source of all political power is the nation, the whole collectivity.
4. For freedom and self-realization, men must identify with a nation.
5. Nations can only be fulfilled in their own states.
6. Loyalty to the nation-state overrides other royalties.
7. The primary condition of global freedom and harmony is the strengthening of the nation-state.