I don’t have an immense amount of knowledge about this subject so I apologize, but I will try to answer your question the best of my ability. I am using a book called The Roots of Nationalism: National Identity Formation in Early Modern Europe, 1600-1815. And maybe someone with more information will be better able to help.

From what I understand, no one particular invented nationalism. Rather it was the result of human nature. Naturally, the human brain likes to define things, to group things, and to differentiate things. When you grow up in a culture, it becomes a part of your identity. The Scottish philosopher, David Hume, in 1748 wrote a essays and made a clear statement about the origins of national character. In his essay “Of National Characters” Hume goes on to say that that national identity was primarily defined along cultural lines. And that national stereotypes and character can be traced back to the Middle Ages. He argues that national identity was a result of imitating each other’s behaviors. More importantly, Hume’s use of “nation” and “national character” seem to suggest that even at the time of him writing that essay, those words were part of common speech. Meaning that even prior to his writings that national identity and similar words had already been “invented.” We can look back upon the Middle Ages to see even back then there were songs in which people had pride toward the land, culture, or people they were from.

In simple terms, people often identify with the culture they were raised and associate with the most. Additionally the human brain enjoys things it is familiar with and also humans are drawn to people with similar thinking and identity. A person that has adopted their cultural identity does not want to believe that their identity bad, they want to think that it’s good, they want to be proud of it. If everyone in a nation believes that all of its aspects such as culture, language, religion, etc. are bad then differences arise amongst people, which means there is no national unity. And therefore with no national unity nothing works. One scholar discussing this subject, Andrew Hastings, suggests that England, before it was England, presented that a sense of national unity was already detectable there by the end of the tenth century which led to its founding. There is an idea shared amongst everyone in a nation, called “imagined community.” Argued by Benedict Anderson, he says that, although members do not know most of their fellow members, they all have an image of their (national) community in their minds. Furthermore, national symbols have existed within the cultures of nations and they were reused in a new historical contexts, without losing the older values attached to these symbols. They contributed to feelings of national unity, power and resilience precisely because of their rootedness in a long and meaningful history

Nationalism has been present since the forming of civilizations as early as the 10th century.

In a deeper regards to your question, nationalism is used merely as a method of gaining support or following for those leading, be it dictators, politicians, or kings. As I said previously, people don’t want to believe their identity and background are bad. Additionally, national unity allows for progress to be made. In general, a group working together to achieve something will be far more successful than a singular person working by themselves. So leaders use nationalism to say “Follow me and we will be great. Work together and we will achieve whatever.” Thus people unify to work toward bettering their community, so they can be proud of it.