I recall reading that what we now understand as nationalism, the correlation between state an nation, was born in the late 18th century with the French Revolution. Before that, territories were thinked of land zones that were ruled by someone. Thats why countries could be divided between the sons of a dead king, and fusioned with the marriage of two leaders. When the idea that power emanated from the people and not the kings was put in practice in France, in a way that threatened the unity of the country, so nationalism emerged as a reason to keep together the territories of the old kingdom.
That's how I understand it, but maybe that's not entirely correct, or maybe it's fully wrong. Either way, I would like to now. Thanks in advance.
Nationalism can be abstracted as what you explained, but it also has a unique history in every nation/country that should he accounted for. Great Britain developed their nationalism in a way that is somewhat similar to what you described, but with notable difference.
For the most part, nationalism took off after and as a result/reaction to the French Revolution. But England (and later Great Britain) had begun to seriously develop a strong sense of nationalism by 1688, following the Glorious Revolution. There emerged a sentiment, at least among the aristocracy and elite merchants, that England and its holdings represented an entity that was Protestant, commercial, maritime, and free, an empire of liberty. They developed logics and arguments in support of this view and further asserted themselves as a force for good in the world. Historians of the British Empire clearly identify this as a nationalist sentiment.
This nationalist sentiment developed in England as a reaction to the Glorious Revolution, wherein James II was forced to flee and was replaced with William and Mary in a constitutional democracy. English thinkers had to rethink what England was as a result this powerful check on the monarchy. It was clear that it was no longer just the throne that held England together, and English and later British thinkers envisioned a nation unified by shared values and goals. In other words, nationalism.
Edit: Grammar
Sources:
David Armitage - The Ideological Origins of the British Empire