I'm not sure I have the right terminology to ask this question.
My sense is that 19th century European nationalism--sometimes called Romantic nationalism? and usually marked as beginning with the French Revolution?--is considered somehow different in degree or kind from other types of ethic and political organizations in that it is somehow hyper or modern or institutional or violent or exclusive (i.e., by explicitly defining itself in dichotomous terms with an analogy between us/them and good/evil) or something relative to earlier, I don't know, "ur-nationalisms".
My question stems from reading and thinking about Ancient Greece. The Greeks, once organized into city-states, seemed to have had some conception of difference between themselves and others, so that Athens and Sparta seemed to understand themselves differently and, at times, antagonistically.
What makes those Greek city-state differences and antagonisms different from the differences and antagonisms of 19th century Europe? And what--other than the printing press, since Athens and Sparta, to continue my previous example, seemed to have imagined themselves differently prior to the printing press--accounts for the origins of the distinction between Ancient Greek city-state "nationalism" and 19th century European nation-state "nationalism" (other than the difference in scale between city-state and nation-state)?
Or am I just wrong about how people imagined their communities in Ancient Greece?
(Obviously I've read Benedict Anderson, but he was writing in the 1960s and I'm not up to date on more recent scholarship in the field.)
There are some earlier discussions of this which I'll link to:
Why is nationalism considered a modern concept
Some questions about nationalism
Nationalisms and early Modern Europe
Does looking back at history reduce the idea of national identity, or enhance it?
I have a better one about early modern Europe somewhere, but search isn't finding it. The tl;dr is that there are people who argue nationalism existed before the French revolution. We can divide them into two or three camps. The first is the "early Modernist" camp, who argue that there are clear nationalisms in Western Europe before 1791: Liah Greenfeld (who argues for Tudor England), Anthony Marx (War of Religion era Spain, France, and England), Philip Gorski (early modern England and the Netherlands), and I feel like there's one more. Rather than "modernists", they're "early modernists". More or less people accept these critiques. The there are people who want to push it back further, to the Medieval era (1066 and all of that), usually starting with England. Their names are escaping me but Roshwald and Hirschi are ones that come to mind. They argue, generally, that people in England (and Germany in Hirschi's case) self-consciously construct themselves as a nation in the mold of Israel in the Book of Kings (the guy who was Roshwald's intellectual forefather, whose name I'm blanking on, argued that the Bible was "Europe's textbook"). The problem here is that nationalism is a mass phenomenon, and even if it's not "mass", it needs to be extended at least to the middle class. Hence, Greenfeld starting with the Tudor ascendency where there's this massive social mobility. Roshwald and Hirschi don't convince me that "nationalism" is actually taught to "the nation", nevermind that there there is a group of people who want the cultural unit (the nation) to equal the governance unit (the state) which is a variation on a commonly used definition (Hechter's variation on Gellner). Then there are people who argue that there have always been nations. Armstrong (who argues for religious groupings, like Christians and Muslims), Anthony D. Smith (who is basically the dean of this school of thought), and Roshwald (who I believe argues for Greek and Roman cases counting). Here, again, I think it matters very much how you define nationalism. If you define nationalism as "having a cultural identity", then you can have nationalism much earlier. But that misses what was unique as we move on from the French Revolution. The opposite of nationalism isn't cosmopolitanism, where everyone is fine and dandy and we all play nice together, the opposite of nationalism is imperialism. Nationalism argues we are the proud and historic nation of X, and we only can have a member of X rule us, and (s)/he must rule in the name of the people. Nationalism was the movement that finally ended imperialism, more or less (first the contiguous empires like the Ottomans, Austria-Hungary, Russia, etc. during the second half of the long 19th century and later overseas empires like the French, English, and Dutch during the middle years of the Cold War). But one important thing about nationalism is that every (male) member of the nation counts, at least in theory. Gellner was wrong about many things, but his insight that nationalism involved a certain interchangeability is key (he thought it was for industrial labor, but that's just wrong, time-wise). I'm not expert, In Greece, I simply don't think you see that. The slave states of Athens and Sparta simply couldn't--you have what you might call a strong caste identity, but I'm not sure I'd be willing to call it a national identity in the modern sense (obviously though others, like Smith and Roshwald, would say that they're more alike than they are different).
Concerning Ancient Greek City states: To my knowledge, Greeks identified themselves with their polis first (Theban, Spartan, Athenian, etc), but had the knowledge that the city-states of the Greek region were fundamentally and culturally different from the civilizations that they made contact with (Secular, republican (in terms of voting) against pious, monarchies).
In my interpretation, I think nationalism was the same, just at different scales. For instance, you might be from the French nation and state (so you're French) while an Ancient Greek counterpart could be born from the Spartan nation and state (so they're Spartan). At the same time, as a French person, you would be a European (or West European), while the Spartan would also be from the Greek region.
It should be noted that Ancient Greeks usually only took up the identity of being someone from the Greek region or civilization when they were identifying AGAINST other civilizations, like their Persian neighbors to the East.