The late 1400s was very far away from Nationalism. You are thinking of the early 1800s. The first truly Nationalist state was France under Napolean or some would say the USA, but even earlier USA barely held itself together for the first 20 years.
Agrarian empires like the Roman one were absolutely massive, juggernauts that just couldn't be stopped. Yet the Middle Ages were a period of small, warring nations, with comparatively much smaller empires arising. Warfare slowly civilized to the point where the common people barely felt the sting.
The peace of Westphalia took place in 1648, creating the notion of an international system law that was bigger than the individual agreements penned between states.
The Napoleonic France of the 1800s represented a sharp break from this world, almost a repeat of the old juggernauts.
What caused empire to stop working and the polities to get ever smaller? I think the answer belongs in what your parent was getting at, people coalesced into ethnic communities and just wouldn't participate in the building of empire anymore. So existing empires broke up and nation-states arose.
The classic 20th C nation-state went hand-in-hand with 20th-C warfare: it needed both massive industry, and every single man. The Ottomans and other relics of earlier times fell apart under the stress of such wars; the states that survived were those with a strong identity. Such identities were partly fostered for this very purpose.
But we seem to have moved on from this, war is again a profession of a few (plus high-tech weapons), conscripting the millions would be of no use. And in parallel our ties to nations are weaker than before, both whether we'd fight for the flag & conversely whether the country is interested in safety nets etc. Our economy is also much less nation-based: GM in its heyday was quite unlike Apple. He calls this emerging form a "market state".
Earlier forms also fought very different wars, he argues that Westphalia (and Waterloo) were among a series of such transitions, each entailing changes to what states are legitimate, and what kind of wars they fight.
Anyway if this sounds interesting then his book "The Shield of Achilles" is cautiously recommended. Someone really needs to write a condensed version... but if you're willing to skip a bit, the ideas are great.
I'm not really sure what drives it, honestly. I'm just talking observationally, about what kind of polities ended up emerging, however they emerged.
We are now living in an era of near absolute nationalism. Polity & nation are nearly synonyms for our generation.
I agree with the criticism that 14th-15th century Spain was not really a national state, but (like you) I think that the distinction is mostly a matter of convention. I'm calling it a proto-nation state.
The emerging Spanish language, culture, and pre-existing religious identity became a core part of what Spain is. Minority languages, customs and religions became a (literal and metaphorical) heresy. They were eventually all given the same ultimatuum as the Jews (again, metaphorically and literally), assimilate into the mainstream religion, language and culture or leave.
- The pressure of mendicant orders.
- The pope.
Some of these processes are literal decrees, banning practices, languages or people. Some are less legible, like "soft power" in modern terms. I'm interested in the long term results, which was (in the case of Spain early on, and many/most places subsequently) a decisive change from multiculturalism (like the sephardi community of Bosnia represents) to national monoculturalism.
That is, over the centuries societal pressures and dynamics were such that a more monolithic identity succeeded a patchwork of language, dialect and culture.
My argument (I suspect we disagree) is that Spain was an earlier example of something that happened widely in the 19th, 20th and is still ongoing: Nation states emerging from empires and advancing towards cultural homogeneity, to the detriment of multiculturalism which existed under most empires.
When Turkey "emerged" this involved a similar process, expulsion (and worse) of Armenians & Greeks, and conformity pressure on other minorities.
A similar thing happened in Poland, which was barely more than 50% polish speaking catholic when my grandmother was born there, and is 9X% polish catholic now.
When Ireland emerged from Britain, migrations and border gerrymandering ensured "Irish Catholic" homegeneity.