Nationalism is one of those -isms that is so entrenched in our modern consciousness that it is hard to imagine a world in which it didn't exist.

What it means, essentially, is that people are asked to (and often do) pledge a primary loyalty to an entity known as a "nation." So this is perhaps why you are getting tripped up — you seem to be accepting that nations are "real" and not (as Benedict Anderson famously called them) "imagined communities."

What is a nation? As the idea emerged, it was a grouping of people designated by shared geography, language, and often other things (culture, religion). It's a hodge-podge of concerns, but the basic idea is that you are grouping fairly large numbers of people under a common heading.

The radical nature in this comes from the fact that there are many other ways people could pledge their loyalty. Take, for example, the Italian situation. The nation that we today call Italy was, until 19th century, a bunch of small kingdoms and city states. Sure, they all spoke similar (but often non-identical) languages that might be called "Italian," but that didn't mean they shared much else, or considered themselves otherwise connected. (Sound radical? It shouldn't — Americans and British don't consider themselves the same nation anymore, despite shared language. Sure, there are shared cultures there, but there is no sense of shared loyalty.) To say, "hey, all you people who speak the same language — you're actually all the same grouping, and should all use the same government, economic system, etc." is actually a pretty radical idea.

It gets more complicated in places where the languages aren't shared at all. Consider the Middle East today, where there is very little sense of Arab nationalism anymore (it used to be a thing), and where the loyalties matter the most are in areas of specific religious sects (e.g. Shiite, Sunni). You can see the power of nationalism here in the people who are trying to assert new nations — like the Kurds, who occupy the land of several existing countries and are saying, essentially, that they constitute a separate grouping that deserves its own state status.

Related to nationalism is the concept of the nation-state, as an aside — the idea that states should be arranged around the idea of nations, not around the idea of mere territory. So the Austro-Hungarian Empire, once a mighty political entity in Europe, was broken into separate nation-states after World War I, each of which much smaller and less formidable than their whole (e.g. Hungary, Slovakia, Austria, etc.).

(Consider, while we are at it, the complicated state of the United States and nationalism. The country is defined, in part, by its plurality of identities — even languages — united under some kind of common banner. But as we know and can see, those claims to shared loyalty can be controversial, and there are many who make their political fortunes by specifically trying to exclude certain sub-groups from that shared collective, rendering them as outsiders/others.)

So the idea of "nations" is pretty tricky, much more so than I think a lot of people realize. Even the idea of "France" being a singular grouping was controversial at the time, even if it seems (from the outside) to be pretty uncontroversial today. There is a difference between saying "I pledge my loyalty to the idea of my shared nation group" and "I pledge my loyalty to my king" or "I pledge my loyalty to my fellow religious congregants" or "I pledge my loyalty to the people I know in my local region," etc.

The issue here isn't that people in the past didn't have loyalties, or that there weren't political units prior to nationalism. They did and there were. The issue is that the political unit and loyalty is not generally what we would identify as a "nation," which is a pretty large entity as far as things go (again, usually defined primarily as a linguistic grouping, but even then that can be super tricky).

The reason we like to talk about these as imagined or constructed entities is that these groupings are anything but "natural," and you see that when you look at the ways in which appeals to nationalism are marshaled and fought against. The Soviet Union, for example, encouraged its individual SSRs to have a sense of Soviet loyalty. As it unraveled, however, the local nationalisms asserted themselves with a vengeance (e.g. Ukrainians insisted they were not Russians — something that has erupted as overt war in recent years). There have been many movements in history where people have said, "hey, you guys, you are actually part of group X, and should give it your loyalty!" and this is often a very radical thing to assert (imagine if I tried to convince you that you owed no loyalty to your state, but instead to something else — an ideology, a religion, a new geographical grouping). Consider also how weird it would be if someone powerful showed up and said, "hey, all of you countries in the same part of the world — you're all actually the same country now, because we can't tell you guys apart and we don't really care." (This is akin to situations where various colonial powers suddenly created new states from nothing — again, a legacy we are today living with as some of these colonial creations, like the state of Iraq, seem to be dissolving and realigning their groupings.)

So this is what it means for nationalism to be new/modern: by the 18th-century this became a common European way for thinking about what was and wasn't a legitimate political entity. By the 19th century you see lots of "unifications" (e.g. Germany, Italy) of places that had previously been smaller entities. By the early 20th century you have an asserted right to "nation-statehood" (that nations should be allowed to have states) and often bizarre colonial state creations as a result. By the mid-20th century there were competitors again to these groupings (e.g. asserted primacy of blood, of ideology, of religion), and by the post-Cold War you see lots of re-contested, argued definitions of whether nations were the adequate groupings or not.

Today even if we do not talk about it that way we define a nation as a form of common genetic ancestry. Basically we usually include people into our nation who we percieve as our closer relatives compared to others who are percieved as farther relatives. This implies language and national identity in general as signs of kinship most likely because they can not be shed easily.

However before the advent of this grouping system several others were used even in Europe. For instance if you think about heretic movements and the reformation itself formerly homogenous groups of people could form distinct clusters within years so different from each other that they did not even consider them worthy of living. You can think of the cathars in France: Definitely cathars were not an ethnic group just a different offshoot of christianity which you could more or less join immediately yet they were considered alien enough to be worthy of a war to exterminate them.

If you want another example in the eastern end of the continent husites who imolemented only some very minor changes in rituals had to fight for their mere survival as these changes were seen as the founding of a new aluen group. Yet in a few decades when the ottoman empire reached the kingdom of Hungary these formerly alien groups were integrated into their former enemies immediately when the approaching muslim empire created a "common christian" udentity where the differences between the sects were tolerated. This is similar to the modern identity-creation of religion in the muslim world.

Another good example is the founding of Hungary at that time our first king (István) took up christianity and his first task was after getting crowned was to quench a revolt led by the followers of the old pagan faith. In this war the king's army consisted of his converted Hungarians and German knights who were total newcomers yet apparently the freshly converted Hungarians felt to be more in a group with them than other Hungarians following the old religion.