The history of the Balkans from say 1850 to 1914 is really interesting. All these different groups were trying to gain the allegiance of what were often called “emerging nationalities.”
In actual fact these nationalities were in the process of being invented. Different groups claimed “historical right” to particular areas because they’d controlled a big chunk of land 600 or 800 years before for a decade or two. My personal favorite is the Serbs claiming they “owned” most of the Balkans because Stephen Dushan had an empire for 25 years or so back in the 1300s. Obviously these claims overlapped greatly.
In Macedonia, for example, Greeks claimed all Macedonians were Greeks because Alexander the Great was from Macedonia. In fact, I think they still do. To an unbiased observer, this is a really, really flimsy basis for such a claim.
Meanwhile, the Macedonians themselves were largely Orthodox Slavs. They spoke a language with various dialects that were close to Serbian on one end of the region and Bulgarian on the other.
So of course Serbs and Bulgarians both claimed all Macedonians were “really” members of their “nationality.”
This leaves out of the mess that people in the Balkans didn’t live in neat blocks by “nationality” as they (mostly) did in Western Europe. They lived in general in villages of various ethnicities scattered across the landscape. Which meant that when one “nation” acquired title to the area, the other ethnic groups imediately became intruders, in a land where they had lived just as long as their now “majority” neighbors.
What a mess!
The Italians and the Greeks could claim all of the Balkans based on the fact that the whole peninsula belonged to the Roman Empire, and at times in the Middle Ages to the Byzantine Empire. The Byzantines called themselves Romans, and the Greeks continued to call themselves Romans for the most part until the 19th century, I think, when part of Greece became an independent kingdom (under a non-Greek king).
The ancient Greeks considered the ancient Macedonians to be barbarians, but the modern Greeks claim that they were Greeks--and have prevented the Republic of Macedonia from using that name at the UN. Most of the Balkans became Slavic in the 6th or 7th century, apart from Greece and Albania, so the modern Macedonians have little to do with the ancient Macedonians. (Perhaps they inherited some DNA...language changes don't always mean that the earlier population is wiped out.)
The modern Macedonian language is very similar to Bulgarian--if the Russians had succeeded in creating a "Big Bulgaria" in 1878 that area would use Bulgarian as their official language. Instead it was later conquered by Serbia in the Balkan Wars and called "South Serbia." Between the wars it was illegal to use the local Macedonian language--people were supposed to use Serbian instead. The Tito government recognized the Macedonians as separate from the Serbs and created the Macedonian literary language (trying to make it as different from Bulgarian as they could). Serbian, Macedonian, and Bulgarian are all related but Macedonian and Bulgarian are closer--they have features in common which are lacking in Serbian (like attaching the article to the end of a noun).
The whole Yugoslav project was an example of intellectuals thinking they know what is best for the people.