>It couldn't have been language
Even though plebs began speaking it only after the introduction of the television, the Italian language that we speak today is a scholarly project that goes all the way back to Dante Alighieri's De Vulgari Eloquentia.
>mutually unintelligible dialects or even used outright different language groups
One thing is to say Italians didn't speak one language, another is making it sound like we needed an army of translators as big as the European Parliament's just to talk to one another, or that German speakers in the far north cannot learn a second language. Think Switzerland.
>It couldn't have been genetics
Since the Romans - no, since before them, in fact - we've been fucking the people we wanted, without taking orders on the subject from tiny-moustached dictators. Speaking of dictators, did you know Mussolini had a Jewish mistress, Margherita Sarfatti?
I advise user against restricting his already limited reproductive and romantic potential any further.
>It couldn't have been a legal precedent
If you mention law, I should remind you that Roman law is the foundation for all Western Law, be it Civil or Common, not just Italy's.
>what is now called Italy
Let the etimologists have their immense controversy over which people gave Italy its name, during millennia of endless human settlement, peopling and migration, people moved, but the peninsula and its sorrounding islands do not.
>From an outsider's view it looks like they just clumped a bunch of different states and ethnicities together and called them one
It's called geography, and it's the same with all nation states. "We have made Italy. Now we must make Italians." And all nation states have their respective ideology, education system, language, scholars, religions, anthem and everything else to make sure a similar "making" of the nation, ever the work in progress, happens.
>So what united them?
Same way Italy ended up as a republic: a combination of warring and voting.