Richard Miles makes a similar claim in his work on Carthage. He contends that there was a "Western Mediterranean" economic/cultural trade zone in which Punic/Etruscan/Greek, & native peoples all mixed together in a huge melting pot of commerce and population movements. Further, he claims that this zone is broken up by political, not socio-economic means.
As for Rome, Miles asserts that it was founded as a "hub" within this zone with Punics, Greeks, and Etruscans all moving to the region. Supposedly this explains why Rome always felt "different" than the other Latin cities. Accordingly, Rome and Roman as a concept only became ingrained as a result of the growth of the power and ambitions of Rome, who sought to craft for themselves a distinct culture from those around them. Please insert reference here comparing Livy to 19th century nationalists writers.
For the record, I think the idea is interesting, particularly that of early inter-connectivity in the Western Med. However, I think Miles takes this idea waaaaaaaay too far.
I like that idea - and have seen quite a lot of recent work (
The Making of the Middle Sea on the Mediterranean and
The Edge of the World on the North Sea spring to mind) making largely the same point: that we tend to mentally divide people up nowadays by landmasses, because we're used to seeing maps of the world where the land has different colours and the sea is all blue, but that for most of history, people have been much more connected than divided by water, and that if you want to understand (for example) prehistoric Morocco, you're often better looking for connections in Spain than to the south. I'd question whether it was uniquely Roman - there are a couple of tombs in various Etruscan cities which include the names of the people buried there, and it's not unusual to find Greek and Latin ones - and, of course, the overwhelming majority of 'Greek' pottery was in fact found in those tombs, and it took a good few years for anyone to realise that it wasn't actually Etruscan.
I'm also not convinced that 'Roman' identity was ever unimportant - if you were a citizen of Rome, it probably mattered a great deal: your citizenship mattered wherever you were in the ancient world. It's certainly true, though (and I expect that this is really where he's going) that the position and 'value' of that Roman identity did change over time. For example, in the early Republic, there was such a thing as 'Latin colonisation' - the idea being that, when the city grew too large or for whatever other reason, the magistrates would take a party of colonists out into the territory and they would found a new city. If you did this as a colony of Roman citizens, you kept all of your civil rights, though you had to physically go to Rome to exercise most of them. However, if you did it as a colony of Latins - that is, the non-citizens from around Rome, who spoke the same language - you did not have any citizen rights (and so there was much less of a limit on how far away such a colony could be), but the state would give you twice as much land. Roman citizens were allowed to give up their citizenship in order to join Latin colonies, and in the early period quite a lot of them did. By the 2nd century BC, however, things changed: being counted as 'Roman' now carried quite a lot of pride and benefits, because Rome was now quite clearly dominant in a way that it hadn't been three centuries before. So they ended up with a severe shortage of people willing to (effectively) sell that identity for more land, with the result that Latin colonisation died out after about 120 BC.
What you also see is a changing sense of where being 'Latin' and 'Italian' stand in relation to being 'Roman'. As I mentioned, the clear point of Latin colonisation is that you are either a 'Latin' or a 'Roman', and 'Italy' was at that point (at best) a geographic term, and perhaps the majority of people who lived in it would not have called themselves 'Romans', as most of their states were, at most, allied to but supposedly independent of Rome. Again, when you go forward a few centuries, that changes: 'Latin' and 'Roman' become interchangeable terms, all Latins are given Roman citizenship, and people give up that insistence that their local identity has to 'trump' Roman identity, because they quite like having the benefits and status of being Roman. By the time you get to Augustus, you have a huge state project designed around creating this idea of 'Italy' which is nothing if not 'Roman' - you can read the
Aeneid and see the poet use 'Italians' and 'Romans' completely and deliberately interchangeably, which might have raised objections even a hundred years before. I think that's where I'd put in Livy alongside nationalist writers - nationalism is all about creating an image of 'the nation' that includes all of the people you want to include, and at some point that has to be invented. For the most obvious example, you have German national anthem, whose first verse draws you a nice mental map of who the 'Germans' are supposed to be:
Deutschland, Deutschland über alles,
Über alles in der Welt,
Wenn es stets zu Schutz und Trutze
Brüderlich zusammenhält.
Von der Maas bis an die Memel,
Von der Etsch bis an den Belt,
Germany, Germany above all else,
Above all else in the world,
when, for protection and defense,
it always stands brotherly together.
From the Meuse to the Memel,
From the Adige to the Belt
It's worth pointing out that not everyone is likely to have automatically or initially agreed with these ideas of 'the nation' - the German one, for example, includes a fair bit of what is now Italy, the Netherlands and Poland.