Please note that Mommsen uses the AUC chronology (Ab Urbe Condita), i.e. from the founding of the City of Rome. You can use this reference table to have the B.C. dates
From: The History of Rome, by Theodor Mommsen
Translated with the sanction of the author by William Purdie Dickson
Italy and the Italians
Lastly, with this military administrative union of the whole peoples dwelling to the south of the Apennines, as far as the Iapygian promontory and the straits of Rhegium, was connected the rise of a new name common to them all--that of "the men of the toga" (-togati-), which was their oldest designation in Roman state law, or that of the "Italians," which was the appellation originally in use among the Greeks and thence became universally current.
The various nations inhabiting those lands were probably first led to feel and own their unity, partly through their common contrast to the Greeks, partly and mainly through their common resistance to the Celts; for, although an Italian community may now and then have made common cause with the Celts against Rome and employed the opportunity to recover independence, yet in the long run sound national feeling necessarily prevailed.
As the "Gallic field" down to a late period stood contrasted in law with the Italian, so the "men of the toga" were thus named in contrast to the Celtic "men of the hose" (-braccati-); and it is probable that the repelling of the Celtic invasions played an important diplomatic part as a reason or pretext for centralizing the military resources of Italy in the hands of the Romans.
Inasmuch as the Romans on the one hand took the lead in the great national struggle and on the other hand compelled the Etruscans, Latins, Sabellians, Apulians, and Greeks (within the bounds to be immediately described) alike to fight under their standards, that unity, which hitherto had been undefined and latent rather than expressed, obtained firm consolidation and recognition in state law; and the name -Italia-, which originally and even in the Greek authors of the fifth century--in Aristotle for instance--pertained only to the modern Calabria, was transferred to the whole land of these wearers of the toga.
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Reference address : https://www.ellopos.net/elpenor/rome/2-07-pyrrhus-rome-italy.asp?pg=84