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
Matters wore a different aspect, when one crossed the Roman frontier. The great Celtic nation, which in the southern districts already began to be crushed by the Italian immigration, still moved to the north of the Cevennes in its time-hallowed freedom. It is not the first time that we meet it: the Italians had already fought with the offsets and advanced posts of this vast stock on the Tiber and on the Po, in the mountains of Castile and Carinthia, and even in the heart of Asia Minor; but it was here that the main stock was first assailed at its very core by their attacks.
The Celtic race had on its settlement in central Europe diffused itself chiefly over the rich river-valleys and the pleasant hill-country of the present France, including the western districts of Germany and Switzerland, and from thence had occupied at least the southern part of England, perhaps even at this time all Great Britain and Ireland;(10) it formed here more than anywhere else a broad, geographically compact, mass of peoples.
10. An immigration of Belgic Celts to Britain continuing for a considerable time seems indicated by the names of English tribes on both banks of the Thames borrowed from Belgic cantons; such as the Atrebates, the Belgae, and even the Britanni themselves, which word appears to have been transferred from the Brittones settled on the Somme below Amiens first to an English canton and then to the whole island. The English gold coinage was also derived from the Belgic and originally identical with it.
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Reference address : https://www.ellopos.net/elpenor/rome/5-07-subjugation-west.asp?pg=13