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
Celts and Romans - Advance of Roman Trade and Commerce into Free Gaul
Of far greater importance was the contact of the Celtic nation with the Roman people, and with the Germans. We need not here repeat-- what has been related already--how the Romans in their slow advance had gradually pressed back the Celts, had at last occupied the belt of coast between the Alps and the Pyrenees, and had thereby totally cut them off from Italy, Spain and the Mediterranean Sea--a catastrophe, for which the way had already been prepared centuries before by the laying out of the Greek stronghold at the mouth of the Rhone. But we must here recall the fact that it was not merely the superiority of the Roman arms which pressed hard on the Celts, but quite as much that of Roman culture, which likewise reaped the ultimate benefit of the respectable beginnings of Greek civilization in Gaul.
Here too, as so often happens, trade and commerce paved the way for conquest. The Celt after northern fashion was fond of fiery drinks; the fact that like the Scythian he drank the generous wine unmingled and to intoxication, excited the surprise and the disgust of the temperate southern; but the trader has no objection to deal with such customers. Soon the trade with Gaul became a mine of gold for the Italian merchant; it was nothing unusual there for a jar of wine to be exchanged for a slave. Other articles of luxury, such as Italian horses, found advantageous sale in Gaul.
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Reference address : https://www.ellopos.net/elpenor/rome/5-07-subjugation-west.asp?pg=44