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
The Roman Policy with Reference to the German Invasion
The less the Celts, left to themselves, were a match for the Germans, the more reason had the Romans carefully to watch over the complications in which the two nations might be involved. Although the movements thence arising had not up to the present time directly affected them, they and their most important interests were yet concerned in the issue of those movements. As may readily be conceived, the internal demeanour of the Celtic nation had become speedily and permanently influenced by its outward relations.
As in Greece the Lacedaemonian party combined with Persia against the Athenians, so the Romans from their first appearance beyond the Alps had found a support against the Arverni, who were then the ruling power among the southern Celts, in their rivals for the hegemony, the Haedui: and with the aid of these new "brothers of the Roman nation" they had not merely reduced to subjection the Allobroges and a great portion of the indirect territory of the Arverni, but had also, in the Gaul that remained free, occasioned by their influence the transference of the hegemony from the Arverni to these Haedui.
But while the Greeks were threatened with danger to their nationality only from one side, the Celts found themselves hard pressed simultaneously by two national foes; and it was natural that they should seek from the one protection against the other, and that, if the one Celtic party attached itself to the Romans, their opponents should on the contrary form alliance with the Germans.
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Reference address : https://www.ellopos.net/elpenor/rome/5-07-subjugation-west.asp?pg=50