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 Celts Attacked in Their Own Land
The victory was complete, and the Romans were firmly resolved to prevent the recurrence of such surprises by the complete subjugation of the Celts on the south of the Alps. In the following year (530) the Boii submitted without resistance along with the Lingones; and in the year after that (531) the Anares; so that the plain as far as the Po was in the hands of the Romans. The conquest of the northern bank of the river cost a more serious struggle.
Gaius Flaminius crossed the river in the newly-acquired territory of the Anares (somewhere near Piacenza) in 531; but during the crossing, and still more while making good his footing on the other bank, he suffered so heavy losses and found himself with the river in his rear in so dangerous a position, that he made a capitulation with the enemy to secure a free retreat, which the Insubres foolishly conceded. Scarce, however, had he escaped when he appeared in the territory of the Cenomani, and, united with them, advanced for the second time from the north into the canton of the Insubres.
The Gauls perceived what was now the object of the Romans, when it was too late: they took from the temple of their goddess the golden standards called the "immovable," and with their whole levy, 50,000 strong, they offered battle to the Romans. The situation of the latter was critical: they were stationed with their back to a river (perhaps the Oglio), separated from home by the enemy's territory, and left to depend for aid in battle as well as for their line of retreat on the uncertain friendship of the Cenomani.
There was, however, no choice. The Gauls fighting in the Roman ranks were placed on the left bank of the stream; on the right, opposite to the Insubres, the legions were drawn up, and the bridges were broken down that they might not be assailed, at least in the rear, by their dubious allies.
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Reference address : https://www.ellopos.net/elpenor/rome/3-03-extension-italy.asp?pg=36