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
Attack on Etruria by the Romans
Subjected to these simultaneous and, as it were, concerted assaults on the part of very different peoples--the Syracusans, Latins, Samnites, and above all the Celts--the Etruscan nation, that had just acquired so vast and sudden an ascendency in Latium and Campania and on both the Italian seas, underwent a still more rapid and violent collapse. The loss of their maritime supremacy and the subjugation of the Campanian Etruscans belong to the same epoch as the settlement of the Insubres and Cenomani on the Po; and about this same period the Roman burgesses, who had not very many years before been humbled to the utmost and almost reduced to bondage by Porsena, first assumed an attitude of aggression towards Etruria.
By the armistice with Veii in 280 Rome had recovered its ground, and the two nations were restored in the main to the state in which they had stood in the time of the kings. When it expired in the year 309, the warfare began afresh; but it took the form of border frays and pillaging excursions which led to no material result on either side. Etruria was still too powerful for Rome to be able seriously to attack it. At length the revolt of the Fidenates, who expelled the Roman garrison, murdered the Roman envoys, and submitted to Lars Tolumnius, king of the Veientes, gave rise to a more considerable war, which ended favourably for the Romans; the king Tolumnius fell in combat by the hand of the Roman consul Aulus Cornelius Cossus (326?), Fidenae was taken, and a new armistice for 200 months was concluded in 329.
During this truce the troubles of Etruria became more and more aggravated, and the Celtic arms were already approaching the settlements that hitherto had been spared on the right bank of the Po. When the armistice expired in the end of 346, the Romans on their part resolved to undertake a war of conquest against Etruria; and on this occasion the war was carried on not merely to vanquish Veii, but to crush it.
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Reference address : https://www.ellopos.net/elpenor/rome/2-04-fall-etruscan-celts.asp?pg=17