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 Attack Rome - Battle on the Allia - Capture of Rome
For a moment, however, it seemed as if the two peoples, through whose co-operation Etruria saw her very existence put in jeopardy, were about to destroy each other, and the reviving power of Rome was to be trodden under foot by foreign barbarians. This turn of things, so contrary to what might naturally have been expected, the Romans brought upon themselves by their own arrogance and shortsightedness.
The Celtic swarms, which had crossed the river after the fall of Melpum, rapidly overflowed northern Italy--not merely the open country on the right bank of the Po and along the shore of the Adriatic, but also Etruria proper to the south of the Apennines. A few years afterwards (363) Clusium situated in the heart of Etruria (Chiusi, on the borders of Tuscany and the Papal State) was besieged by the Celtic Senones; and so humbled were the Etruscans that the Tuscan city in its straits invoked aid from the destroyers of Veii.
Perhaps it would have been wise to grant it and to reduce at once the Gauls by arms, and the Etruscans by according to them protection, to a state of dependence on Rome; but an intervention with aims so extensive, which would have compelled the Romans to undertake a serious struggle on the northern Tuscan frontier, lay beyond the horizon of the Roman policy at that time. No course was therefore left but to refrain from all interference. Foolishly, however, while declining to send auxiliary troops, they despatched envoys.
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Reference address : https://www.ellopos.net/elpenor/rome/2-04-fall-etruscan-celts.asp?pg=20