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
Subjugation of Latium by Etruria
By land also their power seemed to be on the increase. To acquire possession of Latium was of the most decisive importance to Etruria, which was separated by the Latins alone from the Volscian towns that were dependent on it and from its possessions in Campania. Hitherto the firm bulwark of the Roman power had sufficiently protected Latium, and had successfully maintained against Etruria the frontier line of the Tiber. But now, when the whole Tuscan league, taking advantage of the confusion and the weakness of the Roman state after the expulsion of the Tarquins, renewed its attack more energetically than before under the king Lars Porsena of Clusium, it no longer encountered the wonted resistance.
Rome surrendered, and in the peace (assigned to 247) not only ceded all her possessions on the right bank of the Tiber to the adjacent Tuscan communities and thus abandoned her exclusive command of the river, but also delivered to the conqueror all her weapons of war and promised to make use of iron thenceforth only for the ploughshare. It seemed as if the union of Italy under Tuscan supremacy was not far distant.
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Reference address : https://www.ellopos.net/elpenor/rome/2-04-fall-etruscan-celts.asp?pg=2