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
In fact the Boii had already renewed the war in 516, and their chiefs Atis and Galatas had--without, it is true, the authority of the general diet--summoned the Transalpine Gauls to make common cause with them. The latter had numerously answered the call, and in 518 a Celtic army, such as Italy had not seen for long, encamped before Ariminum. The Romans, for the moment much too weak to attempt a battle, concluded an armistice, and to gain time allowed envoys from the Celts to proceed to Rome, who ventured in the senate to demand the cession of Ariminum--it seemed as if the times of Brennus had returned.
But an unexpected incident put an end to the war before it had well begun. The Boii, dissatisfied with their unbidden allies and afraid probably for their own territory, fell into variance with the Transalpine Gauls. An open battle took place between the two Celtic hosts; and, after the chiefs of the Boii had been put to death by their own men, the Transalpine Gauls returned home. The Boii were thus delivered into the hands of the Romans, and the latter were at liberty to expel them like the Senones, and to advance at least to the Po; but they preferred to grant the Boii peace in return for the cession of some districts of their land (518).
This was probably done, because they were just at that time expecting the renewed outbreak of war with Carthage; but, after that war had been averted by the cession of Sardinia, true policy required the Roman government to take possession as speedily and entirely as possible of the country up to the Alps.
Do you see any typos or other mistakes? Please let us know and correct them
Reference address : https://www.ellopos.net/elpenor/rome/3-03-extension-italy.asp?pg=29