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
Outbreak of War between Samnium and Rome-- Pacification of Campania
Differences had already for several years existed between Rome and
Samnium in consequence of the continual aggressions in which the
Romans indulged on the Liris, and of which the founding of Fregellae
in 426 was the latest and most important. But it was the Greeks of
Campania that gave occasion to the outbreak of the contest.
After
Cumae and Capua had become Roman, nothing so naturally suggested
itself to the Romans as the subjugation of the Greek city Neapolis,
which ruled also over the Greek islands in the bay--the only town
not yet reduced to subjection within the field of the Roman power.
The Tarentines and Samnites, informed of the scheme of the Romans to
obtain possession of the town, resolved to anticipate them; and while
the Tarentines were too remiss perhaps rather than too distant for the
execution of this plan, the Samnites actually threw into it a strong
garrison. The Romans immediately declared war nominally against the
Neapolitans, really against the Samnites (427), and began the siege
of Neapolis.