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
He rid himself--in a perfidious manner, it is
true--of the insubordinate army of mercenaries, revived the citizen-
militia, and endeavoured, at first with the title of general,
afterwards with that of king, to re-establish the deeply sunken
Greek power by means of his civic troops and of fresh and more
manageable recruits. With the Carthaginians, who in concert with the
Greeks had driven king Pyrrhus from the island, there was at that time
peace. The immediate foes of the Syracusans were the Mamertines.
They were the kinsmen of those hated mercenaries whom the Syracusans
had recently extirpated; they had murdered their own Greek hosts;
they had curtailed the Syracusan territory; they had oppressed and
plundered a number of smaller Greek towns. In league with the Romans
who just about this time were sending their legions against the
Campanians in Rhegium, the allies, kinsmen, and confederates in crime
of the Mamertines,(2) Hiero turned his arms against Messana.