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
There remained the insular Celts. From the close connection
between them and the Celts of the continent, especially
the maritime cantons, it may readily be conceived that they had
at least sympathized with the national resistance, and that if they
did not grant armed assistance to the patriots, they gave at any rate
an honourable asylum in their sea-protected isle to every one
who was no longer safe in his native land.
This certainly involved
a danger, if not for the present, at any rate for the future; it
seemed judicious--if not to undertake the conquest of the island
itself--at any rate to conduct there also defensive operations
by offensive means, and to show the islanders by a landing
on the coast that the arm of the Romans reached even across the Channel.
