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
If Pompeius acknowledged the oligarchic constitution
and offered to fight for it against Caesar, the republican opposition
might and must recognize him as its general, and in alliance
with him compel the timid majority to a declaration of war.
That Pompeius was not quite in earnest with his fidelity
to the constitution, could indeed escape nobody; but, undecided
as he was in everything, he had by no means arrived like Caesar
at a clear and firm conviction that it must be the first business
of the new monarch to sweep off thoroughly and conclusively
the oligarchic lumber.
At any rate the war would train
a really republican army and really republican generals;
and, after the victory over Caesar, they might proceed
with more favourable prospects to set aside not merely
oneof the monarchs, but the monarchy itself, which was in the course
of formation. Desperate as was the cause of the oligarchy, the offer
of Pompeius to become its ally was the most favourable arrangement
possible for it.