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
Certainly it was not Pompeius alone that was placed at the head of an army, but also his old enemy and Caesar's ally throughout many years, Crassus; and undoubtedly Crassus obtained his respectable military position merely as a counterpoise to the new power of Pompeius. Nevertheless Caesar was a great loser, when his rival exchanged his former powerlessness for an important command. It is possible that Caesar did not yet feel himself sufficiently master of his soldiers to lead them with confidence to a warfare against the formal authorities of the land, and was therefore anxious not to be forced to civil war now by being recalled from Gaul; but whether civil war should come or not, depended at the moment far more on the aristocracy of the capital than on Pompeius, and this would have been at most a reason for Caesar not breaking openly with Pompeius, so that the opposition might not be emboldened by this breach, but not a reason for conceding to him what he did concede.
Purely personal motives may have contributed to the result; it may be that Caesar recollected how he had once stood in a position of similar powerlessness in presence of Pompeius, and had been saved from destruction only by his--pusillanimous, it is true, rather than magnanimous--retirement; it is probable that Caesar hesitated to breakthe heart of his beloved daughter who was sincerely attached to her husband--in his soul there was room for much besides the statesman. But the decisive reason was doubtless the consideration of Gaul. Caesar--differing from his biographers--regarded the subjugation of Gaul not as an incidental enterprise useful to him for the gaining of the crown, but as one on which depended the external security and the internal reorganization, in a word the future, of his country.
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Reference address : https://www.ellopos.net/elpenor/rome/5-08-pompeius-caesar.asp?pg=27