If on the other hand he did break with Caesar and, which was not very probable, effected even now a coalition with the aristocracy, this alliance between opponents, concluded under pressure of necessity and at the last moment, was so little formidable that it was hardly for the sake of averting it that Caesar agreed to those concessions.
A serious rivalry on the part of Crassus with Caesar was utterly impossible.

It is difficult to say what motives induced Caesar to surrender without necessity his superior position, and now voluntarily to concede--what he had refused to his rival even on the conclusion of the league of 694, and what the latter had since, with the evident design of being armed against Caesar, vainly striven in different ways to attain without, nay against, Caesar's will--the second consulate and military power.

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.