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
The New Commandership-in-Chief
The most essential change in the military system consisted in the institution of a permanent military head in the person of the Imperator, who, superseding the previous unmilitary and in every respect incapable governing corporation, united in his hands the whole control of the army, and thus converted it from a direction which for the most part was merely nominal into a real and energetic supreme command.
We are not properly informed as to the position which this supreme command occupied towards the special commands hitherto omnipotent in their respective spheres. Probably the analogy of the relation subsisting between the praetor and the consul or the consul and the dictator served generally as a basis, so that, while the governor in his own right retained the supreme military authority in his province, the Imperator was entitled at any moment to take it away from him and assume it for himself or his delegates, and, while the authority of the governor was confined to the province, that of the Imperator, like the regal and the earlier consular authority, extended over the whole empire. Moreover it is extremely probable that now the nomination of the officers, both the military tribunes and the centurions, so far as it had hitherto belonged to the governor,(36) as well as the nomination of the new adjutants of the legion, passed directly into the hands of the Imperator; and in like manner even now the arrangement of the levies, the bestowal of leave of absence, and the more important criminal cases, may have been submitted to the judgment of the commander-in-chief.
36. With the nomination of a part of the military tribunes by the burgesses (III. XI. Election of Officers in the Comitia) Caesar-- in this also a democrat--did not meddle.
With this limitation of the powers of the governors and with the regulated control of the Imperator, there was no great room to apprehend in future either that the armies might be utterly disorganized or that they might be converted into retainers personally devoted to their respective officers.
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Reference address : https://www.ellopos.net/elpenor/rome/5-11-old-republic-new-monarchy.asp?pg=68