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
While Caesar thus entered the struggle only as commandant of Gaul, without other essential resources than efficient adjutants, a faithful army, and a devoted province, Pompeius began it as de facto supreme head of the Roman commonwealth, and in full possession of all the resources that stood at the disposal of the legitimate government of the great Roman empire. But while his position was in a political and military point of view far more considerable, it was also on the other hand far less definite and firm.
The unity of leadership, which resulted of itself and by necessity from the position of Caesar, was inconsistent with the nature of a coalition; and although Pompeius, too much of a soldier to deceive himself as to its being indispensable, attempted to force it on the coalition and got himself nominated by the senate as sole and absolute generalissimo by land and sea, yet the senate itself could not be set aside nor hindered from a preponderating influence on the political, and an occasional and therefore doubly injurious interference with the military, superintendence. The recollection of the twenty years' war waged on both sides with envenomed weapons between Pompeius and the constitutional party; the feeling which vividly prevailed on both sides, and which they with difficulty concealed, that the first consequence of the victory when achieved would be a rupture between the victors; the contempt which they entertained for each other and with only too good grounds in either case; the inconvenient number of respectable and influential men in the ranks of the aristocracy and the intellectual and moral inferiority of almost all who took part in the matter--altogether produced among the opponents of Caesar a reluctant and refractory co-operation, which formed the saddest contrast to the harmonious and compact action on the other side.
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Reference address : https://www.ellopos.net/elpenor/rome/5-10-brundisium-pharsalus-thapsus.asp?pg=12