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
Position of the Insurgents at Alesia
Thus the junction of the two divisions of the army was happily accomplished. The insurgents meanwhile had consulted as to the farthe conduct of the war at Bibracte (Autun) the capital of the Haeduil the soul of these consultations was again Vercingetorix, to whom the nation was enthusiastically attached after the victory of Gergovia. Particular interests were not, it is true, even now silent; the Haedui still in this death-struggle of the nation asserted their claims to the hegemony, and made a proposal in the national assembly to substitute a leader of their own for Vercingetorix.
But the national representatives had not merely declined this and confirmed Vercingetorix in the supreme command, but had also adopted his plan of war without alteration. It was substantially the same as that on which he had operated at Avaricum and at Gergovia. As the base of the new position there was selected the strong city of the Mandubii, Alesia (Alise Sainte Reine near Semur in the department Cote d'Or)(49) and another entrenched camp was constructed under its walls.
49. The question so much discussed of late, whether Alesia is not rather to be identified with Alaise (25 kilometres to the south of Besancon, dep. Doubs), has been rightly answered in the negative by all judicious inquirers.
Immense stores were here accumulated, and the army was ordered thither from Gergovia, having its cavalry raised by resolution of the national assembly to 15,000 horse. Caesar with the whole strength of his army after it was reunited at Agedincum took the direction of Besancon, with the view of now approaching the alarmed province and protecting it from an invasion, for in fact bands of insurgents had already shown themselves in the territory of the Helvii on the south slope of the Cevennes. Alesia lay almost on his way; the cavalry of the Celts, the only arm with which Vercingetorix chose to operate, attacked him on the route, but to the surprise of all was worsted by the new German squadrons of Caesar and the Roman infantry drawn up in support of them.
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Reference address : https://www.ellopos.net/elpenor/rome/5-07-subjugation-west.asp?pg=125