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 Beginnings of Romanic Development
But the fact that this great people was ruined by the Transalpine wars of Caesar, was not the most important result of that grand enterprise; far more momentous than the negative was the positive result. It hardly admits of a doubt that, if the rule of the senate had prolonged its semblance of life for some generations longer, the migration of peoples, as it is called, would have occurred four hundred years sooner than it did, and would have occurred at a time when the Italian civilization had not become naturalized either in Gaul, or on the Danube, or in Africa and Spain. Inasmuch as the great general and statesman of Rome with sure glance perceived in the German tribes the rival antagonists of the Romano-Greek world; inasmuch as with firm hand he established the new system of aggressive defence down even to its details, and taught men to protect the frontiers of the empire by rivers or artificial ramparts, to colonize the nearest barbarian tribes along the frontier with the view of warding off the more remote, and to recruit the Roman army by enlistment from the enemy's country; he gained for the Hellenico-Italian culture the interval necessary to civilize the west just as it had already civilized the east.
Ordinary men see the fruits of their action; the seed sown by men of genius germinates slowly. Centuries elapsed before men understood that Alexander had not merely erected an ephemeral kingdom in the east, but had carried Hellenism to Asia; centuries again elapsed before men understood that Caesar had not merely conquered a new province for the Romans, but had laid the foundation for the Romanizing of the regions of the west. It was only a late posterity that perceived the meaning of those expeditions to England and Germany, so inconsiderate in a military point of view, and so barren of immediate result. An immense circle of peoples, whose existence and condition hitherto were known barely through the reports--mingling some truth with much fiction--of the mariner and the trader, was disclosed by this means to the Greek and Roman world. "Daily," it is said in a Roman writing of May 698, "the letters and messages from Gaul are announcing names of peoples, cantons, and regions hitherto unknown to us."
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Reference address : https://www.ellopos.net/elpenor/rome/5-07-subjugation-west.asp?pg=141