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
To the question, under what circumstances this new style of tactics, the first national system that on its own proper ground showed itself superior to the Roman, arose among the Parthians, we unfortunately can only reply by conjectures. The lancers and mounted archers were of great antiquity in the east, and already formed the flower of the armies of Cyrus and Darius; but hitherto these arms had been employed only as secondary, and essentially to cover the thoroughly useless Oriental infantry. The Parthian armies also by no means differed in this respect from the other Oriental ones; armies are mentioned, five-sixths of which consisted of infantry. In the campaign of Crassus, on the other hand, the cavalry for the first time came forward independently, and this arm obtained quite a new application and quite a different value.
The irresistible superiority of the Roman infantry in close combat seems to have led the adversaries of Rome in very different parts of the world independently of each other--at the same time and with similar success--to meet it with cavalry and distant weapons. What as completely successful with Cassivellaunus in Britain(6) and partially successful with Vercingetorix in Gaul(7)-- what was to a certain degree attempted even by Mithradates Eupator(8)-- the vizier of Orodes carried out only on a larger scale and more completely.
6. Cf. V. VII. Cassivellaunus
7. Cf. V. VII. The Carnutes ff.
8. Cf. V. II. Renewal of the War
And in doing so he had special advantages: for he found in the heavy cavalry the means of forming a line; the bow which was national in the east and was handled with masterly skill in the Persian provinces gave him an effective weapon for distant combat; and lastly the peculiarities of the country and the people enabled him freely to realize his brilliant idea. Here, where the Roman weapons of close combat and the Roman system of concentration yielded for the first time before the weapons of more distant warfare and the system of deploying, was initiated that military revolution which only reached its completion with the introduction of firearms.
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Reference address : https://www.ellopos.net/elpenor/rome/5-09-crassus-joint-rulers.asp?pg=9