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
As compared with an army like this Parthian one, the Roman army was at a disadvantage strategically, because the cavalry commanded the communications; and at a disadvantage tactically, because every weapon of close combat must succumb to that which is wielded from a distance, unless the struggle becomes an individual one, man against man. The concentrated position, on which the whole Roman method of war was based, increased the danger in presence of such an attack; the closer the ranks of the Roman column, the more irresistible certainly was its onset, but the less also could the missiles fail to hit their mark.
Under ordinary circumstances, where towns have to be defended and difficulties of the ground have to be considered, such tactics operating merely with cavalry against infantry could never be completely carried out; but in the Mesopotamian desert, where the army, almost like a ship on the high seas, neither encountered an obstacle nor met with a basis for strategic dispositions during many days' march, this mode of warfare was irresistible for the very reason that circumstances allowed it to be developed there in all its purity and therefore in all its power. There everything combined to put the foreign infantry at a disadvantage against the native cavalry. Where the heavy-laden Roman foot-soldier dragged himself toilsomely through the sand or the steppe, and perished from hunger or still more from thirst amid the pathless route marked only by water-springs that were far apart and difficult to find, the Parthian horseman, accustomed from childhood to sit on his fleet steed or camel, nay almost to spend his life in the saddle, easily traversed the desert whose hardships he had long learned how to lighten or in case of need to endure. There no rain fell to mitigate the intolerable heat, and to slacken the bowstrings and leathern thongs of the enemy's archers and slingers; there amidst the deep sand at many places ordinary ditches and ramparts could hardly be formed for the camp. Imagination can scarcely conceive a situation in which all the military advantages were more on the one side, and all the disadvantages more thoroughly on the other.
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Reference address : https://www.ellopos.net/elpenor/rome/5-09-crassus-joint-rulers.asp?pg=8