The Wealth of Nations by Part 5 Chapter 1 Page 38

Rome. The militias of some barbarous nations defended themselves much better. The Scythian or Tartar militia, which Mithridates drew from the countries north of the Euxine and Caspian seas, were the most formidable enemies whom the Romans had to encounter after the second Carthaginian war.

The Parthian and German militias, too, were always respectable, and upon several occasions gained very considerable advantages over the Roman armies. In general, however, and when the Roman armies were well commanded, they appear to have been very much superior; and if the Romans did not pursue the final conquest either of Parthia or Germany, it was probably because they judged that it was not worth while to add those two barbarous countries to an empire which was already too large. The ancient Parthians appear to have been a nation of