While the Parthian warfare was admirably adapted for the national defence on the broad plains of inner Asia, it was ill suited for conquest, and, comparatively speaking, ineffective in more contracted and difficult regions.

The Parthian military system had not the elasticity of the Roman--it did not in the same way adapt itself to circumstances, or admit of the addition of new arms, or the indefinite expansion of an old one.

However loose and seemingly flexible, it was rigid in its uniformity; it never altered; it remained under the thirtieth Arsaces such as it had been under the first, improved in details, perhaps, but essentially the same system.

The Romans, on the contrary, were ever modifying their system, ever learning new combinations or new manoeuvres or new modes of warfare from their enemies.

They met the Parthian tactics of loose array, continuous distant missiles, and almost exclusive employment of cavalry, with an increase in the number of their own horse, a larger employment of auxiliary irregulars, and a greater use of the sling.