ty. Their warfare was
suited for defence rather than for attack, unless against contemptible
enemies. They were bad hands at sieges, and seldom ventured to engage in
them, though they would do so if circumstances required it. They wearied
of long campaigns, and if they did not find victory tolerably easy,
were apt to retire and allow their foe to escape, or baffle him by
withdrawing their forces into a distant and inaccessible region. After
their early victories over Crassus and Antony, they never succeeded in
preventing the steady advance of a Roman army into their territory,
or in repulsing a determined attack upon their capital. Still they
generally had their revenge after a short time. It was easy for the
Romans to overrun Mesopotamia, but it was not so easy for them to hold
it; and it was scarcely possible for them to retire from it after an
occupation without disaster. The clouds of Parthian horse hung upon
their retreating columns, straitened them for provisions, galled them
with missiles, and destroyed those who could not keep up with the main
body. The towns upon the line of their retreat revolted and shut their
gates, defying even such commanders as Severus and Trajan. Of the six
great expeditions of Rome against Parthia, one only, that of Avidius
Cassius, was entirely successful. In every other case either the
failure of the expedition was complete, or the glory of the advance was
tarnished by disaster and suffering during the retreat.

The results of invading Parthia would have been even more calamitous
to an assailant but for one weak point in the military system of the
Parthians. They were excessively unwilling to venture near an enemy
at night, and as a general rule abstained from all military movements
during the hours of darkness. As evening approached, they drew off to a
considerable distance from their foe, and left him unmolested to retreat
in any direction that he pleased. The reason of this probably was, not
merely that they did not fortify their camps; but that, depending wholly
on their horses, and being forced to hobble or tether them at night,
they could not readily get into fighting order on a sudden during
darkness. Once or twice in the course of their history, we find them
departing from their policy of extreme precaution, and recommencing
the pursuit of a flying foe before dawn; but it is noted as an unusual
occurrence.

It was also a general principle of Parthian warfare to abstain from