The Parthian tactics were of a simple kind, and differed little from those of other nations in the same region, which have depended mainly on their cavalry.  To surround their foe, to involve him in difficulties, to cut off:  his supplies and his stragglers, and ultimately to bring him into a position where he might be overwhelmed by missiles, was the aim of all Parthian commanders of any military capacity.  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.