The Parthians were essentially a warlike people; and the chief interest which attaches to them is connected with their military vigor and ability. It is worth while to consider at some length the peculiarities of that military system which proved itself superior to the organization of the Macedonians, and able to maintain for nearly three hundred years a doubtful contest with the otherwise irresistible Romans.
We are told that the Parthians had no standing army. When war was proclaimed and the monarch needed a force, he made his immediate vassals acquainted with the fact, and requested each of them to marshal their troops, and bring them to a fixed rendezvous by a certain day. The troops thus summoned were of two kinds, Parthian and foreign. The governors of the provinces, whether tributary kings or satraps, called out the military strength of their respective districts, saw to their arming and provisioning, and, marching each at the head of his contingent, brought a foreign auxiliary force to the assistance of the Great King. But the back-bone of the army, its main strength, the portion on which alone much reliance was placed, consisted of Parthians. Each Parthian noble was bound to call out his slaves and his retainers, to arm and equip them at his own expense, and bring them to the rendezvous by the time named. The number of troops furnished by each noble varied according to his position and his means; we bear in one instance of their amounting to as many as 10,000, while in another recorded case the average number which each furnished was no more than 125. The various contingents had their own baggage-trains, consisting ordinarily of camels, in the proportion (as it would seem) of one to every ten fighting-men.
A Parthian army consisted usually of both horse and foot, but in proportions unusual elsewhere. The foot soldiers were comparatively few in number, and were regarded as of small account. Every effort was made to increase the amount and improve the equipment of the horsemen, who bore the brunt of every fight, and from whose exertions alone victory was hoped. Sometimes armies consisted of horsemen only, or rather of horsemen followed by a baggage train composed of camels and chariots.
The horse were of two kinds, heavy and light. The heavy horsemen wore coats of mail, reaching to their knees, composed of rawhide covered with scales of iron or steel, very bright, and capable of resisting a strong blow. They had on their heads burnished helmets of Margian steel, whose glitter dazzled the spectator. Their legs seem not to have been greaved, but encased in a loose trouser, which hung about the ankles and embarrassed the feet, if by any chance the horseman was forced to dismount. They carried no shield, being sufficiently defended by their coats of mail. Their offensive arms were a long spear, which was of great strength and thickness, and a bow and arrows of unusual size. They likewise carried in their girdle a short sword or knife, which might be used in close combat. Their horses were, like themselves, protected by a defence of scale armor, which was either of steel or bronze.
The light horse was armed with the same sort of bows and arrows as the heavy, but carried no spear and wore no armor. It was carefully trained to the management of the horse and the bow, and was unequalled in the rapidity and dexterity of its movements. The archer delivered his arrows with as much precision and force in retreat as in advance, and was almost more feared when he retired than when he charged his foe. Besides his arrows, the light horseman seems to have carried a sword, and he no doubt wore also the customary knife in his belt.
We are told by one writer that it was a practice of the Parthians to bring into battle a number of led horses, and that the riders from time to time exchanged their tired steeds for fresh ones, thus obtaining a great advantage over enemies who had no such practice. But the accounts which we have of Parthian engagements make no reference to this usage, which we can therefore scarcely suppose to have been adopted to any large extent. It may be doubted, also, if the practice could ever be one of much value, since the difficulty of managing led horses amid the tumult of a battle would probably more than counterbalance the advantage derivable from relays of fresh steeds.
During the later period of the monarchy, the Parthians, who had always employed camels largely in the conveyance of stores and baggage, are said to have introduced a camel corps into the army itself, and to have derived considerable advantage from the new arm. The camels could bear the weight of the mailed warrior and of their own armor better than horses, and their riders were at once more safe in their elevated position and more capable of dealing effective blows upon the enemy. As a set-off, however, against those advantages, the spongy feet of the camel were found to be more readily injured by the tribulus, or caltrop, than the harder feet of the horse, and the corps was thus more easily disabled than an equal force of cavalry, if it could be tempted to pass over ground on which caltrops had been previously scattered.
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.
It was also a general principle of Parthian warfare to abstain from campaigning during the winter. So much depended upon the tension of their bow-strings, which any dampness relaxed, that their rule was to make all their expeditions in the dry time of their year, which lasted from early in the spring until late in the autumn. The rule was, however, transgressed upon occasions. Phraates II. made his attack upon Antiochus Sidetes, while the snow was still upon the ground; and Volagases I. fell upon Paetus after the latter had sent his troops into winter quarters. The Parthians could bear cold no less than heat; though it was perhaps rather in the endurance of the latter than of the former that they surpassed the Romans. The sun’s rays were never too hot for them; and they did not need water frequently or in large quantities. The Romans believed that they increased their ability of bearing thirst by means of certain drugs which they consumed; but it may be questioned whether they really employed any other remedies than habit and resolution.