Herodian (late second, first half third century): Greek historian, author of a History of the Roman Empire since the Death of Marcus Aurelius in which he describes the reign of Commodus (180-192), the Year of the Five Emperors (193), the age of the Severan dynasty (211-235), and the Year of the Six Emperors (238).
The translation was made by Edward C. Echols (Herodian of Antioch's History of the Roman Empire, 1961 Berkeley and Los Angeles) and was put online for the first time by Roger Pearse (Tertullian.Org). The version offered on these pages is hyperlinked and contains notes by Jona Lendering.
Alexander's Persian War
[6.5.1]  After thus setting matters in order, Alexander, considering that the huge army he had assembled was now nearly equal in power and numbers to the barbarians, consulted his advisers and then divided his force into three separate armies. One army he ordered to overrun the land of the Medes after marching north and passing through Armenia, which seemed to favor the Roman cause.
[6.5.2] He sent the second army to the eastern sector of the barbarian territory, where, it is said, the Tigris and Euphrates rivers at their confluence empty into very dense marshes; these are the only rivers whose mouths cannot be clearly determined. The third and most powerful army he kept himself, promising to lead it against the barbarians in the central sector.note[The second army proceeded along the Euphrates, the third along the Tigris. The main aim of this war appears to have been a strike at the Persian heartland. The first army was to proceed through Media Atropatene and may have tried to to use the road from Ecbatana to Susa, which may have been the point where the other armies were to convene as well, before unitedly crossing the Zagros and attacking Ardašir's capital Firuzabad.] He thought that in this way he would attack them from different directions when they were unprepared and not anticipating such strategy, and he believed that the Persian horde, constantly split up to face their attackers on several fronts, would be weaker and less unified for battle.
[6.5.3] The barbarians, it may be noted, do not hire mercenary soldiers as the Romans do, nor do they maintain trained standing armies. Rather, all the available men, and sometimes the women too, mobilize at the king's order. At the end of the war each man returns to his regular occupation, taking as his pay whatever falls to his lot from the general booty.
[6.5.4] They use the bow and the horse in war, as the Romans do, but the barbarians are reared with these from childhood, and live by hunting; they never lay aside their quivers or dismount from their horses, but employ them constantly for war and the chase.
Alexander therefore devised what he believed to be the best possible plan of action, only to have Fortune defeat his design.
[6.5.5] The army sent through Armenia had an agonizing passage over the high, steep mountains of that country. (As it was still summer, however, they were able to complete the crossing.) Then, plunging down into the land of the Medes, the Roman soldiers devastated the countryside, burning many villages and carrying off much loot. Informed of this, the Persian king led his army to the aid of the Medes, but met with little success in his efforts to halt the Roman advance.
[6.5.6] This is rough country; while it provided firm footing and easy passage for the infantry, the rugged mountain terrain hampered the movements of the barbarian cavalry and prevented their riding down the Romans or even making contact with them. Then men came and reported to the Persian king that another Roman army had appeared in eastern Parthianote[Not the real Parthia in northeastern Iran, but the parts of the Parthian Empire east of the Euphrates.] and was overrunning the plains there.
[6.5.7] Fearing that the Romans, after ravaging Parthia unopposed, might advance into Persia, Artaxerxesnote[Artaxerxes is a rendering of Ardašir.] left behind a force which he thought strong enough to defend Media, and hurried with his entire army into the eastern sector. The Romans were advancing much too carelessly because they had met no opposition and, in addition, they believed that Alexander and his army, the largest and most formidable of the three, had already attacked the barbarians in the central sector. They thought, too, that their own advance would be easier and less hazardous when the barbarians were constantly being drawn off elsewhere to meet the threat of the emperor's army.
[6.5.8] All three Roman armies had been ordered to invade the enemy's territory, and a final rendezvous had been selected to which they were to bring their booty and prisoners. But Alexander failed them: he did not bring his army or come himself into barbarian territory, either because he was afraid to risk his life for the Roman empire or because his mother's feminine fears or excessive mother love restrained him.
[6.5.9] She blocked his efforts at courage by persuading him that he should let others risk their lives for him, but that he should not personally fight in battle. It was this reluctance of his which led to the destruction of the advancing Roman army. The king attacked it unexpectedly with his entire force and trapped the Romans like fish in a net; firing their arrows from all sides at the encircled soldiers, the Persians massacred the whole army. The outnumbered Romans were unable to stem the attack of the Persian horde; they used their shields to protect those parts of their bodies exposed to the Persian arrows.
[6.5.10] Content merely to protect themselves, they offered no resistance. As a result, all the Romans were driven into one spot, where they made a wall of their shields and fought like an army under siege. Hit and wounded from every side, they held out bravely as long as they could, but in the end all were killed. The Romans suffered a staggering disaster; it is not easy to recall another like it, one in which a great army was destroyed, an army inferior in strength and determination to none of the armies of old. The successful outcome of these important events encouraged the Persian king to anticipate better things in the future.