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.
[6.4.1]  When Alexander finished speaking, the cheering army promised its wholehearted support for the war. After a lavish distribution of money to the soldiers, the emperor ordered preparations for his departure from the city. He then went before the senate and made a speech similar to the one recorded above; following this, he publicly announced his plans to march out.
[6.4.2] On the appointed day, after he had performed the sacrifices prescribed for departures, Alexander left Rome, weeping and repeatedly looking back at the city. The Senate and all the people escorted him, and everyone wept, for he was held in great affection by the people of Rome, among whom he had been reared and whom he had ruled with moderation for many years.
[6.4.3] Traveling rapidly, he came to Antioch, after visiting the provinces and the garrison camps in Illyricum; from that region he collected a huge force of troops. While in Antioch he continued his preparations for the war, giving the soldiers military training under field conditions.
[6.4.4] He thought it best to send another embassy to the Persian king to discuss the possibility of peace and friendship, hoping to persuade him or to intimidate him by his presence. The barbarian, however, sent the envoys back to the emperor unsuccessful. Then Artaxerxesnote[Artaxerxes is a rendering of Ardašir.] chose four hundred very tall Persians, outfitted them with fine clothes and gold ornaments, and equipped them with horses and bows; he sent these men to Alexander as envoys, thinking that their appearance would dazzle the Romans.
[6.4.5] The envoys said that the great king Artaxerxes ordered the Romans and their emperor to withdraw from all Syria and from that part of Asia opposite Europe; they were to permit the Persians to rule as far as Ionia and Caria and to govern all the nations separated by the Aegean Sea and the Propontic Gulf, inasmuch as these were the Persians' by right of inheritance.
[6.4.6] When the Persian envoys delivered these demands, Alexander ordered the entire four hundred to be arrested; stripping off their finery, he sent the group to Phrygia, where villages and farm land were assigned to them, but he gave orders that they were not to be allowed to return to their native country. He treated them in this fashion because he thought it dishonorable and cowardly to put them to death, since they were not fighting but simply carrying out their master's orders.
[6.4.7] This is the way the affair turned out. While Alexander was preparing to cross the Tigris and Euphrates rivers and lead his army into barbarian territory, several mutinies broke out among his troops, especially among the soldiers from Egypt; but revolts occurred also in Syria, where the soldiers attempted to proclaim a new emperor.note[Taurinus. He fled to the east and died when he tried to cross the Euphrates.] These defections were quickly discovered and suppressed. At this time Alexander transferred to other stations those field armies which seemed better able to check the barbarian invasions.