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.
Rise of the Sasanians
[6.2.1]  In the fourteenth year,note[The fourteenth year would be 235. In fact, the Persian War started in 231.] however, unexpected dispatches from the governors of Syria and Mesopotamia revealed that Artaxerxes,note[Artaxerxes is a rendering of Ardašir, a Persian leader from the Sasanian family, who had indeed overthrown the Parthian royal house, the Arsacids, and had refounded the eastern empire.] the Persian king, had conquered the Parthians and seized their Eastern empire, killing Artabanus [IV], who was formerly called the Great King and wore the double diadem. Artaxerxes then subdued all the barbarians on his borders and forced them to pay tribute. He did not remain quiet, however, nor stay on his side of the Tigris River, but, after scaling its banks and crossing the borders of the Roman empire, he overran Mesopotamia and threatened Syria.
[6.2.2] The entire continent opposite Europe, separated from it by the Aegean Sea and the Propontic Gulf, and the region called Asia he wished to recover for the Persian empire. Believing these regions to be his by inheritance, he declared that all the countries in that area, including Ionia and Caria, had been ruled by Persian governors, beginning with Cyrus, who first made the Median empire Persian, and ending with Darius, the last of the Persian monarchs, whose kingdom was seized by Alexander the Great. He asserted that it was therefore proper for him to recover for the Persians the kingdom which they had formerly possessed.
[6.2.3] When the Eastern governors revealed these developments in their dispatches, Alexander was greatly disturbed by these unanticipated tidings, particularly since, raised from childhood in an age of peace, he had spent his entire life in urban ease and comfort. Before doing anything else, he thought it best, after consulting his advisers, to send an embassy to the king and by his letters halt the invasion and disappoint the barbarian's hopes.
[6.2.4] In these letters he told Artaxerxes that he must remain within his own borders and not initiate any action; let him not, deluded by vain hopes, stir up a great war, but rather let each of them be content with what was already his. Artaxerxes would find fighting against the Romans not the same thing as fighting with his barbarian kinsmen and neighbors. Alexander further reminded the Persian king of the victories won over them by Augustus, Trajan, Verus, and Severus. By writing letters of this kind, Alexander thought that he would persuade the barbarian to remain quiet or frighten him to the same course.
[6.2.5] But Artaxerxes ignored Alexander's efforts; believing that the matter would be settled by arms, not by words, he took the field, pillaging and looting all the Roman provinces. He overran and plundered Mesopotamia, trampling it under the hoofs of his horses. He laid siege to the Roman garrison camps on the banks of the rivers, the camps which defended the empire. Rash by nature and elated by successes beyond his expectations, Artaxerxes was convinced that he could surmount every obstacle in his path.
[6.2.6] The considerations which led him to wish for an expanded empire were not small. He was the first Persian to dare to launch an attack on the Parthian Empire and the first to succeed in winning back that empire for the Persians. Indeed, after Darius had been deprived of his kingdom by Alexander of Macedon, the Macedonians and Alexander's successors divided up the territory by countries and ruled the nations of the East and all Asia for many years.
[6.2.7] When these governors quarreled and the power of the Macedonians was weakened by continual wars, they say that Arsaces the Parthian was the first to persuade the barbarians in those regions to revolt from the Macedonians. Invested with the crown by the willing Parthians and the neighboring barbarians, Arsaces ruled as king. For a long time the empire remained in his own family, down to Artabanus in our time; then Artaxerxes killed Artabanus and took possession of his kingdom for the Persians. After easily subduing the neighboring barbarian nations, the king began to plot against the Roman empire.