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.
Severus Alexander Declares War
[6.3.1]  When the bold actions of this Eastern barbarian were disclosed to Alexander while he was passing the time in Rome, he found these affronts unendurable. Though the undertaking distressed him and was contrary to his inclinations, since his governors there were calling for him, he made preparations for departure. He assembled for army service picked men from Italy and from all the Roman provinces, enrolling those whose age and physical condition qualified them for military service.
[6.3.2] The gathering of an army equal in size to the reported strength of the attacking barbarians caused the greatest upheaval throughout the Roman world. When these troops were gathered in Rome, Alexander ordered them to assemble on the usual plain. There he mounted a platform and addressed them as follows:
[6.3.3] "I wished, fellow soldiers, to make the customary speech to you, the speech from which I, speaking to the popular taste, receive approval, and you, when you hear it, receive encouragement. Since you have now enjoyed many years of peace, you may be startled to hear something unusual or contrary to your anticipations.
[6.3.4] Brave and intelligent men should pray for things to turn out for the best, but they should also endure whatever befalls. It is true that the enjoyment of things done for pleasure brings gratification, but good repute results from the manliness involved in setting matters straight when necessity demands. To initiate unjust actions is not proof of good intentions, but it is a courageous deed to rid oneself of those who are troublesome if it is done with good conscience; one may expect good results if he has done nothing unjust but has avoided injustice.
[6.3.5] The Persian Artaxerxesnote[Artaxerxes is a rendering of Ardašir.] has slain his master Artabanus [IV], and the Parthian Empire is now Persian. Despising our arms and contemptuous of the Roman reputation, Artaxerxes is attempting to overrun and destroy our imperial possessions. I first endeavored by letters and persuasion to check his mad greed and his lust for the property of others. But the king, with barbarian arrogance, is unwilling to remain within his own boundaries and challenges us to battle.
[6.3.6] Let us not hesitate to accept his challenge. You veterans remind yourselves of the victories which you often won over the barbarians under the leadership of Severus and my father, Caracalla. You recruits, thirsting for glory and honor, make it clear that you know how to live at peace mildly and with propriety, but make it equally clear that you turn with courage to the tasks of war when necessity demands.
[6.3.7] he barbarian is bold against the hesitant and the cowardly, but he does not stand up in like fashion to those who fight back; it is not in close-quarter combat that they battle the enemy with hope of success. Rather, they believe that whatever success they win is the result of plundering after a feigned retreat and flight. Discipline and organized battle tactics favor us, together with the fact that we have always been taught to conquer the barbarian."