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.
Negotiations with Parthia
[4.10.1]  Not long after this, Caracalla, desirous of gaining the title Parthicusnote[Which meant that he had defeated the Parthians. Caracalla's remarkable diplomatic move may have had something to do with the Parthian civil war: the rebel Artabanus IV quarreled with his relative, Vologases VI. Caracalla may have offered support to Artabanus.] and of being able to report to the Romans that he had conquered all the Eastern barbarians, even though there was peace everywhere, devised the following plan. He wrote a letter to the king of Parthia (his name was Artabanus [IV]) and sent to him an embassy laden with gifts of expensive materials and fine workmanship.
[4.10.2] He wrote to the king that he wished to marry his daughter; that it was not fitting that he, emperor and son of an emperor, be the son-in-law of a lowly private citizen. His wish was to marry a princess, the daughter of a great king. He pointed out that the Roman and the Parthian empires were the largest in the world; if they were united by marriage, one empire without a rival would result when they were no longer divided by a river.
[4.10.3] The rest of the barbarian nations now not subject to their authority could easily be reduced, as they were governed by tribes and confederacies. Furthermore, the Roman infantry were invincible in close-quarter combat with spears, and the Parthians had a large force of highly skilled horse-archers.
[4.10.4] The two forces, he said, complemented each other; by waging war together, they could easily unite the entire inhabited world under a single crown. Since the Parthians produced spices and excellent textiles and the Romans metals and manufactured articles, these products would no longer be scarce and smuggled by merchants; rather, when there was one world under one supreme authority, both peoples would enjoy these goods and share them in common.
[4.10.5] At first the Parthian king did not approve of the proposals in Caracalla's letters, saying that it was not proper for a barbarian to marry a Roman. What accord could there be when they did not understand each other's language and differed so radically in diet and dress? Surely, the king said, there are many distinguished Romans, one of whose daughters he could marry, just as for him there were the Arsacids;note[The name of the Parthian dynasty.] it was not fitting that either race be bastardized.