Legio III Parthica: one of the Roman legions. Its name indicates that it was recruited to fight against the Parthians.
Arguably, the third Parthian legion is the least known of all the legions of the Principate. It is certain, however that it was founded in 197 by the emperor Lucius Septimius Severus, together with two sister legions, I Parthica and II Parthica. Severus needed these units during his projected campaign against the Parthian Empire. The expedition was very successful and culminated in the sack of the Parthian capital Ctesiphon. According to the Roman historian Cassius Dio,note[Cassius Dio, Roman History 55.24.4.] the First and the Third remained in the region, because Severus annexed Mesopotamia (northern Iraq) and converted this area into a province. The second Parthian legion was transferred to the Alban mountain near Rome, where it served as the empire's strategic reserve.
Mesopotamia was not an ordinary province, because its governors were prefects from the equestrian order, not senatorial governors. Accordingly, the commander of III Parthica was not a senator but a Roman knight.
III Parthica was stationed in a fortress called Rhesaena on the Upper Chaboras (modern Khabur). Archaeological excavations, which may give more information about the legion's history, will be difficult because the site is in the military zone between Syria and Turkey. In fact, the frontier is almost through the site, which has, accordingly, a Turkish and an Arabic name: Ceylanpinar and Ras al-'Ayn. However, there are coins with the legend LE III P S (Legio III Partica Severiana).
At Rhesaena, the Third controlled the road between the former Arab principalities Edessa and Nisibis, and defended the empire against the Parthians or -after the fall of their empire- the Sasanian Persians. It is interesting to note that Rhesaena is far behind the actual frontier. Perhaps Severus was already experimenting with the system of defense in depth that was introduced by Gallienus after 260 and brought to perfection by Constantine I the Great.
The Third must have taken part in the expeditions of third century, like the one led by Severus' son Caracalla and his successor Macrinus (217), and the war waged by Severus Alexander against the new, Sasanian Persian empire. The Sasanians had invaded the Roman empire in 230 and had installed an emperor in Emessa, but Severus Alexander was able to restore order and invaded Iraq. In 243, this war was renewed, and at Rhesaena, the Third legion, no doubt reinforced, defeated the Persian army.
Coins bearing the legend L III PIA may refer to this victory, and prove that the unit had received the surname Pia, "pious". Some caution, however, is in place, because similar coins were minted by Decius (249-251), who is not known for an eastern campaign. Other coins, from Sidon, suggest that veterans of III Parthica were settled in that city.
In 244, the Romans again invaded Iraq, but their emperor Gordian III died and was succeeded by Philip the Arab, who owed his throne to the Sasanian king Shapur I.
Even worse was to come. In 256 Shapur captured Satala (the fortress of XV Apollinaris), and two years later he reached the Black Sea and sacked Trapezus. When the Roman emperor Valerian tried to restore order and invaded Iraq, he was defeated and captured. Captive Roman soldiers were ordered to build a bridge at modern Shushtar. These Roman defeats are commemorated on several Sasanian rock reliefs.
However, under the emperors Odaenathus of Palmyra (261-267) and Diocletian (284-305), the Romans restored their fortunes and in 298, a peace treaty was concluded in which the Persians had to give up territories in northern Mesopotamia. The third legion must have played a role during these campaigns, but we have almost no information about them. A tantalizing piece of evidence is a gold piece that was found in the Roman theater of Orange, minted by the Gallic emperor Victorinus in 271, and mentioning LEG III PARTHICA. It is of course possible that a subunit was in the West, but it is not very likely.
It was not uncommon that subunits of a legion were sent to other parts of the empire. A speculator is known from Rome. The soldier who buried his son in Cilicia, however, was probably not on a foreign mission, because Cilicia is close to Mesopotamia. (It is also possible that this soldier belonged to I Parthica.) A soldier buried in Isauria, a notoriously unquiet region, may have been killed in action.
According to the Notitia Dignitatum (East, 35), written at the beginning of the fifth century, the third Parthian legion was at Apadna in Osrhoene, near the confluence of the rivers Chaboras and Euphrates.
According to the coin from Orange, the emblem of this legion was a centaur.
- K.O. Castelin, The Coinage of Rhesaena in Mesopotamia (= Numismatic Notes and Monographs 108) (1946)
- Emil Ritterling, "Legio" in: Realencyclopädie of Klassischen Altertumswissenschaft, XII (1925)
- Catherine Wolff, "Legio III Parthica" in: Yann Le Bohec (ed.): Les Légions de Rome sous le Haut-Empire, tome I (2000), pp.251-252