Balāš V succeeded his father in 190/1 or 193 and reigned until 207/8. During these years the Roman empire was itself afflicted with the “Parthian” malady. After the murder of Commodus in 192, three rival army commanders set their sights on the throne. Later two of them, Pescennius Niger and Septimius Severus, remained to fight it out. The Parthians gave some help, but not much, to Pescennius Niger who commanded the legions in Syria. The historian Herodian (3.1, pp. 1-3) implies that the Parthian great king could only urge his vassals to send troops. It would therefore appear that the great king did not have a large force at his own disposal. On the other hand Balāš V may have supported the rebels in the Roman-ruled territory of Osrhoene who, together with the army of the Parthian vassal-state of Adiabene, laid siege to the Roman-held city of Nisibis (Hanslik, col. 1851, and Debevoise, p. 256, reckon that he did, but Ziegler, p. 130, holds that the siege was an independent venture by the king of Adiabene, not a deliberate treaty breach by Balāš V).

In 194 Septimius Severus finally achieved victory in the Roman civil war, and in the following year he marched into western Mesopotamia with the aim of recovering the lost territories (see Hasebroek, pp. 73ff.). The course of events cannot be traced with precision, mainly because it is not known whether various incidents took place in this campaign or in Severus’s second campaign which began in 197. In any case Osrhoene and Nisibis were reconquered. The only uncertain point is whether it was at this time that Osrhoene was made a Roman province and that a new province of Mesopotamia comprising areas east of Osrhoene was constituted. (This is the opinion of Ziegler, p. 131, and Bertinelli, p. 39; but see Magie, II, pp. 1543-54.) Adiabene also was conquered or at least was the scene of a victory, and another victory was won over some Arabs, as indicated by Severus’s assumption of the titles “Parthicus Adiabenicus” and “Parthicus Arabicus.” The prospects for the Parthians were far from favorable when the emperor had to call off his campaign and return home to deal with the army commander in Gaul, Clodius Albinus, who set himself up as counter-emperor. After defeating Albinus near Lugdunum (Lyon) early in 197, Septimius Severus resumed his war with the Parthians. In the meantime Balāš V had suppressed revolts in Iran and defeated the pro-Roman king Narses of Adiabene. He then marched into Mesopotamia and besieged but failed to capture Nisibis. When Roman reinforcements arrived, the Parthians gave up the siege. The Romans then took the offensive, advancing down the Euphrates to Seleucia and Babylon, which they occupied without resistance, though they had to fight hard before Ctesiphon fell to them, probably late in the year 198 (Bertinelli, pp. 37ff.). The emperor now assumed the title “Parthicus Maximus.” The Romans were unable, however, to hold onto their gains. Difficulties in obtaining adequate food supplies and reinforcements obliged them to withdraw. On the way back, Septimius Severus attempted to seize Hatra, but like Trajan was unsuccessful. The Roman troops probably spent the winter in northern Mesopotamia, and in the spring of 199 they made a second attempt on Hatra but were again repulsed and suffered heavy losses. They then finally withdrew to Syria. It seems likely that a peace treaty between the two powers was concluded in 199, though the ancient sources are silent on the subject. While Septimius Severus had not been able to make any large and lasting gains, he at least obtained a secure frontier with the Parthian empire. Two new Roman provinces were now constituted, namely Osrhoene (less a small area including the capital Edessa, which apparently continued to be a vassal kingdom) and Mesopotamia, and three new legions (I-III Parthicae) were mustered to garrison and defend these provinces. Recent archeological investigations (see Bertinelli, pp. 41ff.) have made it possible to trace parts of the Roman defense line. This limes ran through Alaina (Tell Hayal), Singara (Balad Senjar) and further east through sites at Zagurae (Ain Sinu) to Vicat (Tell Ibra); the finds do not show whether the frontier ran to Ad Flumen Tigris (Mosul), which would have been its rational terminus.

Roman-Parthian relations remained peaceful in the rest of the reigns of Balāš V, who died in 206/7, and Septimius Severus, who died in 211. Only after the accession of the latter’s son Caracalla did things change.