Roman–Parthian War of 161–166

In the late summer or autumn of 161, a few months after the death of Antoninus Pius, Vologases IV drove the Roman client king out of Armenia and installed the Arsacid Pacorus. Marcus Sedatius Severianus, governor of Cappadocia, led a legion into Armenia but was trapped at Elegeia by the Parthian general Chosroes and took his own life as his army was destroyed; the army of the governor of Syria, Attidius Cornelianus, was also defeated. In 162 Lucius Verus set out for the East and made Antioch his headquarters, while his generals fought the war.

In 163 Statius Priscus took the Armenian capital, Artaxata, and Rome installed as king Sohaemus, a Roman senator of Arsacid descent. In 165 Avidius Cassius led his army down the Euphrates, won a major battle at Dura-Europos and reached the twin cities on the Tigris: Seleucia, still a largely Greek city, was sacked — the official version claimed that its people had broken faith first — and Ctesiphon was taken and its royal palace set on fire. Lucius took the titles Armeniacus, Parthicus Maximus and Medicus, and on his return the two emperors celebrated a joint triumph. Rome kept Dura-Europos and territory in upper Mesopotamia around Nisibis.

Cassius’s army came back short of supplies and suffering from a disease caught at Seleucia, whose siege is dated to the winter of 165–166. The traditional Roman view blamed the epidemic that then spread through the empire, the Antonine Plague, on soldiers who had violated a temple in the city, although the first recorded case, at Smyrna, is dated 165.