In AD 161, after a long and largely peaceful reign, Antoninus Pius died, leaving the 40 year old Marcus Aurelius to take his place. The Senate clearly favored the mature Marcus over his 31 year old joint heir Lucius Verus, who had an almost Neronian reputation for personal indulgence (such as cavorting with actors), and attempted to name Marcus as sole emperor to replace Antoninus. Marcus Aurelius however insisted on following the wills of both Hadrian and Antoninus by having his adopted brother Lucius Verus secured as ‘co-emperor’. He married his daughter Lucilla to Verus to further cement the relationship in AD 164.
Despite the peaceful and easy transition from Antoninus to Aurelius and Verus, including an all important surplus treasury, the new emperors faced several immediate crises. The flooding of the Tiber River introduced a temporary famine that was overcome through the personal intervention of the emperors. In Britannia war loomed with restless tribes and along the Danube the Chatti crossed into Raetia, perhaps as a forebear of later Germanic incursions to come. These incidents were effectively managed by appointed legates, but in the east, old rivalries with Parthia would require far more attention. Disagreement between the two powers over accession issues in Armenia had been kindling since the later years of Antoninus reign and had in fact been a matter of contention dating back to the reign of Nero over a century earlier.
With the death of Antoninus, Vologaesus III king of Parthia may have viewed the establishment of a Roman diarchy as a sign of weakness. Compounding this issue may have been the fact that neither of the two emperors had acquired any military experience whatsoever. Whatever the case may have been, Vologaesus seized a perceived moment of Roman weakness and installed his own candidate upon the Armenian throne. Rome’s response was swift but initially ineffective. A Roman legion under Severianus marched from Cappadocia into Armenia and was routed at Elegeia prompting the Parthians to invade Roman territory. The governor of Syria Attidius Cornelianus suffered defeats as well, pressuring the Romans for definitive personal involvement from the imperial family. Marcus Aurelius dispatched Lucius Verus to Parthia to oversee the war and to give it an air of heightened importance, but Verus was more inclined to enjoy himself on the trip than to prepare for war. As reported in the Historia Augusta, “Verus, after he had come to Syria, lingered amid the debaucheries of Antioch and Daphne and busied himself with gladiatorial bouts and hunting.” Aurelius was fully aware of his ‘brother’s’ inadequacies and Verus’ presence was more a statement indicating the importance of the campaign than an indication of military command.
Fortunately, despite Verus’ indulgences, his legates were focused on the task at hand. Statius Priscus, Avidius Cassius and Martius Verus were entrusted with command of the legions while Marcus Aurelius conducted affairs of the state back in Rome. Though the details provided by the ancients are scant, the Historia Augusta credits Priscus with an invasion of Armenia that took the capital of Artaxata. Avidius Cassius was credited by Cassius Dio as having led the overall campaign. After withstanding the early attacks of Vologaesus, Cassius advanced deep into Mesopotamia, eventually razing Seleucia and the Parthian palaces in Ctesiphon. Though the involvement of Martius Verus is limited only to the mention of his name by the ancients, it was he who later as governor of Cappadocia interceded on behalf of Marcus Aurelius against the revolt of the afore-mentioned Avidius Cassius. This however was some years off, and for now, the 5 year campaign (161 – 166 AD) against Parthia proved to be as decisive as any war in recent Roman history. A Roman candidate once again sat the Armenian throne and Parthia had been thoroughly defeated.
Lucius Verus and Marcus Aurelius were both honored with the titles Armeniacus and Parthicus, as Verus returned to Rome to celebrate a triumph. However, with the return of his army came a terrible plague (presumably smallpox thanks in large part to the descriptions of the ancient physician Galen) which spread throughout the empire. While the plagues devastating effects are debated (as far as total death toll) there is no question that the next few years were predominately focused on efforts to defeat it. Of its potentially 5 million victims over the course of the next 15 years its most notorious victim in the early stages was likely Lucius Verus himself. After both he and Aurelius had personally marched north to investigate Germanic incursions along the Danube, they found the plague was spreading rapidly among the legions. Returning to Italy in AD 169, Verus fell ill and at the age of 38 years the junior emperor died, leaving Rome once again with a single emperor… Marcus Aurelius.