Antoninus Pius would rule the empire until 161, when he died, and was succeeded by Marcus Aurelius, who later raised his adoptive brother Verus to co-emperor.

As emperor, the majority of his reign was occupied by his direction of the war with Parthia which ended in Roman victory and some territorial gains.

When his father died on 1 January 138, Hadrian chose Titus Aurelius Antoninus as his new heir, giving him the title of caesar.

[5] As a prince and future emperor, Verus received careful education from the famous grammaticus Marcus Cornelius Fronto.

[15] Soon after the emperors' accession, Marcus's eleven-year-old daughter, Annia Lucilla, was betrothed to Lucius (in spite of the fact that he was, formally, her uncle).

"[20] Fronto returned to his Roman townhouse at dawn on 28 March, having left his home in Cirta as soon as news of his pupils' accession reached him.

Reflecting on the speech he had written on taking his consulship in 143, when he had praised the young Marcus, Fronto was ebullient: "There was then an outstanding natural ability in you; there is now perfected excellence.

[34] Severianus had fallen under the influence of Alexander of Abonoteichus, a self-proclaimed prophet who carried a snake named Glycon around with him, but was really only a confidence man.

[35] Alexander was father-in-law to the respected senator Publius Mummius Sisenna Rutilianus, then-proconsul of Asia, and friends with many members of the east Roman elite.

[37] Severianus led a legion (perhaps the IX Hispana)[38] into Armenia, but was trapped by the great Parthian general Chosrhoes at Elegeia, a town just beyond the Cappadocian frontiers, high up past the headwaters of the Euphrates.

[39] There was threat of war on other frontiers as well—in Britain, and in Raetia and Upper Germany, where the Chatti of the Taunus mountains had recently crossed over the limes.

Publius Julius Geminius Marcianus, an African senator commanding X Gemina at Vindobona (Vienna), left for Cappadocia with detachments from the Danubian legions.

"[54] He encouraged Marcus to rest, calling on the example of his predecessors (Pius had enjoyed exercise in the palaestra, fishing, and comedy),[55] going so far as to write up a fable about the gods' division of the day between morning and evening—Marcus had apparently been spending most of his evenings on judicial matters instead of at leisure.

"[58] Fronto sent Marcus a selection of reading material, including Cicero's pro lege Manilia, in which the orator had argued in favor of Pompey taking supreme command in the Mithridatic War.

[59] "You will find in it many chapters aptly suited to your present counsels, concerning the choice of army commanders, the interests of allies, the protection of provinces, the discipline of the soldiers, the qualifications required for commanders in the field and elsewhere [...][notes 6]"[61] To settle his unease over the course of the Parthian War, Fronto wrote Marcus a long and considered letter, full of historical references.

There had been reverses in Rome's past, Fronto writes, at Allia, at Caudium, at Cannae, at Numantia, Cirta, and Carrhae;[62] under Trajan, Hadrian, and Pius;[63] but, in the end, Romans had always prevailed over their enemies: "always and everywhere [Mars] has changed our troubles into successes and our terrors into triumphs".

[69][notes 8] Moreover, he was far more qualified than his praetorian partner, Cornelius Repentinus, who was said to owe his office to the influence of Pius's mistress, Galeria Lysistrate.

[74] Lucius selected his favorite freedmen, including Geminus, Agaclytus, Coedes, Eclectus,[75] and Nicomedes, who gave up his duties as praefectus vehiculorum to run the commissariat of the expeditionary force.

[82] He stopped in Ephesus, where he is attested at the estate of the local aristocrat Publius Vedius Antoninus,[83] and made an unexpected stopover at Erythrae.

[84] The journey continued by ship through the Aegean and the southern coasts of Asia Minor, lingering in the famed pleasure resorts of Pamphylia and Cilicia, before arriving in Antioch.

[114] In the middle of the war, perhaps in autumn 163 or early 164, Lucius made a trip to Ephesus to be married to Marcus' daughter Lucilla.

[122] I Minervia and V Macedonica, under the legates M. Claudius Fronto and P. Martius Verus, served under Statius Priscus in Armenia, earning success for Roman arms during the campaign season of 163,[123] including the capture of the Armenian capital Artaxata.

[117] Detachments from Cappadocian legions are attested at Echmiadzin, beneath the southern face of Mount Ararat, 400 km east of Satala.

It would have meant a march of twenty days or more, through mountainous terrain, from the Roman border; a "remarkable example of imperialism", in the words of Fergus Millar.

[131] In 163, while Statius Priscus was occupied in Armenia, the Parthians intervened in Osroene, a Roman client in upper Mesopotamia, just east of Syria, with its capital at Edessa.

[132] (The Edessene coinage record actually begins at this point, with issues showing Vologases IV on the obverse and "Wael the king" (Syriac: W'L MLK') on the reverse).

[135][notes 13] Soon after the conquest of the north bank of the Euphrates, other Roman forces moved on Osroene from Armenia, taking Anthemusia, a town south-west of Edessa.

[141] By the end of the year, Cassius' army had reached the twin metropolises of Mesopotamia: Seleucia on the right bank of the Tigris and Ctesiphon on the left.

The citizens of Seleucia, still largely Greek (the city had been commissioned and settled as a capital of the Seleucid Empire, one of Alexander the Great's successor kingdoms), opened its gates to the invaders.

[143] Cassius' army, although suffering from a shortage of supplies and the effects of a plague contracted in Seleucia, made it back to Roman territory safely.