Avidius Cassius, marching down the Euphrates in 165, defeats the Parthians at Dura-Europus, which city the Romans incorporate into the province of Syria.

A descendent of the Seleucid king Antiochus IV Epiphanes, Cassius is the son of Gaius Avidius Heliodorus, a noted orator who was Prefect of Egypt from 137 to 142 under Hadrian, and wife Junia Cassia Alexandria.

He was born in the town of Cyrrhus in Syria, although he once called Alexandria his 'paternal city'.

It is assumed that Cassius began his career during the reign of Antonius Pius.

Possibly elected as a quaestor in 154, it is assumed that the young vir militaris had been stationed in the final years of Pius’s reign as a legatus in one of the legions stationed along the Danube in Moesia Inferior, watching over the Sarmatians.

Certainly by 161, he was noted as a legatus in the legions.

A strict disciplinarian as the Legatus (General) of Legio III Gallica, he quickly comes to prominence under the emperor Marcus Aurelius during the Parthian War.

Cassius has by year’s end traveled to the south and crossed Mesopotamia at its narrowest point and proceeded to attack the twin Parthian cities on the Tigris River, Seleucia on the right bank, and Ctesiphon, the Parthian capital, on the left.

He captures Ctesiphon and burns the palace of Vologases, and ...

....although Seleucia had opened its gates to the Romans, Lucius destroys it as well, leaving a blot on reputation.

Excuses are sought, or invented: the official version (promulgated, according to the Historia Augusta, by Asinius Quadratus) has it that the Seleuceni broke faith first.

Whatever the case, the sacking marks a particularly destructive chapter in Seleucia's long decline.

During the sacking, Roman troops steal the statue of Apollo Comaeus from its temple; they eventually bring it back to Rome, where it is installed at the temple of the Palatine Apollo.

This blasphemy may have been on Marcus' mind when he called a lectisternium, a great meal offered to the gods, at the beginning of the Marcomannic Wars in 167, to ward off the evils then being visited on the state.

In dire need of supplies, and with his soldiers showing the first signs of having contracted the plague at Seleucia, ...

...Cassius marches back to Syria with the spoils of the campaign.

Sending details of his achievements to Rome, he is rewarded with elevation to the Senate, although officially the campaign is credited to his commander-in-chief, Lucius Verus.

Verus even orders Cassius to write up the events of the campaign so that Verus can write himself into the official history of the campaign.

Lucius is awarded with a triumph on his return from the campaign against Parthia; the parade is unusual because it includes the two emperors, their sons and unmarried daughters as a big family celebration.

Marcus Aurelius' two sons, Commodus, five years old and Annius Verus, three, are elevated to the status of Caesar for the occasion.

A major public confrontation between Imperial Academy students and eunuchs in 166 evolves into a major incident.

The governor of the capital province (modern western Henan and central Shaanxi), Li Ying, had arrested and executed a fortuneteller named Zhang Cheng, who had had his son kill a man, having predicted that a general pardon was coming.

Li is arrested, and some two hundred Academy students sign a petition requesting his release—which further angers Emperor Huan, who has the students arrested.

Only after about a year, and Dou Wu's intercession, are Li and the students released, but all of them have their citizenship rights stripped.

This incident is later known as the first Disaster of Partisan Prohibition.

As recorded in the Hou Hanshu, a possible contact between Rome and Han China occurs in 166 when a Roman traveler visits the court of Emperor Huan, claiming to be an ambassador representing a certain Andun, who can be identified either with Marcus Aurelius or his predecessor Antoninus Pius.

Rafe de Crespigny asserts that this was most likely a group of Roman merchants.

(de Crespigny, Rafe.

[2007].

A Biographical Dictionary of Later Han to the Three Kingdoms (23–220 AD).

Leiden: Koninklijke Brill.)

Other travelers to Eastern-Han China include Buddhist monks who translate works into Chinese, such as An Shigao of Parthia, and Lokaksema from Kushan-era Gandhara, India.

Germanic tribes and other nomadic people have begun in the 160s to launch raids along the northern border, particularly into Gaul and across the Danube.

This new impetus westwards is probably due to attacks from tribes farther east.

A first invasion of the Chatti in the province of Germania Superior had been repulsed in 162.

Far more dangerous is the invasion of 166, when the Marcomanni of Bohemia, clients of the Roman Empire since 19, cross the Danube together with the Lombards and other German tribes.

Fronto's son-in-law Aufidius Victorinus had been stationed during the early 160s as a legate in Germany, there with his wife and children (another child had stayed with Fronto and his wife in Rome).

The condition on the northern frontier looks grave.

A frontier post has been destroyed, and it appears as though all the peoples of central and northern Europe are in turmoil.

There is corruption among the officers: Victorinus has to ask for the resignation of a legionary legate who is taking bribes.

Experienced governors have been replaced by friends and relatives of the imperial family.

L. Dasumius Tullius Tuscus, a distant relative of Hadrian, is in Upper Pannonia, succeeding the experienced M. Nonius Macrinus.

Lower Pannonia is under the obscure Ti. Haterius Saturnius.

M. Servilius Fabianus Maximus is shuffled from Lower Moesia to Upper Moesia when Iallius Bassus joins Lucius in Antioch.

Lower Moesia is filled by Pontius Laelianus' son.

The Dacias are still divided in three, governed by a praetorian senator and two procurators.

The peace cannot long hold; Lower Pannonia does not even have a legion.

Cassius is made suffect consul, a position he holds outside of Rome, in May of 166.

During this year, he and Verus launch a new campaign against the Parthians, this time across the northern Tigris river, and into Media, and rumors (ultimately false) reach Rome that Cassius had crossed the Indus River along with the Third Syrian legion.

By the end of 166, Cassius has been made imperial legate of the Roman province of Syria.

When imperial forces move east under the command of Emperor Lucius Verus after the forces of Vologases IV of Parthia attack Armenia, the Romans' defense of the eastern territories is hampered when large numbers of troops succumb to what will become known as the Antonine Plague.

The Parthian capitals of Seleucia and Ctesiphon had been captured by the Roman general Avidius Cassius in 165 or 166.

Most likely around the same time, Roman legions invade Media and Adiabene.

However, the Romans are forced to withdraw in 166, due to suffering heavy losses by a plague that has erupted in Seleucia.

The war soon ends afterwards, with Vologases IV having lost a vast part of northern Mesopotamia to the Romans.

A force of six thousand Langobardi and Lacringi had invaded Pannonia in late 166 or early 167.

Local forces (vexillations of the Legio I Adiutrix commanded by Candidus and the Ala I Ulpia Contariorum commanded by Vindex) had defeated the invasion with relative ease, but they mark the beginning of what is to come.

In their aftermath, the military governor of Pannonia, Marcus Iallius Bassus, initiates negotiations with eleven tribes.

The Marcomannic king Ballomar (r. 166 –178), a Roman client, acts in these negotiations as a mediator.

In the event, a truce is agreed upon and the tribes withdraw from Roman territory, but no permanent agreement is reached.