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.