Two brothers were in charge of Iberia (modern Georgia) and Armenia: Pharasmanes and Mithridates. Rhadamistus was the son of Pharasmanes and wanted to become king of Iberia, but his father lived quite long, and he therefore decided to overthrow his uncle. According to the Roman historian Tacitus, Rhadamistus
was tall and handsome, remarkable for his bodily strength, versed in the national accomplishments, and in high repute with the neighbouring peoples.note[Tacitus, Annals 12.44.]
Having gathered support in Armenia, Rhadamistus besieged his uncle in Garni, east of the capital Artaxata.note[Tacitus, Annals 12.45.] The fortress was betrayed and Rhadamistus became king (51 CE), which was the immediate cause of a Parthian intervention.
In the next year, 52, the Parthian king Vologases I invaded Armenia, occupied its capitals Artaxata and Tigranocerta, and made his younger brother Tiridates king of Armenia.note[Tacitus, Annals 12.50.] This was a breach of a treaty that the Parthian king Phraates IV had concluded with the Roman emperor Augustus in 20 BCE: the Romans were allowed to appoint the Armenian king. Vologases may have thought that the old Roman emperor Claudius (r.41-54) would not adequately respond to this deliberate provocation. Indeed, Rome did not immediately send troops. Still, Vologases got the problems he had been asking for.
In Media and Hyrcania, his son Vardanes II revolted,note[Tacitus, Annals 13.7.] and it is certainly possible that he had Roman support. In any case, the Hyrcanian problems would continue for several years, required (according to Tacitus) "numerous campaigns",note[Tacitus, Annals 15.1.] and made it impossible for Vologases to intervene in Armenia. His supply lines were now threatened. To make problems worse, the Armenian winter turned out to be severe. As a consequence, an epidemic broke out,note[Tacitus, Annals 12.50.] and Vologases was forced to return home.
This allowed Rhadamistus to regain control of Armenia. However, early in 55, the Armenians expelled him and invited Tiridates to come back.note[Tacitus, Annals 13.6.]