The war took place as a delicate balance of power between the Roman and Parthian empires was in place in the Caucasus. Rome was then ruled by Claudius, Parthia by Vologases I. Two brothers then ruled the Caucasian kingdoms, Pharasmanes I in Iberia, Mithridates in Armenia. They were both dependent on Roman support, which had installed Mithridates on the Armenian throne in 35 AD. However, 15 years later, trust between the brothers had deteriorated, which Tacitus blames on the intrigues of Pharasmanes' son Rhadamistus.

Fearing usurpation by Rhadamistus, his father convinced him to make war upon his uncle and claim the Armenian throne for himself. The Iberians invaded with a large army and surrounded Mithridates at the fortress of Gorneas (Garni), which was garrisoned by the Romans under the command of Caelius Pollio, a prefect, and Casperius, a centurion. Rhadamistus was unable to take the fortress by assault or by siege. Pollio, swayed by bribery from Rhadamistus, betrayed Mithridates and induced the Roman soldiers to threaten the capitulation of the garrison. Under this threat, Mithridates left the fortress in order to make peace with Rhadamistus. Rhadamistus then executed Mithridates and his sons, despite a promise of non-violence, and became King of Armenia. Of this usurpation, Tacitus wrote "Rhadamistus might retain his ill-gotten gains, as long as he was hated and infamous; for this was more to Rome's interest than for him to have succeeded with glory".

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