The Iberians invaded with a large army, with him surrounding and starting a siege against 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.

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".

[3] Faced with this upset of the regional balance and regarding the event as unrightful appropriation, Vologases I of Parthia invaded in 52 AD to proclaim his brother Tiridates king of Armenia.

The conflict lasted for two years, until winter, plague and shortage of supplies forced Tiridates to retire from Armenia, which allowed Rhadamistus to return with his army strengthened and reassert rule.

However, he would eventually be deposed due to being oppressive and punishing the populace for what he perceived was treason, when they allowed the Parthians to enter their towns unopposed.