Fearing usurpation by Rhadamistus, his father convinces him to declare war upon his uncle and claim the Armenian throne for himself. The Iberians invade with a large army and force Mithridates to retreat into the fortress of Gorneas (Garni), which is garrisoned by the Romans under the command of Caelius Pollio, a prefect, and Casperius, a centurion. Rhadamistus is unable to take the fortress by assault or by siege. Pollio, swayed by bribery from Rhadamistus, induces the Roman soldiers to threaten the capitulation of the garrison. Under this threat, Mithridates leaves the fortress in order to make peace with Rhadamistus. Rhadamistus then executes Mithridates and his sons, despite a promise of non-violence, and becomes King of Armenia. Of this usurpation, Tacitus writes "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". Shortly after the Iberians are expelled from Armenia by a rebellion of the Armenian nobility which is supported by the Parthian Empire. Both kingdoms would then take sides respectively during the Roman–Parthian War of 58–63.