Roman–Parthian Wars participant Osroene
Roman Republic vs Parthia
When Pompey took charge of the war in the East, he re-opened negotiations with Phraates III; they came to an agreement and Roman–Parthian troops invaded Armenia in 66/65 BC, but soon a dispute arose over Euphrates boundary between Rome and Parthia. Pompey refused to recognize the title of "King of Kings" for Phraates, and offered arbitration between Tigranes and the Parthian king over Corduene. Finally, Phraates asserted his control over Mesopotamia, except for the western district of Osroene, which became a Roman dependency.