Relations between the Roman and Iranian states were established c. 92 BC. It was in 69 BC that the two states clashed for the first time; the political rivalry between the two empires would dominate much of Western Asia and Europe until 628. Initially commencing as a rivalry between the Parthians and Rome, from the 3rd to mid-7th centuries the Roman Empire (later the Byzantine Empire) and its rival Sassanid Persia were recognized as two of the leading powers in the world.
Relations during the Republic
The first direct contact between the Roman Republic and the Parthians was c. 92 BC, when Lucius Cornelius Sulla, while proconsul in Cilicia, met the Parthian ambassador Orobazus. Plutarch reports that he managed to take the central seat between the Parthian Ambassador and an ambassador from Pontus, and concluded a treaty that set the Euphrates as the boundary between the two powers. Orobazus was executed on his return to Parthia for allowing Sulla to outmaneuver him, and Sulla himself later came under criticism for being too high-handed in his treatment of such a powerful nation.
The first time the Romans came into direct military contact with Parthia came when Lucullus invaded Armenia in 69 BC, leading to diplomatic friction and clashes on the frontier between Armenia and Parthia. Over the following decades both empires became entangled in each other's civil wars, perhaps beginning with Crassus’s disastrous invasion of Parthia. Parthia was later involved in the civil war after the assassination of Julius Caesar. In 42 BC, when Antony placed a legion in Syria, Cassius’ envoy Labienus joined forces with king Orodes of Parthia and, led by Pacorus, attacked the Levant and the Asia Minor. However, this was not to last as Antony successfully sent his general Publius Ventidius Bassus to recover the lost territory. After some difficulty dealing with local Parthian appointee kings, the Romans finally subdued the regained province and installed Herod the Great as king. Antony’s forces attempted a crossing of the Euphrates at the city of Zeugma but were held back by Parthian defences and had to settle for annexing the Armenian kingdom after deposing its king.
Relations during the Julio-Claudian dynasty
Augustus was loath to seek further conflict with Parthia. However, the coveted standards were still held by the Parthians and this was of great concern to Augustus, forcing him to regain them through a less conventional method. Augustus set up Musa of Parthia as a client Queen. Indeed Musa was an Italian servant-girl who was given to the Parthian monarch Phraates IV () as a gift by the Roman Emperor Augustus. Musa quickly became queen and a favourite of Phraates IV, giving birth to Phraataces (Phraates V) about 19 BC. In 2 BC, Musa had Phraates IV poisoned and made herself along with Phraates V the co-rulers of the Parthian Empire. After a short rule, the Parthian nobility, angered by Phraates V's recent acknowledgement of Roman suzerainty in Armenia and his mother's Italian descent, deposed them both from the throne in 4 AD and installed Orodes III as king. Phraates V and Musa fled to Rome, where Augustus welcomed them.
The next half century saw relations between the two nations antagonistic but not overtly hostile, with the Romans unsuccessfully supporting a series of pretender kings, including Claudius in 49 CE, indicating the extent to which Rome was attempting to influence Parthian politics for its own ends. However, during the reign of Nero, Vologases I invaded Armenia and installed his own brother on the throne, disrupting the balance of influence which had hitherto existed there. The ensuing war was ended by a compromise which allowed the Parthian prince Tiridates and his descendants to reign in Armenia on condition that he and his successors received their crown from the Roman emperor and ruled as his clients.
Strabo described the Parthian Empire as the only rival existing to Rome.
Relations during the Flavian dynasty
During Vespasian’s rule Parthia seemed to make some attempts to strengthening the ties between the two powers, such as asking to form an alliance at the Caucasus against belligerent Sarmatian tribes and offering assistance to Vespasian against the short lived emperor Vitellius once it became clear that Vespasian would rule. However, both of these Vespasian refused.