Roman–Iranian relations
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.
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.
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.
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.
The Antonine plague, possibly of smallpox, which was sweeping Parthia at the time now spread to the Roman army, leading to their withdrawal: the plague killed nearly half of the population in the Italian peninsula (for some months there were nearly 3000 deaths each day in the city of Rome[7]) and the Romans' legions in the eastern territories of their empire were hampered when large numbers of troops succumbed to the disease.
With the plague the Roman Empire stopped to grow and started a process of slow disintegration in the next centuries, according to historians like Theodor Mommsen.
After submitting a request to marry the daughter of Persian king Artabanus IV (potentially allowing an heir to assume control of both empires), Caracalla massacred the diplomatic party sent to arrange the marriage and attempted an invasion of Persia in 216 AD.
[11] In the 5th century, Romans provided a subsidy as the Sassanians requested, to construct defenses in Derbent, through which incursions from tribes of the northern steppes endangered both empires.
[12] Due to the increase in diplomatic relations between the Empires throughout the medieval period, a visual culture of kingship grew between the Romans and the Persians in the 6th and 7th centuries.
[13] In addition, alongside the advancement of diplomatic ties, familial, often flowery and ornate, language also became a significant metaphor to describe the various features of the Romano-Persian political relationship, since it was something both Empires had substantial indigenous predecessors for.
