Romans in Persia is related to the brief invasion and occupation of western and central areas of Parthia (modern-day Iran) by the Romans during their empire. Emperor Trajan was even temporarily able to nominate a king of western parts of Parthia, Parthamaspates, as ruler of a Roman "client state" in Parthia.
Characteristics
The Romans, having expanded into the eastern Mediterranean, came in contact with their historical rivals in the Middle East: the Parthians and Persians, whose respective empires occupied modern Iran and surrounding areas. The Iranian empires that faced the Romans had different names, related to the origin of the dynasties in control of their empires in different historical periods: first were the Medians (Media Atropatene), then Parthians and finally Sassanians.
In 64 BC Pompey conquered the remaining Seleucid territories in Syria, extinguishing their state and advancing the Roman eastern frontier to the Euphrates, where Romans met for the first time the territory of the Parthians.
Mark Antony, in his quest to avenge the battle of Carrhae defeat, conquered in 33 BC some areas of Atropatene (northern Iran) and Armenia but soon lost it: that was the first time that Romans occupied a Persian territory, even if temporarily.
Furthermore, probably in 20 BC, Augustus is said to have nominated Ariobarzanes II, the son of Artavasdes, to be king of Media Atropatene, creating a semi-authonomous "vassal state" of Rome in what is now northwestern Iran. But soon, around 19 AD, The Parthians took again full control of Atropatene.
Although warfare between the Romans and the Parthians/Sassanids lasted for seven centuries, the frontier remained largely stable in those centuries. A game of tug of war ensued: towns, fortifications, and provinces were continuously sacked, captured, destroyed, and traded. The line of stalemate shifted in the 2nd century AD with Trajan: it had run along the northern Euphrates until then. The new line ran northeast, across Mesopotamia to the northern Tigris. There were also several substantial shifts further north, in Armenia and the Caucasus.
A new series of wars began in the 2nd century AD, during which the Romans consistently held the upper hand over Parthia. In 113 AD the Roman Emperor Trajan decided that the moment was ripe to resolve the "eastern question" once and for all time by the decisive defeat of Parthia and the annexation of Armenia: his conquests mark a deliberate change of the Roman Policy towards Parthia, and a shift of emphasis in the "grand strategy" of the empire.
Finally, one characteristic of the Roman presence in Persia is that Roman emperors dreamed of conquering all Persia from Trajan to Galerius, while Parthian/Sassanian kings never tried to conquer Rome, Italy or southeastern Europe according to historian Theodor Mommsen.
Roman Parthia
Romans were able to conquer the westernmost part of Parthia under emperor Trajan.
Indeed, in 113 AD Trajan invaded Parthia, marching first on Armenia. In 114 AD Trajan annexed Armenia to the Roman empire, after defeating and killing Parthamasiris, relative of Osroes I of Parthia. Then he turned south into Parthia itself, taking the cities of Babylon, Seleucia and finally the capital of Ctesiphon in 116 AD.
He deposed Osroes I and put his own puppet ruler Parthamaspates on the throne. In Mesopotamia Osroes' brother Mithridates IV and his son Sanatruces II took the diadem and fought against the Romans, but Trajan marched southward to the Persian Gulf, defeated them, and declared Mesopotamia a new province of the empire. In the process, he also captured the great city of Susa in Khuzestan.
Those months of 116 and 117 saw western Parthia as a client state of the Romans. Trajan originally planned to annex Parthia as part of the Roman Empire, but ultimately decided instead to place Parthamaspates on his father's throne as a Roman client, doing so in 116.
Trajan created even the province of Assyria, whose eastern border were never well defined by historians: it is possible that included northwestern Elam, a semi-autonomous Persian province (now Īlām Province of Iran) that welcomed his conquest of Mesopotamia.
Following the death of Trajan and Roman withdrawal from the area, Osroes easily defeated Parthamaspates and reclaimed the Persian throne. Hadrian acknowledged this fait accompli, recognized Osroes, Parthamaspates king of Osroene, and returned Osroes' daughter who had been taken prisoner by Trajan (as a warranty to control his client state).
