Roman–Parthian Wars
During the Roman Liberators' civil war of the 1st century BC, the Parthians actively supported Brutus and Cassius, invading Syria, and gaining territories in the Levant.
A Roman counter-attack under Statius Priscus defeated the Parthians in Armenia and installed a favored candidate on the Armenian throne, and an invasion of Mesopotamia culminated in the sack of Ctesiphon in 165.
Parthian enterprise in the West began in the time of Mithridates I; during his reign, the Arsacids succeeded in extending their rule into Armenia and Mesopotamia.
[6] 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.
The bulk of his force was either killed or captured; of 42,000 men, about half died, a quarter made it back to Syria, and the remainder became prisoners of war.
It is also mentioned by Plutarch that the Parthians found the Roman prisoner of war that resembled Crassus the most, dressed him as a woman and paraded him through Parthia for all to see.
The following year, the Parthians launched raids into Syria, and in 51 BC mounted a major invasion led by the crown prince Pacorus and the general Osaces; they besieged Cassius in Antioch, and caused considerable alarm in the Roman provinces in Asia.
During the ensuing Liberators' civil war, the Parthians actively supported Brutus and Cassius, sending a contingent which fought with them at the Battle of Philippi in 42 BC.
They swiftly overran Syria, and defeated Roman forces in the province; all the cities of the coast, with the exception of Tyre admitted the Parthians.
Pacorus then advanced into Hasmonean Judea, overthrowing the Roman client Hyrcanus II and installing his nephew Antigonus (40–37 BC) in his place.
Soon Labienius was driven back to Syria by Roman forces, and, though his Parthian allies came to his support, he was defeated, taken prisoner and then put to death.
[12] With Roman control of Syria and Judaea restored, Mark Antony led a huge army into Caucasian Albania (just east of Armenia), but his siege train and its escort were isolated and wiped out, while his Armenian allies deserted.
[14] The decision of the Parthian king Artabanus II to place his son, Arsaces, on the vacant Armenian throne nearly led to a war with Rome in 36 AD.
Artabanus III reached an understanding with the Roman general, Lucius Vitellius, renouncing Parthian claims to a sphere of influence in Armenia.
[15] A new crisis was triggered in 58, when the Romans invaded Armenia after the Parthian king Vologases I forcibly installed his brother Tiridates on the throne there.
[3] In 114, Trajan invaded Armenia, annexed it as a Roman province, and killed Parthamasiris who was placed on the Armenian throne by his relative, the king of Parthia, Osroes I.
[18] In 115, the Roman emperor overran northern Mesopotamia and annexed it to Rome as well; its conquest was deemed necessary, since otherwise the Armenian salient could be cut off by the Parthians from the south.
His grand scheme for Armenia and Mesopotamia were ultimately "cut short by circumstances created by an incorrect understanding of the strategic realities of eastern conquest and an underestimation of what insurgency can do.
He decided that it was in Rome's interest to re-establish the Euphrates as the limit of its direct control, and willingly returned to the status quo ante, surrendering the territories of Armenia, Mesopotamia, and Adiabene back to their previous rulers and client-kings.
