Pompeian–Parthian invasion of 40 BC

After the defeat of the Parthian-backed Pompeians in the Liberators' civil war by Mark Antony and Octavian, Orodes II sent a Parthian force under Prince Pacorus I and the Pompeian general Quintus Labienus in 40 BC to invade the eastern Roman territories while Antony was in Egypt. Roman soldiers in Syria, many of whom were former Republicans fighting in the last civil war, joined the force, and the Levant and much of Asia Minor were swiftly overrun by Pacorus I and Labienus, respectively. In 39 BC, Antony sent Ventidius, who defeated and executed Labienus in a counter-attack, and then drove Pacorus I out of the Levant. A second Parthian invasion of Syria by Pacorus I resulted in the latter's death and a decisive Parthian defeat. Antony later began a campaign with a massive force against Parthia, but he was defeated.

Background
The Parthians had defeated and killed Marcus Licinius Crassus (a member of the First Triumvirate along with Julius Caesar and Pompey), at the Battle of Carrhae. They also had relations with Pompey, though never supported him militarily in the latter's conflict with Caesar. After Pompey's death, Caesar planned an invasion of Parthia, which was never implemented due to his assassination.

In 42 BC, Parthian forces supported the "Liberators" (the leaders of Caesar's assassination) against the Caesarians under Mark Antony and Octavian in the Battle of Philippi in the Liberators' civil war. After the Liberators' defeat, in an attempt to resurrect the Pompeian cause, general Quintus Labienus, who had been sent to Parthia to ask for assistance in the last civil war, joined the Parthians, and the Parthian king Orodes II sent him together with his son Pacorus I to invade eastern Roman territories while Mark Antony was in Egypt with Cleopatra.

Invasion
Orodes II sent his son Pacorus I and Labienus as the commanders of a large Parthian army to invade Roman territory in early 40 BC (or late 41 BC, according to some scholars). According to Vagi, the invasion required some 20,000 horsemen. Many Roman forces in Syria defected to Labienus. Antony's commander in Syria, Lucius Decidius Saxa, fled, first to Antioch, and then to Cilicia, where he was captured by Labienus and executed. Several Roman aquilae were then captured by the Parthians. (These aquilae, together with ones captured after the Battle of Carrhae, were later returned after Augustus' negotiations with the Parthians.) Apamea and Antioch then surrendered. After that, the two commanders split: Pacorus invaded Palestine and Phoenicia, while Labienus launched a "blitzkrieg" in Asia Minor, capturing much of the region and being hailed as Imperator. Pacorus, who had gained a reputation for military talent and moderation, swiftly took all the cities along the Levantine coast with the exception of Tyre (which was notoriously difficult to capture), reaching Gaza by May 40 BC, and receiving homage from the Nabataeans. At the same time, his general Barzapharnes led a force inland. The pro-Parthian Jewish leader Antigonus II Mattathias sent a large subsidy to Pacorus I, and the latter supported him in the fight against the pro-Roman Jewish leaders Hyrcanus II and Phasael, successfully installing him as the new king of Judea. Hyrcanus II and Phasael were captured while trying to negotiate with the Parthians, and were deported to Parthia, while Herod, another leader, fled.

Mark Antony left Egypt for Greece and sent Publius Ventidius Bassus to Asia Minor. He scored two victories with minimal forces north of the Taurus Mountains in 39 BC (Battle of the Cilician Gates, Battle of Amanus Pass), capturing and executing Labienus. He then drove the Parthians out of Syria. Another Parthian invasion of Syria in 38 BC under Pacorus I resulted in a decisive defeat at the Battle of Mount Gindarus in Cyrrhestica, with Pacorus I being killed and the Parthian presence in Syria being brought to an end.

Coins minted by Labienus survive from this period, and were probably minted in Antioch.

In 38 BC, Mark Antony finally began the campaign against Parthia with a large force, but it resulted in a defeat with heavy Roman losses.