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, the 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.