The Parthian empire had once been an ally of Rome. Parthians and Romans had fought together to finish off the Seleucid Empire in 63 BC, and enjoyed peaceful relations after. This all changed in 54 BC, when the ambitious Roman Marcus Licinius Crassus launched an unprovoked invasion of Parthian Syria with the intent to march on Seleucia and conquer the Parthian empire. Instead, his army was annihilated in the Syrian desert at the Battle of Carrhae. Of Crassus’ 38,000 men, only 8,000 or so made it back to Roman territory. 20,000 Romans were killed, and 10,000 were prisoners in Parthia.
The immediate result of the campaign was a Parthian invasion of Roman Syria under the command of a general named Osaces and Pacorus, the son of the Parthian ruler Shah Orodes II. The death of Crassus and many of his officers left Gaius Cassius Longinus as the ranking Roman commander in Syria. While the Parthians besieged Antioch, Pacorus was recalled to Parthia by his father. Cassius rallied the remaining Roman troops in the area and broke the siege, then defeated the Parthians again at Antigonea. In this battle, Osaces was killed and his troops dispersed.
The first round of Roman-Parthian conflict thus ended in a status quo ante bellum. As a result, like the United States and USSR 2000 years later, the two superpowers of the ancient Near East in the 1st century BC saw continued direct war as too risky and destructive when compared to its potential benefits. Therefore, the struggle between them for regional supremacy turned from confrontation to war by proxy. Struggle between armies was replaced by each side meddling in each other’s internal struggles, supporting rebel factions and fighting proxy battles with client states.
The first shot of the proxy war came from Cassius’ replacement as governor of Syria. Marcus Calpurnius Bibulus arrived in 51 BC to take control of the province from Cassius. He sought to divide the Parthians against each other so as to preclude further invasions of Roman territory. Bibulus befriended a Parthian satrap named Ornodapates, who carried an old grudge against Orodes. Using Ornodapates as a go-between, Bibulus constructed a plot to stage a coup d’etat, overthrow Orodes and install his son Pacorus on the throne in his stead. The plot failed, but the resulting strife temporarily distracted Parthia from any westward expansion.
While the Parthians were otherwise preoccupied, the political situation in Rome was spiraling out of control. Once allies, Julius Caesar and Ganeus Pompey were now enemies. In 49 BC, their rivalry and refusal to disband their armies spilled over into open civil war. Julius Caesar rapidly marched on Rome, forcing Pompey to withdraw to Greece without a fight. Pompey spent the winter of 49-48 BC regrouping in Greece and preparing for a decisive showdown against Caesar.