During the 1st century AD, a force of the Parthian Empire destroys a vexillation of a Roman auxiliary cohort sent to construct a fort on the banks of the Euphrates in the Kingdom of Palmyra. The garrison is slaughtered. Meanwhile, with tensions rising between Rome and Parthia, the Legio X Fretensis, Legio III Gallica and the Legio VI Ferrata are drilling for war in Syria. Prefect Macro and Centurion Cato are drilling the Second Illyrian, an auxiliary cohort, attached to the Legio X Fretensis for the looming war.
Cato and Macro were sent to Syria by Narcissus to gather proof that the governor of Syria, Longinus was planning to use the Syrian legions to usurp the Emperor Claudius. During their time in Antioch, Crisups, a Roman legionary, murders an auxiliary leading to Crispus' execution, much to the chagrin of the legionaries. A Parthian convoy arrives, delivering the head of Centurion Castor, the soldier who commanded the Euphrates fort, and warns of Parthian intervention, should Rome continue to be seen to be annexing Palmyra. Shortly thereafter, a Roman soldier arrives at the behest of Lucius Sempronius, a Roman ambassador to Palmyra, informing Longinus that Palmyra has descended into civil war.
Artaxes, the son of Palmyran king Vabathus, has raised an army and laid siege to the Palmyran loyalists in the Royal Citadel. Fearing the Parthians will arrive before the Romans can, Longinus sends the Second Illyrian and a cohort of the Legio X Fretensis to reinforce the loyalists. Along the way, the Roman force is aided by Prince Balthus, who covets the Palmyran throne, despite not being Vabathus' first born. The Romans and Balthus' men fight their way through to the city and manage to reinforce the loyalist troops, mainly composed of Greek mercenaries. Following a banquet to celebrate the successful defence of a rebel assault, Amethus, one of Vabathus' sons is found murdered, with Balthus being the prime suspect. Meanwhile, Cato meets Sempronius' daughter, Julia, and the two fall in love after an uneasy start.
After a rebel bombardment, the loyalist food stores are all but destroyed. Cato attempts to bluff Artaxes into standing down, however before Artaxes can respond, Longinus arrives with two legions and several auxiliary units. Longinus privately reveals to Cato and Macro that they were never meant to reach Palmyra, and were meant to die in the desert, removing Narcissus' spies that had frustrated his plans. Against the advice of Cato, Longinus leads the legions into the desert, determined to destroy Artaxes and his Parthian allies. During a night attack, Longinus panics, orders a retreat and leaves the army at the mercy of the Parthian horsemen. On the suggestions of Cato, the army manages to trap the Parthians, destroying their army. Balthus orders his brother, Artaxes, killed, leaving him the sole heir to Vabathus' throne.
Back in Palmyra, it is revealed Balthus had ordered his slave, Carpex, to murder Amethus. Balthus is arrested to be put to death. With no heir, Sempronius reveals that the empire will annex Palmyra and absorb it into the province of Syria. Macro and Cato are released from Narcissus' employment, ending their posting in the East. Sempronius later gives Cato his consent to marry his daughter, Julia.

Question: What forces defeat the Roman forces building a fort on the riverbank of the Euphrates?
Answer: Parthian forces