On such flat terrain, the Legion proved to have no viable tactics against the highly mobile Parthian horsemen, and the slow and vulnerable Roman formations were surrounded, exhausted by constant attacks, and eventually crushed.

Crassus, a member of the First Triumvirate and the wealthiest man in Rome, had been enticed by the prospect of military glory and riches and decided to invade Parthia without the official consent of the Senate.

[citation needed] The war against Parthia resulted from political arrangements intended to be mutually beneficial for Marcus Licinius Crassus, Pompeius Magnus, and Julius Caesar, the so-called First Triumvirate.

In March and April 56 BC, meetings were held at Ravenna and Luca, in Caesar's province of Cisalpine Gaul, to reaffirm the weakening alliance formed four years earlier.

It was agreed that the Triumvirate would marshal its supporters and resources to secure legislation for prolonging Caesar's Gallic command and to influence the upcoming elections for 55 BC, with the objective of a second joint consulship for Crassus and Pompey.

[3] The Triumvirate aimed to expand their faction's power by traditional means: military commands, placing political allies in office and advancing legislation to promote their interests.

Legislation passed by the tribune Trebonius (the Lex Trebonia) granted extended proconsulships of five years, matching that of Caesar in Gaul, to the two outgoing consuls.

[4] Meanwhile, in Parthia, a war of succession had broken out in 57 BC after King Phraates III had been killed by his sons Orodes II and Mithridates IV, who then began fighting each other over the throne.

In the first stage, Orodes emerged victorious and appointed his brother as king of Media (his de facto governor) as a compromise.

[6] Gabinius sought to interfere in the succession dispute on behalf of Mithridates so that Rome could make him its puppet king and seize control of Parthia in the process.

Orodes, now unopposed in his own realm, marched north to invade Rome's ally Armenia, where King Artavasdes II soon defected to the Parthian side.

The Roman legions had easily crushed the numerically superior armies of other eastern powers such as Pontus and Armenia, and Crassus expected Parthia to be an easy target.

[11] Cicero, however, suggested an additional factor: the ambitions of Marcus's son, the talented Publius Crassus, who had commanded successful campaigns in Gaul under Caesar.

[13] The tribune Ateius Capito put up strenuous opposition and infamously conducted a public ritual of execration as Crassus prepared to depart.

[15] Publius Crassus joined him in Syria during the winter of 54–53 BC and brought with him the thousand Celtic cavalry troopers from Gaul who remained loyal to their young leader until their death.

In response, the Parthian king, Orodes II, divided his army and took most of the soldiers, mainly foot archers with a small amount of cavalry, to punish the Armenians himself.

Crassus' commanding general, Cassius, recommended that the army be deployed in the traditional Roman fashion, with infantry forming the centre and cavalry on the wings.

However, Plutarch wrote in his accounts that the Romans were met with a shower of arrows that passed through every kind of cover, hard and soft alike.

The Parthian cataphracts exploited that weakness and repeatedly charged the Roman line, which caused panic and inflicted heavy casualties.

[27] When the Romans tried to loosen their formation to repel the cataphracts, the latter rapidly retreated, and the horse archers resumed shooting at the legionaries, who were now more exposed.

Crassus, deeply shaken by his son's death, ordered a retreat to the nearby town of Carrhae and left behind 4000 wounded, who were killed by the Parthians the next morning.

Surena proposed a truce to allow the Roman army to return to Syria safely in exchange for Rome giving up all territory east of the Euphrates.

The 10,000 Roman prisoners of war appear to have been deported to Alexandria Margiana (Merv) near the Parthian Empire's northeastern border in 53 BC, where they reportedly married local people.

The prisoners, Dubs proposed, were resettled by the Parthians on their eastern border and may have fought as mercenaries at the Battle of Zhizhi between the Chinese and the Xiongnu in 36 BC.

To date, no artifacts that might confirm a Roman presence, such as coins or weaponry, have been discovered in Zhelaizhai, and Dubs' theories have not been accepted by the vast majority of historians.

[43] Alfred Duggan used the possible fate of the Roman prisoners as the kernel of his novel Winter Quarters, which suggested that they were employed as frontier guards on the eastern border of the Parthian Empire.

[44] The capture of the golden aquilae (legionary battle standards) by the Parthians was considered a grave moral defeat and evil omen for the Romans.

Gaius Stern has claimed that the death nearly cut the links the First Triumvirate enjoyed with the blue-blooded aristocracy, leaving the entire state vulnerable to the friction that eventually turned into civil war.

[47] Thus, an immediate effect of the battle may have been the elimination of certain private checks and balances (such as Crassus's relationship to Metellus Pius Scipio) that had kept a lid on political tensions.