Roman victory at Carrhae

The Battle of Carrhae was an important battle between the Parthian Empire and the Roman Republic: it was fought near the town of Carrhae, in 53 BC. The Roman general Marcus Licinius Crassus decisively defeated a Parthian army led by the Parthian Spahbod ("general") Surena. It was the first of the battles between the Roman and Parthian empires.

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. Rejecting an offer from the Armenian King Artavasdes II to allow Crassus to invade Parthia via Armenia, Crassus marched his army directly through the deserts of Mesopotamia. His army clashed with Surena's force near Carrhae, a small town in modern-day Turkey.

Political background in Rome
The war in Parthia resulted from political arrangements intended to be mutually beneficial for 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 their 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 Pompeius. The leaders of the triumvirate aimed to expand their faction's power through traditional means: military commands, placing political allies in office, and advancing legislation to promote their interests. Pressure in various forms was brought to bear on the elections: money, influence through patronage and friendship, and the force of a thousand troopers brought from Gaul by Crassus's son Publius. The faction secured the consulship and most, though not all, of the other offices sought. 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. The Spanish provinces would go to Pompeius; Crassus arranged to have Syria, with the transparent intention of going to war with Parthia. Marcus Licinius CrassusThe notoriously wealthy Marcus Crassus was around sixty and hearing-impaired when he embarked on the Parthian invasion. Some Romans objected to the war against Parthia. Cicero called it a war nulla causa (“with no justification”), on the grounds that Parthia had a treaty with Rome. The tribune Ateius Capito put up strenuous opposition, and infamously conducted a public ritual of execration as Crassus prepared to depart.

Crassus left Rome on November 14, 55 BC. Publius Crassus joined him in Syria during the winter of 54–53 BC, bringing with him the thousand Celtic cavalry troopers from Gaul.

Build up to war
Crassus arrived in Syria in late 55 BC and immediately set about using his immense wealth to raise an army. He assembled a force of seven legions (about 35,000 heavy infantry). In addition he had about 4,000 light infantry, and 4,000 cavalry, including the 1,000 Gallic cavalry Publius had brought with him. With the aid of Hellenic settlements in Syria and support of about 6,000 cavalry from Artavasdes, the Armenian king, Crassus marched on Parthia. Artavasdes advised him to take a route through Armenia to avoid the desert and offered him reinforcements of 16,000 cavalry and 30,000 infantry. Crassus refused the offer and decided to take the direct route through Mesopotamia, and capture the great cities in the region. In response, the Parthian king Orodes II divided his army and he took most of the soldiers, mainly foot archers with a small amount of cavalry, to punish the Armenians and sent the rest of his forces, 9,000 horse archers and 1,000 cataphracts under the command of the general Surena, to scout out and harass Crassus' army.

Crassus received directions from the Arab chieftain Ariamnes, who had previously assisted Pompey in his eastern campaigns. Crassus trusted Ariamnes, but Ariamnes was in the pay of the Parthians. He urged Crassus to attack at once, falsely stating that the Parthians were weak and disorganized. He then led Crassus's army into the most desolate part of the desert, far from any water. Crassus then received a message from Artavasdes, claiming that the main Parthian army was in Armenia and begging him for help. Crassus ignored the message and continued his advance into Mesopotamia. He encountered Surena's army near the town of Carrhae.

The battle
After being informed of the presence of the Parthian army, Crassus deployed his army in the traditional Roman fashion, with infantry forming the center and cavalry on the wings. This formation would allow his forces to maneuver, but at the risk of being outflanked. The Roman forces advanced and came to a stream. Crassus' generals advised him to make camp, and attack the next morning in order to give his men a chance to rest. Publius, however, was eager to fight and managed to convince Crassus to confront the Parthians immediately.

The Parthians went to great lengths to intimidate the Romans. First they beat a great number of hollow drums and the Roman troops were unsettled by the loud and cacophonous noise. Surena then ordered his cataphracts to cover their armor in cloths and advance. When they were within sight of the Romans, they simultaneously dropped the cloths, revealing their shining armor. The sight was designed to intimidate the Romans, but Surena was impressed by the lack of effect it had. Though he had originally planned to shatter the Roman lines with a charge by his cataphracts, he judged that this would not be enough to break them at this point. Thus, he sent his horse archers to surround the Roman square. Crassus sent his skirmishers to drive the horse archers off, but they were driven back by the latter's arrows. The horse archers then engaged the legionaries. The legionaries were protected by their large shields and armor, but these could not cover the entire body. Therefore the majority of wounds inflicted were nonfatal hits to exposed limbs. The Romans repeatedly advanced towards the Parthians to attempt to engage in close-quarters fighting, but the horse archers were always able to retreat safely, loosing Parthian shots as they withdrew. The legionaries then formed the testudo formation, in which they locked their shields together to present a nearly impenetrable front to missiles. However, this formation severely restricted their ability to fight in melee combat. The Parthian cataphracts exploited this weakness and repeatedly charged the Roman line, causing panic and inflicting heavy casualties. When the Romans abandoned the formation, the cataphracts withdrew and the horse archers resumed shooting.

Crassus now hoped that his legionaries could hold out until the Parthians ran out of arrows. However, Surena used thousands of camels to resupply his horse archers. Upon realizing this, Crassus dispatched his son Publius with 1,300 Gallic cavalry to advance and provoke an attack from the horse archers. The Gauls were immediately surrounded and attacked by the mounted archers. Once they were engaged, their position was fixed and they were vulnerable to an approach by the Roman legionaries. Crassus ordered his troops to advance and envelope the horse-archers. The Parthians were now forced into close-quartered combat against the legionaries and suffered heavily for it, for they were unsuited for such combat. The Parthian cavalry's will eventually broke and panic spread, many of the horse archers being killed as they tried to escape. Surena was eventually slain along with his bodyguards, and the remaining Parthians broke and fled. Overall the Roman army had achieved a complete victory.

Roman casualties amounted to about 200 killed and 1000 wounded, while the Parthians suffered over 9,400 killed.

Aftermath
The Battle of Carrhae was a decisive Roman victory and marked the beginning of the end of the Parthian empire. Crassus advanced further into Mesopotamia, defeating Orodes II, with the rest of the Parthian Army, and sacking Ctesiphon later that year. Crassus would however never return to Rome as he contrated an illness and died before his goverorship of Syria came to an end. His death permanently unraveled the alliance between Caesar and Pompey. Within two years of Crassus' death, Caesar would cross the Rubicon and begin a civil war against Pompey and the legitimate government of the Republic.