Warmatrix

The Battle of Carrhae in 53 BCE is recorded as one of the worst defeats of the Roman army. It is an example of how a pure cavalry force can beat an infantry army, if the conditions are right - in this case mostly because of grave tactical errors by the Roman general.
The Roman consul Marcus Licinius Crassus, envious of glory won by contemporaries like Pompey and Caesar, wanted a military triumph of his own. He mounted an invasion of Parthia. In Syria he mustered an army of seven legions: 33,000 soldiers and 4,000 non-combatants, plus 4,000 light infantry, 3,000 local cavalry and 1,000 Gallic mercenary cavalry. Declining an offer from help from the Armenian king Artavasdes, he marched straight into Mesopotamia, aiming for the big cities there. The Parthian king Orodes II took most of the forces available to him with him in a punitive expedition against Armenia and left 9,000 light and 1,000 heavy cavalry behind under the head of the Surena family, simply called 'the Surena', to deal with the Romans. It seems likely that the king intended only to delay the Romans, not defeat them.
Crassus left behind some troops as garrisons and seems to have invaded with some 30,000 legionaries, not 37,000. He listened to advice from a local chieftain called Ariamnes, who was, unknown to Crassus, in pay of the Parthians. Ariamnes urged the Romans to press on towards what seemed to be a small Parthian force and lured them deep into a desert. Then their guide disappeared. The Parthians suddenly showed up, having first killed almost all Roman scouts who rode ahead. Crassus, intimidated, formed his infantry into a hollow square. Shortly before noon he managed to reach the river Balissus, where some thirst could be quenched. Instead of halting, Crassus was swayed by his son Publius to move on. Then the Surena moved to attack. He sent in his horse archers to pepper the Romans with arrows. The Roman scuta protected them fairly well, but sometimes an arrow got through and the constant hail of missiles took a rising toll. The Parthians had a supply train carrying thousands of nasty barbed arrows, to prevent his troops running out of missiles. The Romans responded by making charges, but the Parthians nimbly wheeled back and retreated before they could come to blows. Next the Romans tried their testudo formation, in which a group of soldiers holds their shields to all sides. These were broken up by the Parthian cataphracts. Despite their success they refrained from engaging in hand-to-hand combat, as it proved that in that kind of fight they were no match for the Roman heavy infantry.
Publius tried to break out with 8 cohorts, 500 archers and 1,300 Gallic cavalry. The Parthians, by means of feint attacks and retreats, lured them away from the main army. The horses kicked up massive amounts of dust, so that the two Roman forces could not see each other. Publius' force was weakened by arrows and when sufficiently damaged, the Parthians surrounded them and cut them to pieces. Crassus, aware that his son was in big trouble, ordered a general advance, but was confronted with Publius' head stuck onto a spear. The Parthians kept on disrupting the Roman lines with arrow fire and cataphract charges. The Roman army was by now badly damaged, exhausted, demoralized and had nearly broken down. Only nightfall saved them. Crassus retreated to the town of Carrhae, reaching it only by midnight. He was forced to leave thousands of wounded men behind. Many others got lost in the dark and were captured by the Parthians in the morning.
The next day Crassus again trusted a local guide to lead him on a way out, but again it was a Parthian spy, who now led him into a maze-like swamp. At daylight, some other surviving Roman units linked up with him. The Parthians soon tracked the leaders down and the Surena offered a truce. Forced with a possible mutiny of his troops, Crassus went to meet him with a small delegation. During the meeting a fight erupted in which the Roman generals were killed. The remaining Romans tried to flee back westward. Most were killed or captured. Total Roman losses are estimated by the historian Plutarch at 50% dead and 25% surrendered, plus 7 legionary eagles lost. Only 25% of the army made it back to Syria, where they held off Parthian counterattacks by making good use of wooded terrain.
The Romans lost because they had an inexperienced army, did not scout properly and severely underestimated the effectiveness of the Parthians in the flat desert terrain. The advance beyond the river was a big error from Publius (and Crassus for agreeing with it) and his subsequent attempt at breakout was so foolish that it cost him his head.
Carrhae was one of the first clashes between Romans and Persians that would lead to a long string of wars between the two sides. The Parthians do not seem to have held a big grudge against the Romans. They employed many of the soldiers who surrendered on their eastern border, where they fought well against the Chinese, surrendered only after a good fight and were said to have subsequently become part of the Chinese army. The Surena was not rewarded by his king, but murdered, because Orodes was jealous of his achievements. On the other side of the Old World, in the Roman republic, the death of Crassus destroyed the first triumvirate and may have been a factor in disturbing the political balance and leading to the civil war that put Julius Caesar in power.