After receiving word from his scouts about the presence of a Parthian army near Carrhae in 53 BC, Crassus seems to have panicked. His troops were exhausted and poorly prepared for battle after a long and fast march through the Mesopotamian desert. He didn't allow his men to rest or make camp, but instead began to form lines for battle.
Advised by his officer staff to stretch out in classic formation with the infantry flanked by cavalry, Crassus instead ordered hollow square formations to allow defense against flanking. He commanded the middle while his son Publius and another officer, Cassius, commanded the wings. They advanced toward the smaller and less impressive Parthian force far too confident.
As they approached with 35,000 men, the Parthian force seemed to be only about 10,000 men, mainly light horse archers. However, as they approached, the Parthian commander Surena ordered cavalry positioned at the rear uncover their concealed armor. The vaunted Parthian cataphracts were small in number, but their heavy armor was definitely an impressive and intimidating sight. As the battle opened a hail of Parthian arrows pinned down the Romans. Crassus ordered his son Publius to attack the archers with his Gallic cavalry and a force of infantry. Initially, Publius drove back the horse archers, but found himself far away from the main Roman body. The Parthians cut off his force, surrounding it with horse archers and the cataphracts. Though the Gauls fought bravely and ferociously, Publius was overwhelmed, and the cataphracts seemed invincible. Trapped away from his father and the army, Publius ordered his own death at the hand of one of his men, and the Roman force was butchered.
Crassus meanwhile got word that his son was in trouble, just as pressure was diverted from his own lines to that of Publius' force. Crassus reformed his lines in the traditional Roman style and ordered a general advance. As this was getting under way however, the Parthians who had defeated the Gallic cavalry rode in front of the Romans with the head of Publius on a spear. The Roman advance was stopped fast by the Parthians, and the already rattled Crassus, seems to have lost the will to fight. His legates, Cassius and Octavius ordered a retreat intended to save the army during the night, desperately leaving the wounded on the field. Remaining cavalry fled the battle immediately, leaving Crassus without scouts. They rode first to Carrhae to inform the garrison of the battle and then hurried on to Zeugma to avoid the disaster that was sure to come.
In the confusion and desperation of the Roman retreat, as many as 4,000 wounded legionaries were put to the sword as the Parthians came in pursuit the following morning. Another 4 cohorts had been separated from the main body and were surrounded and killed, save for 20 men who were allowed to flee for displaying exceptional bravery. Crassus and the remaining Roman army reached the relative safety of Carrhae and probably prepared for a siege.
Crassus however, was still obviously unsettled. Once again a Parthian spy duped him, this time into fleeing the safety of the town. The spy led the Romans to inescapable terrain and the Parthian main force approached. They offered a parlay, including an offer of peace if Crassus himself joined the negotiation. At first he refused, but the legionaries, afraid and exhausted, threatened his life if he didn't accept the offer. At the meeting, the Parthians seized and executed Crassus and the Roman party, sending the Romans into further disarray.
In the end, the great bulk of the Roman army was hunted down and killed or captured. Nearly 20,000 were killed and another 10,000 captured. Of the original force, only about 5,000 men under Cassius, and the cavalry that departed early, managed to escape. The Parthians meanwhile, settled the Roman prisoners in an eastern territory called Sogdia. Interestingly, the Han Chinese later captured this area and the Roman transplants were likely among the first westerners to meet the Chinese directly.
The death of Crassus helped signal the end of the triumvirate between he, Caesar and Pompey, but even if he had lived its doubtful that civil war wouldn't have erupted eventually anyway. As the Romans were too pre-occupied with western concerns and the political turmoil that was about to erupt, the situation with Parthia was largely ignored for nearly another 30 years. Parthian king Orodes II ordered the death of Surena shortly thereafter, and the Parthians did little to press their advantage in eastern Roman territories. The lost standards of Crassus' lost legions remained in a Parthian temple Rome's first emperor, Augustus, negotiated their return in 20 BC.