While the infantry centres of both armies collided, the battle was decided by the actions of the cavalry of each side. First off, the cataphracts and the right-wing cavalry led by Antiochus charged into the Roman legionaries stationed on the left with such force that they not only broke through the frontlines, but also then completely routed the Roman infantry contingent. Most likely due to his Roman pride, Livy downplayed the collapse of the Roman legion in question, yet still made it known that Antiochus and the cataphracts were very successful in the attack as he recounted the significant event:
For Antiochus, who commanded the right wing, having observed that the enemy, through confidence in the river, had placed no reserve there, except four troops of horse, and that these, keeping close to the infantry, left an open space on the bank of the river, made a charge on them, with a body of auxiliaries and cataphracts. He not only attacked them in front, but having surrounded the wing in the direction of the river, pressed them in flank also; until the routed cavalry first, and then the infantry that were next to them, fled with precipitation to the camp.
Although Livy mentions cavalry, it is important to note that the vast majority of the Roman left wing that the cataphracts overcame was comprised of legionaries. In The Syrian Wars, Appian reinforced Livy’s account, for he also stated that `Antiochus, on the right, broke through the Roman line of battle, dismembered it, and pursued a long distance’. This feat alone was particularly impressive for at this point in history, Roman legionaries were definitely considered some of the best, if not the best, infantry troops in the world; a legion defeated by a frontal assault in that manner was a rarity for the time. However, Antiochus followed up that brilliant action by attacking the Roman camp, which was guarded by Thracian and Macedonian allies of Rome, instead of striking the legionaries of the centre in the rear or the flanks. If he had done that, victory could have easily belonged to the Seleucids at that moment, but instead Antiochus and his cavalry were tied up in their combat with the camp garrison and the remainder of the routed legionaries who managed to rally and come to the aid of their allies at the fortified camp.
At the same time that Antiochus led his charge on the right wing, the chariots he had placed in front of his left wing simultaneously advanced towards the Roman right wing, predominately composed of cavalry forces. Yet the light infantry stationed among the horsemen of the Roman army launched such a devastating barrage of javelins and arrows that the Seleucid charioteers were unable to withstand it, and thus they were forced to retreat with their vehicles back into their own lines. Chaos ensued, for the cataphracts were preparing to make their assault right as the chariots came rushing back towards them, which caused the armoured horsemen to completely lose their momentum. This allowed the cavalry on the Roman right flank to fully exploit the confusion on the Seleucid side and quickly overcome the lumbering heavy cavalrymen stuck in their tracks. Livy recounted the disastrous sequence of events for the Seleucids, emphasizing the amount of armour worn by the cataphracts as a major contributing factor to the defeat of the Seleucid left wing:
But that futile affair was soon the cause of real loss. For the auxiliaries in reserve, which were posted next, being terrified at the turn and disorder of the chariots, betook themselves to flight, leaving all exposed as far as the post of the cataphracts; to whom when the Roman cavalry, after dispersing the reserves, approached, they did not sustain their first onset. Some fled, and others, being delayed by the weight of their coverings and armour, were put to the sword. The whole left wing then gave way, and the auxiliaries, posted between the cavalry and the phalanx, being thrown into confusion, the terror spread even to the centre.
In his account of the battle, Appian also stated that the heavy weight of the cataphract armour prevented them from successfully reacting to the unexpected retreat of the Seleucid chariots:
The horses being wounded in great numbers charged with their chariots upon their own ranks. The dromedaries were thrown into disorder first, as they were next in line to the chariots, and after them the cataphracts who could not easily dodge the scythes on account of the weight of their armour.
Appian continued to stress how much their extensive armour negatively affected the cataphracts:
[The Roman cavalry] made so heavy a charge that they put to flight not only those, but the adjoining squadrons and the cataphracts, who were already thrown into disorder by the chariots. The greater part of these, unable to turn and fly quickly, on account of the weight of their armour, were captured or killed.
After crushing the cataphracts, and the rest of the Seleucid forces of the left wing, the horsemen of the Roman army did what Antiochus failed to do to achieve victory by slamming into the flank of the Seleucid phalanx. Yet, miraculously, the Seleucid pikemen held firm and withstood the onslaught from the legionaries in the front and the Roman cavalry attacking the flank. However, Antiochus had placed a small reserve of elephants to protect the rear of the army, which was then the target of severe missile fire from the archers and javelineers within the Roman ranks. It did not take long for the barrage to cause the huge beasts to panic and then rampage into their own infantry troops in front of them, causing the Seleucid phalanx to collapse. The routed Seleucid infantrymen fled to their camp, which was stoutly defended first by the reserves and then by the flood of retreating men who poured into the encampment, allowing many of the pikemen to escape the slaughter; although thousands of the Seleucid troops were slain on the field. The Romans had won the battle.
Magnesia was only the second major battle that the new Seleucid cataphracts had participated in and for the first time their major weakness was fully exposed. Although nearly invulnerable due to their extensive armour, those same defences could also be their Achilles’ heel, for it drastically limited their mobility, normally one of the greatest assets of cavalry troops. From battles such as Magnesia, and in future conflicts, the later kingdoms and empires that utilized cataphracts had to learn that for the elite heavy cavalrymen to be the most effective, they must be supported by light infantry and/or cavalry in order to protect their flanks, or even more importantly, to drive off attacks whenever the armoured horsemen were immobile. On the other hand, the cataphracts’ spectacular charge that caused the rare collapse of a Roman legion also reinforced how deadly the new heavy cavalry troops could be when used properly on the battlefield with shock tactics.
As the first cataphracts ever created, the least is known about the Seleucid version of the heavy cavalry warriors. The most complete depiction of the arms and armour of the Seleucid cataphract is located on the Balustrade Reliefs of the temple of Athena Polias Nikephoros at Pergamum. One of the most impressive features shown among the armament is a metal mask with an incredibly detailed face, including a sculpted beard that was attached to the helmet of the rider. The rest of the cataphract armour included a Hellenistic-style cuirass with traditional pinions and pteruges (strips hanging from the shoulders and lower edge respectively) attached, as well as laminated armour that covered the entire arm (manica) that was made of articulated metal or rawhide hoops that overlapped down the limb and were often riveted to inner straps. An almost complete set of cataphract armour featuring physical remains of iron manica, like those depicted on the Pergamum frieze, have been found during excavations carried out by the French at the site of Ai Khanoum, an ancient Hellenistic city located in modern day Afghanistan. Similar laminated armour that covered the arms probably also protected the legs as well, along with Hellenistic–style greaves. A figurine of a warrior from Syria, now located in the Louvre, has also been discovered and is depicted wearing similar armour to the images on the reliefs at Pergamum. The figure is shown wearing a Hellenistic-style cuirass as well, most likely made of leather, with pteruges, and the arms of the heavily armoured soldier were covered with manica. Last but not least, the helmet of the Syrian figurine has the same metal face mask with the intricate sculpted beard. However, the armour of the cataphract slightly changed in the later years of the Seleucid Empire by the reign of Antiochus IV Epiphanes (r. 175-164 BC), in that the type of cuirass had been exchanged to mainly become a Roman type of mail coat instead of the previous Hellenistic corselet.
The primary weapon wielded by the mounted warrior may have first been the lighter xyston-style lance but later it predominately became the kontos (heavy lance) that was most likely similar in length, at around 3.6 m (12 ft.), but may have been thicker and weighed more. The horse of the Seleucid cataphract also wore a metal facemask, but for the steed it was most likely constructed of bronze and ornamented with a feather crest. It is also clear that the heavy warhorse was provided with lamellar armour to protect the chest; however, it is likely that the armour was even more extensive for the cataphract than what is depicted on the reliefs at Pergamum. The further protection was probably either a half–trapper or even a full body trapper worn by the horse, made of metal scales that overlapped upon a fabric backing, which fastened over the chest of the beast. The half–trapper only covered the shoulders and chest of the mount, while the full trapper provided armour for nearly the entire body; remains of horse trapper armour of the fuller type has been found at sites such as Dura Europos. Another possible addition to the cataphract equipment was an armoured saddle that provided a defence for the thighs of the riders, influenced by earlier Achaemenid models. A fragment dated to either the 4th or 3rd century BC from a terracotta relief flask found at the site of Khumbuz–Tepe, located in southern Khwarezm, contains a depiction of this unique heavy cavalry equipment.
It is probable that the Seleucid cataphracts were predominately recruited from among the Iranian population of the empire, not the ruling Macedonian/Greek people. Not only did the Iranian peoples have a much stronger background in cavalry warfare than the Hellenistic westerners, but they had also faced cataphract–type soldiers in combat on many more occasions, and their ancestors may have even fought as the elite heavy cavalry troops before the conquest of Alexander the Great that resulted in the creation of the Seleucid Empire. On the other hand, it is curious to note that when Livy described the cavalry units at the Battle of Magnesia in 190 BC, he clearly identifies the origins of many of them, but does not do so in his description of the cataphracts. Similarly, when the cataphracts are mentioned in the accounts of Polybius, he also does not use an ethnic term to describe them, which he often does for many other contingents of warriors. Therefore, it may also be possible that the cataphract units were comprised of regulars, primarily of Macedonian origin, that were recruited from the military colonies, not foreign allies or mercenaries. Yet regardless of their origin, the cataphracts were most likely provided with armour in the style of the region the soldier fought in; in the east, the armour would have been predominately lamellar and scale armour types, while plate armour cuirasses were more common in the west.
After the Battle of Magnesia, the Seleucids continued to utilize cataphract troops, yet due to their expensive cost to maintain in the field, there numbers were reduced. As a result of their defeat to the Romans, the empire lost important territory, specifically Asia Minor, as well as a significant portion of its army, and was, therefore, greatly weakened. With fewer available men and decreased resources, the amount of cataphracts in the Seleucid army may have been reduced by thousands of soldiers. However, the empire was still powerful and the new emperor, Antiochos IV, attempted to prove that with a grand display of his military might. In 166 BC, the emperor held a festival at the city of Daphne in Syria with games and a majestic parade of all his best troops in their finest armour. In the account of Polybius, the cataphracts made an appearance in the procession:
Next came the cataphract cavalry, both men and horses acquiring that name from the nature of their panoply; they numbered 1,500. All the above men had purple surcoats, in many cases embroidered with gold and heraldic designs.
It is possible that the number of cataphracts was cut drastically from the 6,000 soldiers at Magnesia down to 1,500 men at Daphne, though there is also a chance that the heavy cavalrymen at the parade were only the base unit, while the overall numbers of the cataphracts were increased during times of war.
The cataphracts continued to fade within the Seleucid army as the empire declined throughout the later half of the second century and into the first century BC. After the Romans seized much of the western territories of the empire, the Parthians then conquered substantial Seleucid lands in the east. The empire fractured into several different kingdoms after the death of Antiochus IV in 164 BC, allowing the Iranian tribal kingdom to easily conquer each former Seleucid region individually. From 160 to 140 BC, the ruler of the Parthians, Mithridates I (r. 165-132 BC), managed to seize most of the eastern regions, including Media, Persis, Elymais, Characene, Assyria, Babylonia and Gedrosia, transforming his kingdom into a full-fledged empire. The Seleucids first attempted to retake their lost territories in a failed campaign led by King Demetrius II (r. 146-138 BC) in 138 BC, and then again in 129 BC by his brother, Antiochus VII Sidates (r. 138-129 BC). After some initial success, however, Antiochus VII ultimately failed to overcome the Parthians. By the first century BC, the once–formidable Seleucid empire was so weak that its land was reduced to the royal city of Antioch and part of Syria. In 96 BC, King Antiochus VIII Grypus (r. 125-96 BC) was murdered, which plunged the fragile kingdom into a state of anarchy that it would never recover from. Once the powerful king of Armenia, Tigranes the Great (r. 95-55 BC), was made aware of the chaotic situation in the Seleucid lands, he quickly exploited the situation and took control of the dying kingdom in 83 BC. Even though the Seleucids underwent a minor resurgence a little over a decade later and regained their autonomy, Roman general Pompey the Great annexed Syria in 63 BC, thus the Seleucid Empire finally ended. Yet even though the Hellenistic state had collapsed, the Seleucids had certainly made their mark on the militaries of the ancient world with the creation of the cataphracts. When Asclepiodotus wrote his military treatise, Tactica, in the first century BC, the ancient writer made sure to include the new heavy cavalrymen in his work:
Now the cavalry, which fights at close quarters, uses a very heavy equipment, fully protecting both horses and men with defensive armour, and employing, like the hoplites, long spears.