During the Syrian War, after being defeated in Greece, Hannibal's old ally Antiochus III retreated to Asia. To his surprise, in late 190 BC the Romans crossed over into Asia for the first time, where Antiochus was gathering his forces. The Roman army was under the nominal command of the inexperienced consul, Lucius Cornelius Scipio, but actually led by his brother Publius Scipio Africanus, the conquerer of Hannibal. At the request of Eumenes II, King of Pergamon, whose capital was under seige by the Seleucids, Scipio boldly advanced on Antiochus' position.
Pursued in mid-December by the Romans up the river Phrygius, the Seleucids established a camp near Magnesia in in Lydia, Asia Minor, where Antiochus had difficulty squeezing his forces into the restricted space available in the Hermus (Hermos) River valley. Hannibal was present but must have been under a cloud following his defeat at sea by Eudamus of Rhodes - and is not reported as commanding any detachments of the force.
The Roman army including Greek and Macedonian allies and numbered about 30,000 troops and 2,800 cavalry, including 800 from Pergamon, under king Eumenes II. Antiochus' army numbered some 70,000-80,000, of whom 12,000 were cavalry, some heavily armoured; the force included elephants and scythe-chariots. Despite his overwhelming advantage in numbers, Antiochus, knowing the power of the legions, was loath to fight, staying in a strongly fortified camp on Mount Sipylos near the town of Magnesia.
Scipio Africanus fell sick and returned to the sea coast. The army was now commanded by Gnaeus Domitius Ahenobarbus. Domitius repeatedly offered battle while Antiochus refused it. After a week, Antiochus, perhaps seeing that his men were becoming demoralised by his lack of action in the face of such a small opposition force, finally came out to fight.
In the first division Antiochus placed his light troops, bowmen, mounted archers, Arabs, and scythed-chariots. In the second were the heavy cavalry (cataphractae), the Gallic (Celtic) and Cappadocian infantry, and the 16,000 man phalanx, which was his elite unit. His 54 elephants were located in the spaces between the phalanx and cavalry.
Antiochus put his phalanx of 16,000 in the centre, 32 ranks deep, divided into 10 sections with 2 elephants in each gap. On the right, he put 1,500 Galatian infantry, 3,000 heavily armoured cavalry, and another 1,000 regular cavalry, followed by 16 elephants, the royal bodyguard, 1,200 mounted archers, 3,000 light infantry and 2,500 archers, and finally 4,000 slingers and archers. On the left, 3,500 allied infantry; 2,700 light-armed auxilliaries; 3,000 heavily armoured cavalry and 1,000 regular, and in front of that his scythe chariots and camel-mounted Arab archer-swordsmen; then a mass of light-armed troops and 16 elephants.
The Roman army formed with a small cavalry force on its left flank resting on a river. In the centre stood the Roman legions (c. 22,000). On the right were some 3,000 light-armed Achaean and Pergamene infantry, then most of the cavalry under the command of Eumenes of Pergamum and finally 500 more light-armed infantry. On the left they posted a small cavalry force and relied on the steep banks of the river for protection on that side. They kept their 16 African elephants behind the lines as a reserve, since they knew Antiochus had a far greater number of Indian elephants (54).
As the armies were arrayed for battle, it is said that Antiochus turned to Hannibal, who accompanied his entourage, to enquire whether these forces grandly arrayed in their gold, jewels and rich silks would be enough for the Romans. "Indeed they will be more than enough," replied Hannibal, "even though the Romans are the greediest nation on earth!"
Eumenes opened the battle by sending in archers, slingers, and dart-throwers, with some cavalry support, to shower the enemy scythe chariots and Arab camel corps. The missiles ended up throwing the horses and camels into a panic, disposing of that threat before the battle proper began. The panicked chariots also caused most of the supporting troops on Antiochus' left to take flight, leaving the armoured cavalry holding that wing.
He then launched all his Roman cavalry against the Gallic and Cappodocian infantry of the second division and routed these also.
Meanwhile, Antiochus himself had led his right wing cavalry against the Roman left flank and defeated it, driving it from the field, with the Seleucid horse in pursuit. Antiochus' charge continued on to an attack on the Roman Camp, which was successfully defended by the tribune Aemilius. Thus the main body of Syrian infantry, the phalanx, was deprived of cavalry support from either right or left.
Eumenes, commanding the Roman right wing. Antiochus' heavy cavalry, without its proper auxiliary support, was broken immediately (many of them cut down because their heaviness prevented effective evasion), and the Romans proceeded to hit the light-armed troops between the cavalry, breaking the Seleucid left wing.
In the centre, the Seleucid pike phalanx was forced to halt and form front on both sides, as well as in front with elephants in the intervals. In this position it was assailed by the archers and slingers, and made an easy target. The phalanx thereupon retired in good order until the elephants in its gaps became frightened and disrupted the formation and panic began to spread into Antiochus' phalanx.
Romans moved their infantry forward, began pelting Antiochus' phalanx with their javelins. Roman soldiers moved out of the way of the elephants and cut their hamstrings as opportunity allowed.
At this, the whole army broke up and the slaughter of the Seleucid infantry began. The Seleucid force was essentially annihilated before the cavalry under Antiochus could return to the field.
Thus, just like in the previous battles of Raphia and Ipsus, the lack of control over the cavalry arm lost the battle. Just as at Raphia 27 years before, Antiochus' pursuit was over-long and by the time he returned to the centre of the battlefield his cause was lost.
On the Roman left, however, Antiochus was able to press on the enemy's lightly-guarded flank, which had moved away from the river banks to maintain contact with the centre drove the Roman cavalry and some of the infantry back to the Roman camp. When the Romans approached the camp, they were ordered to return to battle, and reinforced by reserves from the camp and a detachment of cavalry from the right (where Antiochus by now was routed) turned to make a stand. When Antiochus saw the soldiers he had been pursuing turn about to face him again, with reinforcements approaching, he turned and fled
By now, all of Antiochus' army was in a dangerously chaotic rout. Antiochus' pursuit on his right meant that now both flanks of his phalanx were pretty much unprotected Romans pursued, slaughtered, and took Antiochus' camp.
Antiochus' losses said to be some 50,000 infantry and 3,000 cavalry, with 1,400 captured along with 15 elephants Roman losses were only 300 infantry and 49 cavalry. Magnesia left Antiochus more or less impotent and sued for peace, concluding a treaty - on Roman terms - two year later.
Gnaeus Manlius Valso replaced Lucius Scipio as commander to carry out mop-up operations. Lucius returned to Rome, where he conducted an even more elaborate and ostentatious triumph that his brother had after Zama.
Publius received the thanks of the Senate and the title "principes Senatus". Lucius took the surname Asiaticus.
Two years later Antiochus was forced to agree to a humiliating peace, which settled the fate of Greece and effectively ended Seleucid influence in the Mediterranean. The battle established the Attalid dynasty of Pergamon as the dominant power in Asia Minor.
References: Livy, 37, 37-44 Appian, Syrian Wars, 30-36