The battle of Magnesia, in the winter of 190 B.C., saw a badly outnumbered Roman army defeat the army of the Seleucid Emperor Antiochus III (the Great), forever altering the balance of power in the eastern Mediterranean.
The war between Rome and Antiochus had grown out of a combination of Roman fears of the Great King’s ambitions in Greece and Italy, and the dissatisfaction for the Aetolian League with the spoils they had gained at the end of the Second Macedonian War. Antiochus had recently re-established his family’s power in Asia Minor, and had some claims in Thrace, at the eastern edge of Europe. This worried the Romans, who believed that Greece was under their protection. The Aetolian League had then asked for assistance from Antiochus, and in 192 B.C. he had taken a small army into Greece. This force had been defeated at Thermopylea in 191 B.C., and Antiochus had been forced to retreat to Asia Minor.
The Romans sent an army under the command of Lucius Comelius Scipio and Scipio Africanus to invade Asia Minor. With the help of Eumenes II of Pergamum and the Rhodians they had won a series of naval victories, while with the help of Philip V of Macedonia their army had marched through Macedonia and Thrace, and crossed the Hellespont, invading Asia Minor by the land route.
Antiochus made a series of attempts to negotiate with the Romans, but they demanded that he abandon all claim to anywhere in Asia Minor (north and west of the Taurus Mountains), giving Antiochus little choice but to fight. He had not repeated his mistake of 192, and at the same time as negotiating with the Romans had gathered together a great army from every part of his vast (if somewhat unstable) empire. In the autumn of 190 Antiochus then withdrew east across Asia Minor, searching for a suitable battlefield. He found what he was looking for at Magnesia ad Sipylum.
The Roman army was 30,000 strong, with 6-7,000 auxiliaries and at least 2,800 cavalry, while the core of the army was made up of the legions. Eumese of Pergamum commanded the cavalry, the consul Cn. Domitius Ahenobarbus the legions, while Scipio Africanus was in overall command.
Antiochus had by far the larger army, at least 70,000 strong, but it was far more chaotic in nature. He massively outnumbered the Romans in cavalry, with at least 12,000. to their 2,800. He also had 20,000 light infantry, 54 elephants and a number of scythed chariots. It was a typical Hellenistic army – large, impressive but unwieldy – but it was also built around a solid core, a phalanx at least 16,000 strong.
Antiochus lined up his army on the banks of the River Phrygius, with his phalanx in the centre and cavalry and light troops on each flank. He took personal command of the cavalry on the right, clossest to the river.
The Romans arranged their army with their entire cavalry force on their right, furthest away from the river, in an attempt to prevent Antiochus from outflanking them.
Both Antiochus and the Romans were successful on their right. Antiochus led his cavalry in an attack which broke the Roman left, clossest to the river, and threatened to reach their camp.
On the Roman right Eumenes was equally successful, breaking through the Syrian chariots and light troops, then charging and defeating 3,000 heavy cataphracts. Antiochus’ phalanx was suddenly vulnerable.
As the Romans were well aware the phalanx was deadly if it could operate on its own terms, charging in a dense block, but at Magnesia the Syrian phalanx would not get that chance. The Roman legions under Domitius Ahenobarbus attacked from the front, using their darts and pila to good effect, while Eumenes attacked from the flanks. The Syrian phalanx was falling back towards its camps when 22 elephants posted between its separate sections stampeded. The phalanx lost its cohesion, the Roman legions broke into it, and a slaughter followed. Antiochus is said to have lost 50,000 men, two thirds of his entire army.
In the aftermath of the battle Antiochus realised that there was no point continuing the fight, and sued for peace, offering to abandon all of his possessions west and north of the Taurus Mountains. His army was gone, and most of his recent conquests in Asia Minor went over to the Romans when they learnt of his defeat.
The peace agreement was formally agreed in the peace of Apamea. For the moment the Romans were not interested in having a permanent presence in Asia Minor. Most of the Greek cities of Aisa Minor taken from Antiochus were given their freedom, while large areas were given to Pergamum and Rhodes. The Seleucid Empire was still a powerful force further east, but Antiochus and his successors would never return to Asia Minor.
In only eight years, at Magnesia and Cynoscephalae, the Roman Republic had defeated two of the three successor states of Alexander the Great, and had already taken Ptolemaic Egypt under their protection.