The Battle of Magnesia took place in either December 190 or January 189 BC. It was fought as part of the Roman–Seleucid War, pitting forces of the Roman Republic led by the consul Lucius Cornelius Scipio Asiaticus and the allied Kingdom of Pergamon under Eumenes II against a Seleucid army of Antiochus III the Great. The two armies initially camped northeast of Magnesia ad Sipylum in Asia Minor (modern-day Manisa, Turkey), attempting to provoke each other into a battle on favorable terrain for several days.
When the battle finally began, Eumenes managed to throw the Seleucid left flank into disarray. While Antiochus' cavalry overpowered his adversaries on the right flank of the battlefield, his army's center collapsed before he could reinforce it. Modern estimates give 10,000 dead for the Seleucids and 5,000 killed for the Romans. The battle resulted in a decisive Roman-Pergamene victory, which led to the Treaty of Apamea that ended Seleucid domination in Asia Minor.
Background
Following his return from his Bactrian (210-209 BC) and Indian (206-205 BC) campaigns, Antiochus forged an alliance with Philip V of Macedon, seeking to jointly conquer the territories of the Ptolemaic Kingdom. In 198 BC, he was victorious in the Fifth Syrian War, taking over Coele-Syria and securing his southeastern border. He then focused his attention on Asia Minor, launching a successful campaign against coastal Ptolemaic possessions. In 196 BC, Antiochus used the opportunity of Attalus I's death to assault cities controlled by the Attalid dynasty. Fearing that Antiochus would seize the entirety of Asia Minor, the independent cities of Smyrna and Lampsacus appealed for protection from the Roman Republic. In the early spring of 196 BC, Antiochus' troops crossed to the European side of the Hellespont and began rebuilding the strategically important city of Lysimachia. In October 196 BC, Antiochus met with a delegation of Roman diplomats in Lysimachia. The Romans demanded that Antiochus withdraw from Europe and restore the autonomous status of Greek city-states in Asia Minor. Antiochus countered by claiming that he was simply rebuilding the empire of his ancestor Antiochus II Theos and criticized the Romans for meddling in the affairs of the Asia Minor states whose rights were traditionally defended by Rhodes.
In late winter 196/195 BC, Rome's erstwhile chief enemy, Carthaginian general Hannibal, fled from Carthage to Antiochus' court in Ephesus. Despite the emergence of a pro-war party led by Scipio Africanus, the Roman Senate exercised restraint. The Seleucids expanded their holdings in Thrace from Perinthus to Maroneia at the expense of the Thracian tribesmen. Negotiations between the Romans and the Seleucids resumed, coming to a standstill once again over differences between Greek and Roman law on the status of disputed territorial possessions. In the summer of 193 BC, a representative of the Aetolian League assured Antiochus that the Aetolians would take his side in a future war with Rome, while Antiochus gave tacit support to Hannibal's plans of launching an anti-Roman coup d'état in Carthage. The Aetolians began spurring the Greek states to jointly revolt under Antiochus' leadership against the Romans, hoping to provoke a war between the two parties. The Aetolians then captured the strategically important port city of Demetrias, killing the key members of the local pro-Roman faction. In September 192 BC, the Aetolian general Thoantas arrived at Antiochus' court, convincing him to openly oppose the Romans in Greece. The Seleucids raised 10,000 infantry, 500 cavalry, 6 war elephants, and 300 ships for their campaign in Greece.
Prelude
The Seleucid fleet sailed via Imbros and Skiathos, arriving at Demetrias where Antiochus' army disembarked. The Achaean League declared war on the Seleucids and Aetolians with the Romans following suit in November 192 BC. Antiochus forced Chalcis to open its gates to him, turning the city into his base of operations. Antiochus then shifted his attention towards rebuilding his alliance with Philip V of Macedon, which had been shattered after the latter was decisively defeated by the Romans at the Battle of Cynoscephalae in 197 BC. Philip expected that the Romans would emerge victorious in the conflict and counted on territorial rewards as well as the writing off of war reparations that he owed them; the Seleucids could provide neither, so Antiochus' overtures were rejected and Philip aligned himself with the Romans. Between December 192 and March 191 BC, Antiochus campaigned in Thessaly and Acarnania.