The war ignited in a Greece that was not a formal Roman province, but rather a collection of 'free' city-states under Roman influence following the defeat of Macedon in the Second Macedonian War. Dissatisfied with this Roman-led order, the Aetolian League invited Antiochus III to 'liberate' Greece, and he crossed into the peninsula in 192 BC. He successfully captured Chalcis and established it as his base of operations, spending the winter there to strengthen his position. However, his initial forces were small, and he failed to gain widespread support, allowing the Romans to respond effectively.
In 191 BC, the Roman forces under Manius Acilius Glabrio decisively defeated Antiochus's army at the Battle of Thermopylae . Antiochus, outnumbered and outmaneuvered, was forced to retreat from Greece back to Asia Minor.
The next phase of the war involved the Romans carrying the fight into Asia. A Roman fleet, with Rhodian and Pergamene allies, gained control of the Aegean Sea. Then, in 190 BC, a large Roman army, nominally led by Lucius Cornelius Scipio Asiaticus but heavily influenced by his more famous brother Scipio Africanus, crossed the Hellespont.
The decisive engagement took place at the Battle of Magnesia , near Sardis. The Roman-Pergamene forces inflicted a crushing defeat on the much larger Seleucid army. Antiochus III's forces, despite having war elephants and a numerically superior army, were routed, marking the end of Seleucid ambitions in Asia Minor and Europe.
The war concluded with the Treaty of Apamea in 188 BC. This treaty severely curtailed Seleucid power, forcing them to cede all territories west of the Taurus Mountains to Rome and its allies (Pergamum and Rhodes), pay a massive war indemnity, and surrender their war elephants and fleet. Antiochus was also required to hand over Hannibal, though Hannibal managed to escape.