The abortive invasion of Greece, which revealed Antiochus as an open and declared enemy of Rome, made necessary military intervention to back up Roman demands for the evacuation of Thrace and the Ionian cities. Antiochus at first tried to fortify the Hellespont against the expected crossing by Roman expedition, but, after losing control over the Aegean in the naval battle of Myonnesus (off Teos), he realized that the naval supremacy of the Romans would enable them to bypass his land fortifications in the Dardanelles, and he abandoned them. After the failure of further negotiations, both sides were determined to seek a solution by arms. Antiochus mobilized his army from Sardis along the Hermos Valley to Magnesia-ad-Sipylum against the Romans, who were advancing from the Pergamon area to Thyateira in the Hyrcanian Plain, and thence along the Phrygios southwards. Magnesia, which in 281 B.C. had been the site of the battle of Curupedion, which was decisive in opening the way to the west for the Seleucids, was now to see the beginnings of Roman expansion in the east and Seleucid decline.

The main sources for the battle are the parallel versions of Livy 37.37–44 and Appian Syr. 30–6, based solely on Polybius, who himself made extensive use of Rhodian, Achaean and Pergamene sources, probably in addition to reports from eyewitnesses.