The battle of Myonnesus was the decisive naval battle of the War between Rome and Antiochus III, and saw a combined Roman and Rhodian fleet defeat Antiochus’ main surviving fleet.
In the aftermath of an earlier naval defeat at Corycus Antiochus had decided to build up his main fleet at Ephesus, under the command of Polyxenidas, while he sent the exiled Hannibal to build a second fleet in Phoenicia. His best chance of success was to force the allies to divide their fleets, allowing him to defeat them in detail, preferably after his own two fleets had combined. That plan ended when Hannibal’s new fleet was defeated by the Rhodians at Eurymedon, but that still left Polyxenidas’s fleet intact.
The allies were still forced to divide their fleets, for a large Roman army under the Scipio brothers was marching through Thrace towards the Hellespont, where it would need naval protection to cross into Asia. While the Roman fleet (commanded by Lucius Aemilius Regillus) and the Rhodian fleet (under Eudamas) remained at Samos, watching Polyxenidas at Epheses, the third allied fleet, that of Eumenes of Pergamum, was sent into the northern Aegean to support the army.
Antiochus decided to make one final attempt to defeat the Roman fleet while he still had a numerical advantage in ships. Polyxenidas had 89 large warships available at Ephesus, while the Romans had 58 large ships available for immediate action, supported by 22 smaller Rhodian ships, a total of 80 warships.
In order to draw the Romans out of Samos, Antiochus attacked the nearby city of Notium, one of Rome’s friends in the area. As expected the Romans left port and made for Notium. On the morning of the battle they were docked in the harbour of Teos, west of Myonnesus. An attempt to trap the Romans in the harbour failed, and the two fleets formed up in line of battle, with the Romans to the west and the Syrians to the east, and the Roman left clossest to the shore. Polyxenidas hoped to use his numerical advantage in larger warships to outflank the Roman right. If he was successful, then at least half of the Romans ships would have found themselves under attack by two enemies.
Until the outflanking manoeuvre was completed Polyxenidas’s ships would be fighting at a great disadvantage, for the superior Roman infantry meant that in any boarding action they were likely to be victorious. On the Syrian right, facing the Roman fleet, this was exactly what happened, and Polyxenidas’s ships were slowly overwhelmed.
The battle was decided on the Syrian left. Here Polyxenidas was facing Eudamas and the smaller Rhodian ships. Eudamas quickly realised that if he could slow down the Syrian outflanking manoeuvre for long enough, the Romans would win the battle on their flank, and come to his aid. Using the greater speed and manoeuvrability of his ships, he was completely successful in this, preventing Polyxenidas from carrying out his flanking attack. The Roman right wing broke through the centre of the Syrian line then turned to attack the Syrian left. Under attack from both the Roman right and the Rhodians, the Syrians fled. Seeing their left wing gone, the surviving ships on the Syrian right took advantage of a lucky west wind to escape back into Ephesus.
Myonnesus ended as an overwhelming Roman victory. While they only lost one Rhodian and two Roman ships, Polyxenidas lost 42 of his original 89 ships (twenty nine captured and thirteen sunk). Antiochus had lost his final fleet, and with it his best chance of stopping the Roman army from crossing into Asia Minor. Within a few months Antiochus suffered an equally devastating defeat on land, at Magnesia, and was forced to sue for peace.