Livy tells us, the Roman army was well aware that the Seleucid army and their king, Antiochus, were encamped near Thyatira. After a five day march they arrive at Thyatira to find that the king has moved his camp. The Romans follow the track until they reach the neighborhood of Magnesia and Sipylum, where Antiochus has built camp with a fosse, double ramparts, walls, and turrets. The Roman army sets up camp on the western bank of the Phrygius which is four miles from Antiochus. When the Roman army arrives and starts to set up camp, they are attacked by 1,000 cavalry across the river. There is confusion at the beginning, but soon reinforcements on the Roman side cause the enemy to retreat across the river. The Romans pursue the attackers and slay those who are not able to escape fast enough.
After two days of silence on both sides, the Romans cross the river and set up a new camp two and a half miles from Antiochus. Again, while the Romans are building their camp, 3,000 infantry and cavalry attack the Roman advanced guard which had far less troops. The Romans are able to hold the attack and kill 100 enemies and capture 100 more.
For the following four days, both armies stand ready for battle, and on the fifth the Roman army advances to the middle of the battlefield, but Antiochus keeps his troops within a mile of their camp. That night the Roman consul Scipio calls for a war council and they debate whether to continue fighting or make a new camp to hold through the winter. The Romans decide to fight, and that they will rush the enemies camp if they do not choose to fight on the battlefield. A scout is sent to survey the enemy camp.
The next day, the Roman army advances to the half point on the battlefield, and according to Livy, Antiochus decides to advance his troops so to not lower their morale any further.
According to Appian, consul Scipio had 30,000 strong altogether, which consisted of 10,000 Roman legionaries, 10,000 Italian allies, 3,000 Achaean peltasts, and 3,000 cavalry.
Antiochus had 70,000 strong, and the most notable of his troops were those that made up a 16,000 man Macedonian phalanx. The rest of of the army had a disorganized and inconsistent make up. Antiochus had twenty-two elephants on the flanks of each section of his army.
At onset of battle, Antiochus sent his chariots in a daring charge, but the Romans counter the Seleucid’s charge by simultaneously launching missiles, arrows, and charging cavalry of their own. All the commotion caused by the Roman counterattack caused the enemy chariots to flee the battlefield, followed by the auxiliary troops located behind the chariot force. Soon, the enemy’s front center had taken a beating and the reserves had been outflanked. But closer to the river, Roman troops had begun to retreat back to their camp due to an advancing Antiochus. At the Roman camp, the fleeing troops are stopped by the camp guard with the threat of death. With additional, fresh troops from the camps guard, the Romans make a counter and defeat both wings of the enemy army. The Romans find more resistance then anticipated at the enemy camp, but it is finally stormed and taken. Later delegates from Thyatira, Magnesia, and Sipylum surrender their cities.
50,000 infantry, 3,000 cavalry, 1,500 prisoners, and 15 elephants and their drivers captured.
There were many wounded, but only 300 infantry and 49 cavalry were killed.
The Battle of Magnesia marked the end of the Roman-Syrian War. It increased the Romans control over Asia Minor. It also crippled the already divided Seleucid Empire, and prevented Antiochus from expanding his control to the west.