The Battle of Thermopylae was fought in 191 BC between a Roman army led by consul Manius Acilius Glabrio  and a Seleucid force led by King Antiochus III the Great. The Romans were victorious, and as a result, Antiochus was forced to flee Greece. It described by Appain (included below) and by Livy at 36.16-19.
Appian's account:
    Antiochus marched against the Thessalians and came to Cynoscephalae, where the Macedonians had been defeated by the Romans, and finding the remains of the dead still unburied, gave them a magnificent funeral. Thus he curried favor with the Macedonians and accused Philip before them of leaving unburied those who had fallen in his service.
    Until now Philip had been wavering and in doubt which side he should espouse, but when he heard of this he joined the Romans at once. He invited Baebius, their nearest general, to a rendezvous and gave pledges anew of faithful alliance against Antiochus. Baebius praised him for this, and felt emboldened to send Appius Claudius straightway with 2000 foot through Macedon into Thessaly.
    When Appius arrived at Tempe and from that point saw Antiochus besieging Larissa, he kindled a large number of fires to conceal the smallness of his force. Antiochus thought that Baebius and Philip had arrived, and became panic-stricken, abandoned the siege on a pretext of bad weather, and retreated to Chalcis.
    There he fell in love with a pretty girl, and, although he was above fifty years of age and was supporting the burden of so great a war, he celebrated his nuptials with her, gave a public festival, and allowed his army to spend the whole winter (191 BC) in idleness and luxury. When spring came he made a descent upon Acarnania, where he perceived that idleness had unfitted his army for every kind of duty. Then he repented of his marriage and his public festival. Nevertheless he reduced a part of Acarnania and was besieging the rest of its strongholds when he learned that the Romans were making a passage of the Adriatic. Then at once he returned to Chalcis.
    [§17] The Romans crossed hastily from Brundusium to Apollonia with the forces that were then ready, being 2000 horse, 20,000 foot, and a few elephants, under the command of Manius Acilius Glabrio. They marched to Thessaly and relieved the besieged cities. They expelled the enemy's garrisons from the towns of the Athamanians and made a prisoner of that Philip of Megalopolis who was still expecting the throne of Macedon. They also captured about 3,000 of the soldiers of Antiochus.
    While Manius was doing these things, Philip made a descent upon Athamania and brought the whole of it under subjection, king Amynander fleeing to Ambracia.
    When Antiochus learned these facts, he was terrified by the rush of events and by the suddenness of the change of fortune, and he now perceived the wisdom of Hannibal's advice. He sent messenger after messenger to Asia to hasten the coming of Polyxenidas. Then from all sides he drew in what forces he had. These amounted to 10,000 foot and 500 horse of his own, besides some allies, with which he occupied Thermopylae in order to put this difficult pass between himself and the enemy while waiting for the arrival of his army from Asia.
    The passage at Thermopylae is long and narrow, flanked on the one side by a rough and inhospitable sea and on the other by a deep and impassable morass. It is overhung by two mountain peaks, one called Tichius and the other Callidromus. The place also contains some hot springs, whence comes the name Thermopylae, "hot gates".
    [§18] There Antiochus built a double wall on which he placed engines. He sent Aetolian troops to occupy the summits of the mountains to prevent anybody from coming around secretly by way of the hill called Atropos, as Xerxes had come upon the Spartans under Leonidas, the mountain paths at that time being unguarded. One thousand Aetolians occupied each mountain. The remainder encamped by themselves near the city of Heraclea.
    When Manius saw the enemy's preparations he gave the signal for battle on the morrow and ordered two of his tribunes, Marcus Cato and Lucius Valerius, to select such forces as they pleased and to go around the mountains by night and drive the Aetolians from the heights as best they could. Lucius was repulsed from Mount Tichius by the Aetolians, who at that place fought well, but Cato, who moved against Mount Callidromus, fell upon the enemy while they were still asleep, about the last watch. Nevertheless there was a stiff fight here, as he was obliged to climb over high rocks and precipices in the face of an opposing enemy.
    Meantime Manius was leading his army against Antiochus' front in straight lines, as this was the only way possible in the narrow pass. The king placed his light-armed troops and peltasts in front of the phalanx, and drew up the phalanx itself in front of the camp, with the archers and slingers on the right hand next to the foot-hills, and the elephants, with the guard that always accompanied them, on the left near the sea.
    [§19] Battle being joined, the light-armed troops assailed Manius first, rushing in from all sides. He received their onset bravely, first yielding and then advancing and driving them back. The phalanx opened and let the light-armed men pass through. It then closed and pushed forward, the long pikes set densely together in order of battle, with which the Macedonians from the time of Alexander and Philip have struck terror into enemies who have not dared to encounter the thick array of long pikes presented to them.
    At this juncture the Aetolians were seen fleeing from Callidromus with loud cries, and leaping down into the camp of Antiochus. At first neither side knew what had happened, and there was confusion among both in their uncertainty but when Cato made his appearance pursuing the Aetolians with shouts of victory and was already close above the camp of Antiochus, the king's forces, who had been hearing for some time back fearful accounts of the Roman style of fighting, and who knew that they themselves had been enervated by idleness and luxury all winter, took fright.
    Not knowing how large Cato's force was, it was magnified to their minds by terror. Fearing for the safety of their camp they fled to it in disorder, with the intention of defending it against the enemy. But the Romans were close at their heels and entered the camp with them. Then there was another flight of the Antiocheans as disorderly as the first. Manius pursued them as far as Scarphia, killing and taking prisoners. Returning thence he plundered the king's camp, and by merely showing himself drove out the Aetolians who had broken into the Roman camp during his absence.
    [§20] The Romans lost about 200 in the battle and the pursuit; Antiochus about 10,000, including prisoners. The king himself, at the first sign of defeat, fled precipitately with 500 horse as far as Elateia, and from Elateia to Chalcis, and thence to Ephesus with his bride Euboea, as he called her, with his ships; but not all of them, for the Roman admiral made an attack upon some that were bringing supplies, and sunk them.
    When the people of Rome heard of this victory, so swiftly and easily gained, they offered sacrifice, being satisfied with their first trial of the formidable reputation of Antiochus. To Philip, in return for his services as an ally, they sent his son Demetrius, who was still a hostage in their hands.
