Rome for sometime before the second Macedonian War had viewed Antiochos III with some suspicion. His kingdom, which included most of the West Asian empire of Alexander, was vast and his resources, formidable. Though Rome was aware that Antiochos had refused aid to Philip in the war, he remained a bete noir disturbingly poised to move into Europe by taking the old kingdom of Lysimakos in Thrace. Rome, in her newly assumed role of, ” the protector of Greek liberty”, declared her intention to liberate the Greek cities of Ionia. This thinly disguised threat of war was directed to Antiochos. Philip wasted no time in allying himself with Rome. In 194, Philip sent troops to the aid of Flamininus to help him in suppressing Nabis, king of Sparta, who briefly rebelled against the Roman presence in Greece.
The Aetolians, unhappy with the rewards of their alliance with Rome against Macedon, openly revolted in 192 at the spring meeting of The Aetolian League when they called upon Antiochos to come to Greece and free them from Rome. They delivered a direct challenge by declaring that they would next meet on the banks of The Tiber. The Aetolians opened hostilities by taking the Roman occupied city of Demetrias. Flamininus and the Roman navy sailed for Asia Minor to engage Antiochos before he could attack.
Antiochos led a small expedition into Greece still hoping to form an alliance with Macedon and Rome sent 3,000 men who landed at Appolonia where its commander, Baebius, met with Philip. The Romans with Philip’s aid installed a base of operations in Larissa and waited for the main Roman army while Antiochos spent the winter accessing the situation and attending lavish entertainments. When, in the spring of 191, a Roman army of 2,000 cavalry and 20,000 infantry arrived, the forces of Antiochos were speedily defeated at Thermopylae and the king, at once, sailed for Asia.
Philip and the Romans now turned their attention to the allies of Antiochos in Greece. Rome wanted the alliance with Philip to continue and promised much in return for his aid, even releasing his son and the Macedonian hostages from their six years of captivity in Rome. They agreed that Philip would keep all the territories he captured in Greece. It was a Roman promise; in the end they saw to it that he got very little. Indeed, their enemies, the Aetolians ended up with more than Philip as the Romans planned a balance of power in Greece. Philip was promised a release from the last of the indemnity payments for the second Macedonian war and it was granted in the following year when the Roman armies marched through Macedon on their way to Asia. The Romans were surprised to find that Philip had repaired the roads all along their route and even bridged the rivers to facilitate their advance, he, himself accompanied them to the Hellespont in the company of the commanders, Lucias Cornelius Scipio and his great brother, Scipio Africanus.