Second Macedonian War
Philip had long been interested in the Greek city states but as long as these states were allied with Rome he did not dare attack them. Instead, he began to take control over parts of Illyria. Rome responded by threatening war, and Philip withdrew his forces.
Macedon now started expanding on territory claimed by the Greek city states. These states called for help to their ally Rome, and Rome responded by giving Philip an ultimatum: he had to accept Roman rule over Macedon, in essence making Macedon a Roman province. Philip refused, and the Second Macedonian War began.
The decisive battle was at Cynoscephalae in Thessaly in 197 BC, when the legions of Titus Flamininus defeated Philip's Macedonian phalanx. Macedonian control of Greece was shattered, and at the Isthmian Games in Corinth in 196 BC, Flamininus proclaimed the freedom of Greece, although in fact Greece had now become a Roman protectorate in all but name.
Aside from the control over Greece, another reason for the war was that Rome saw Philip as a traitor: Philip had supported Hannibal of Carthage, which had led to the First Macedonian War.