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Roman supremacy in the Greek East

Rome's preoccupation with its war with Carthage provided an opportunity for Philip V of Macedonia, in the north of the Greek peninsula, to attempt to extend his power westward. He sent ambassadors to Hannibal's camp in Italy, to negotiate an alliance as common enemies of Rome. But Rome discovered the agreement when Philip's emissaries were captured by a Roman fleet. The First Macedonian War saw the Romans involved directly in only limited land operations, but they achieved their objective of occupying Philip and preventing him from aiding Hannibal.

The past century had seen the Greek world dominated by the three primary successor kingdoms of Alexander the Great 's empire: Ptolemaic Egypt, Macedonia and the Seleucid Empire. In 202, internal problems led to a weakening of Egypt's position, disrupting the power balance among the successor states. Macedonia and the Seleucid Empire agreed to an alliance to conquer and divide Egypt. Fearing this increasingly unstable situation, several small Greek kingdoms sent delegations to Rome to seek an alliance. Rome gave Philip an ultimatum to cease his campaigns against Rome's new Greek allies. Doubting Rome's strength, Philip ignored the request, and Rome sent an army of Romans and Greek allies, beginning the Second Macedonian War. In 197, the Romans decisively defeated Philip at the Battle of Cynoscephalae, and Philip was forced to give up his recent Greek conquests. The Romans declared the "Peace of the Greeks", believing that Philip's defeat now meant that Greece would be stable, and pulled out of Greece entirely.

With Egypt and Macedonia weakened, the Seleucid Empire made increasingly aggressive and successful attempts to conquer the entire Greek world. Now not only Rome's allies against Philip, but even Philip himself, sought a Roman alliance against the Seleucids. The situation was exacerbated by the fact that Hannibal was now a chief military advisor to the Seleucid emperor, and the two were believed to be planning outright conquest not just of Greece, but also of Rome. The Seleucids were much stronger than the Macedonians had ever been, because they controlled much of the former Persian Empire and had almost entirely reassembled Alexander the Great's former empire.

Fearing the worst, the Romans began a major mobilization, all but pulling out of recently conquered Spain and Gaul. This fear was shared by Rome's Greek allies, who now followed Rome again for the first time since that war. A major Roman-Greek force was mobilized under the command of the great hero of the Second Punic War, Scipio Africanus, and set out for Greece, beginning the Roman–Seleucid War. After initial fighting that revealed serious Seleucid weaknesses, the Seleucids tried to turn the Roman strength against them at the Battle of Thermopylae, but were forced to evacuate Greece. The Romans pursued the Seleucids by crossing the Hellespont, the first time a Roman army had ever entered Asia. The decisive engagement was fought at the Battle of Magnesia, resulting in complete Roman victory. The Seleucids sued for peace, and Rome forced them to give up their recent Greek conquests. Rome again withdrew from Greece, assuming (or hoping) that the lack of a major Greek power would ensure a stable peace. In fact, it did the opposite.