After the Syrian War against Antiochus III, the Romans were not idle in the east, but the situation in its north and west took priority for some time. In Cisalpine Gaul, rebellious Celts had to be put down once again, and the province was given a major overhaul in terms of roads, colonies and various infrastructure projects. Further west, In Hispania, trouble with the Turdenati and Celtiberians continued almost non-stop since the end of the Second Punic War. The father of the famous Gracchi brothers, Tiberius Sempronius Gracchus, put a short term end to the violence in 179 BC, but more problems were soon to come.
While Spain proved a difficult province to govern, the situation in the east was no better. Relations between Rome and Macedonia never improved since the end of the first two Macedonian Wars, and Philip V passed his dislike of Rome onto his son, Perseus. Perseus took the throne of Macedonia in 179 BC upon the death of his father, and immediately set about undermining regional Roman authority. He arranged a marriage alliance with Antiochus III, an obvious enemy of Rome from the Syrian war, and instituted debt relief in his Kingdom. This policy was upsetting to the Roman Senate and wealthy businessmen who supported the local governments which would end up on the financial losing end of this policy. Militarily, Perseus moved into territories north of Macedon and south into Greece violating terms of earlier treaties confining Macedonia to its traditional frontiers. Finally, he attempted to re-establish relations with the Achaean league and other Greek city-states, threatening Rome's jurisdiction.
By 172 BC, Eumenes, King of Pergamum and a loyal Roman ally, began making accusations against Perseus that he was violating Pergamum's territory. Diplomatic relations went in favor of Perseus for the simple reason that the majority of the Senate was disturbed by the prospect of yet another war. The winds of change wouldn't take long to alter Roman opinion however, as a perceived assassination attempt on Eumenes and continuing trouble in Illyria were both attributed to Perseus. Roman reaction was swift despite their consternation over yet another offensive war. Legions were sent to Epirus to prepare Macedonian invasion routes, and diplomats were sent to various Greek cities to ensure their continuing support and loyalty.
The Consul Q. Marcius Philippus established himself on the Macedonian frontier putting an end to any thoughts of open rebellion among the Boeotian League, a conglomerate of tribes allied to Perseus. By the spring of 171 BC war was declared and the Consul P. Licinius Crassus moved a force to Thessaly where he was defeated in a minor engagement. He blamed the Aetolians, another thorn in the Roman side since the war with Antiochus and arrested several of their leaders. A praetor C. Lucretius Gallus, and later L. Hortensius sacked several enemy and allied Greek towns inciting rage among the Greeks.
The war turned into a political internal struggle for the next several years as accusations, reparations and convictions were laid down upon corrupt Roman officers for their conduct towards allies. The Roman army was an undisciplined mass, growing fat and happy on plundering defenseless Greek towns, and the Consuls of 170 and 169 BC, A. Hostilius Mancinus and Q. Marcius Philippus respectively, did little to actually bring the war to Perseus and seemed rather content to rape the province of whatever wealth they could gather.
168 BC finally proved to be decisive. L. Aemilius Paullus, Consul for the year, arrived and immediately set about training and organizing his army. Paullus managed to force Perseus to battle on June 22, 168 at Pydna. The Macedonians were caught on broken ground, disadvantageous to the immobile phalanx and had little chance of victory. The Romans slaughtered 20,000 of the Macedonian force, taking 6,000 prisoners and 5,000 from nearby forces. Perseus escaped the carnage, but his allies in the region quickly submitted to Rome, and without an army, he had no choice but to surrender.
Perseus was paraded in Paullus' triumph and later banished to the small Italian town of Alba Fucens, where he lived the rest of his days in obscurity and miserable conditions. The Romans however, weren't satisfied with only punishing Perseus. Too many local inhabitants jumped to Macedonia's aid whenever they started trouble and it was time for it to end. Macedonia was broken into 4 Republics, where each was denied the right to conduct commerce with each other. Most local magistrates of any significance were removed from power and shipped off to Rome with Perseus. The Illyrian Kingdom of King Genthius, an ally of Perseus, was also broken up in a similar fashion.
The worst victim of the Roman victory, however, was Epirus. Paullus needed to make an example to the entire region of what would happen in the case of resisting Rome and the aid Epirus provided to Perseus provided a perfect opportunity. A resulting decree from the Senate authorizing the plunder of the coastal region and Paullus took full and brutal advantage. 70 towns in the territory were required to give up all the gold and silver they had in order to stave off Roman retribution. When the treasure was delivered however, Paullus paid no mind to his suggestion that providing the plunder would protect the inhabitants. He ordered an assault on all 70 of these towns crushing the walls and capturing and slaughtering as many people as he could. The result was a massive slaughter in which countless lives were lost and over 150,000 Epirotes were sold into slavery.