The Third Macedonian War started in 169 BC after a number of actions on the part of Perseus of Macedon incited Rome to declare war. At first the Romans achieved a number of small victories, largely due to Perseus' refusal to consolidate his armies. By the end of the year the tide had changed dramatically and Perseus had regained most of his losses, including the important religious city of Dion. Perseus then established himself in an unassailable position on the river Elpeus, in northeastern Greece.
The next year command of the Roman expeditionary force passed to Lucius Aemilius Paullus, an experienced solder who was one of the consuls for the year. In order to force Perseus from his ground, Paulus sent a small force (8,200 foot and 120 horse) under the command of Scipio Nasica to the coast, a feint to convince Perseus that they were attempting a riverborne flanking manuver. Instead, that night Scipio took his force south, and over the mountains to the west of the Roman and Macedonian armies. They moved as far as Pithium then swung northeast to take the Macedonians in the rear.
A Roman deserter, however, made his way to the Macedonian camp and Perseus sent Milo with a force of 12,000 to block the approach road. The encounter that followed sent Milo and his men back in disarray towards the main Macedonian army. After this Perseus moved his army northwards and took up a position near Katerini, a village south of Pydna. It was a fairly level plain and was very suited to the operation of the phalanx.
Paullus then had Scipio rejoin the main force, while Perseus deployed his forces for what appeared to be an attack from the south by Scipio. The Roman armies were actually to the west, and when they advanced they found Perseus fully deployed. Instead of joining battle with troops tired from the march, they encamped to the west in the foothills of Mount Olocrus.
Next day, the armies waited until the afternoon when fighting actually began. The exact cause of the start of the battle differs across reports; one story is that Paulus waited until late enough in the day for the sun not to be in the eyes of his troops, and then sent an unbridled horse forward to bring about alarm. More likely it was the result of some Roman foragers getting a little too close and being attacked by some Thracians in Perseus' army.
In terms of numbers the two armies appear quite evenly matched. The Romans had 38,000 men, of which 33,400 were infantry, including two legions. The Macedonians had 44,000 soldiers, of which 21,000 were phalangites. The cavalry numbers were roughly equal, about 4,000 each. The two armies were drawn up in their usual fashions. The Romans had placed the two legions in the middle, with the allied Latin, Italian and Greek infantry flanking them. The cavalry had been placed on the wings, with the Roman right being supplemented by 22 elephants. The phalanx took up the centre of the Macedonian line, with the elite 3,000-strong Guard formed to the left of the phalanx. Lighter peltasts, mercenaries and Thracian infantry guarded the two flanks of the phalanx, while the Macedonian cavalry were also, rather unevenly, split between the two wings. The strongest contingent was on the Macedonian right, where Perseus commanded the heavy cavalry (including his elite Sacred Squadron), and the Thracian Odrysian cavalry were deployed.
The two centres engaged at about 3pm, with the Macedonians advancing on the Romans a short distance from the Roman camp. Paulus claimed later that the sight of the phalanx had filled him with alarm and amazement. The Romans tried to beat down the enemy pikes or hack off their points, but with little success. Unable to get under the thick bristle of spikes, the Romans were beaten back, and some of their allies abandoned the field.
But as the phalanx pushed forward, the ground became more uneven as it moved into the foothills, and the line lost its cohesion. Paulus now ordered the legions into the gaps, attacking the phalangites on their exposed flanks. At close quarters the longer Roman sword and heavier shield easily prevailed over the short sword (more of a dagger) and lighter armor. They were soon joined by the Roman right, which had succeeded in routing the Macedonian left.
Seeing the tide of battle turn, Perseus fled with the cavalry on the Macedonian right. According to Plutarch, Perseus' cavalry had yet to engage, and both the King and his cavalry were accused of cowardice by the surviving infantry. There weren't too many of these, however; the 3,000 Guard fought to the death, and in total the Macedonians suffered about 25,000 dead or captured of their 40,000.
Perseus later surrendered to Paullus, and was paraded in triumph in Rome. He was then imprisoned. The Macedonian kingdom was dissolved, and replaced with three republics. In time these were also dissolved, and Macedonia became a Roman province.
Although the battle is often considered to be a victory of the Roman legion's flexibility over the phalanx's inflexibility, the loss is clearly due to a failure of command on the part of Perseus. The legion's move into the gaps on the flanks of the phalanx should not have been able to take place, since the Macedonian version of the phalanx had light troops to guard against just such a problem and Perseus had them on the field at the start of the battle. The phalanx had also clearly been doing what it was designed to do, hold the enemy center while the cavalry and other light forces form for a flanking attack. However, this attack never came, and Perseus' splitting of the cavalry to both flanks suggests it never could have. As an illustration of tactics, it demonstrates only that bad commanders lose battles.