The Romans drew up their battle line, and Plutarch gives a stirring description of the Macedonian army as it swung into action against them:
First came the Thracians... huge men with brightly polished shields, wearing black tunics and their legs protected by greaves. As they advanced they brandished their straight spears, heavy with iron, [which they carried] on their right shoulders. After the Thracians marched the mercenary soldiers, each armed in their native style, with the Paeonians mingled amongst them. The third unit was of picked men, the bravest and strongest native Macedonians, in the prime of life, gleaming with golden armour and wearing scarlet coats. As these formed up, they were followed from the camp by the troops of the Brazen Shield phalanx. The whole plain seemed alive with flashing steel and gleaming brass; and the hills resounded with their shouts.
As far as we can tell of the order of battle, Perseus took the right wing, together with his guard cavalry and a special experimental anti-elephant corps. These men apparently wore armour with spikes protruding at angles from their curiasses, apparently in the hope that the elephants would find attacking them the equivalent of a human walking barefoot over a lawn of unshelled chestnuts. According to a writer called Posidonius (quoted by Plutarch), the king was suffering from a horse kick which he had taken so violently on the thigh that the bruising forced him to fight without armour, spiky or otherwise.
Paulus lined himself and his legionaries against the ‘Bronze Shield’ phalanx, and the second legion, under an ex-consul called Albinus, squared up to the second phalanx, which Livy calls the ‘White Shields’.
The battle began on the right wing, where the elephants made short work of the anti-elephant corps. This success was followed by the Latin allied troops who began to push back, and on occasion rout the opposing Macedonians. Another set of Italian allies, the Pelignians, were amongst the first to face the phalanx. They hesitated, nonplussed by the impossibility of penetrating the hedge of spears, until one of their commanders seized their standard and threw it into the enemy ranks. This inspired the Pelignians to rush onto the Macedonian spears, where they were promptly slaughtered.
Paulus himself later admitted that the sight of the oncoming phalanx gave him palpitations. As the wall of spear-points bore down on the Romans, the legionaries were totally unable to get at the enemy. They, like the Pelignians before them, were impaled by the oncoming pikes, and were unable to beat them aside and get to grips with the men wielding them. Seeing the famed Roman legions slowly forced into giving ground, some of the fainter-hearted of Rome's allies began to quietly slip away, assuming the battle to be lost.
In the middle of a heated engagement, the son of Cato the Censor lost his sword. Not wanting to report this disgrace to his formidable father, young Cato persuaded a group of his friends to hurl themselves back into the melee and help him to find it. So ferocious was this inspired charge that the Macedonians were forced to give ground and Cato retrieved his weapon. Across the battlefield similar minor triumphs and disasters were taking place, with the inevitable result that the line of engagement became somewhat ragged as the Macedonians pressed forward in some areas, and were stalled in others.
Paulus realized this gave the more flexible Roman formations an opportunity. Rather than fight legion to phalanx, he ordered his men to break into smaller units, and to concentrate upon those places where gaps had opened in the enemy formation. It is highly probable that this order gave sanction to what the legionaries were already doing, as Roman legionaries generally showed considerable initiative on the field, and some, at least, were veterans of the Spanish wars, and accustomed to fighting in small co-ordinated groups.
This revealed the essential difference between phalanx and legion. Where the legion was forced back it gave ground, losing men, but not cohesion. But once even a small party of legionaries got in among the phalangites, they caused chaos out of all proportion to their number, and created a gap into which ever more Romans could pour. At close quarters, a 16-foot pike was little use against the dreaded Roman gladius and, after the front rank, most phalangites were lightly armed in any case. At one point after another the spear-hedge dissolved and the Macedonian advance came to a confused halt. As soon as the legionaries had chewed their way into the phalanx far enough to start taking enemy soldiers from the side, the phalanx lost all cohesion and collapsed like a stack of cards.
Perseus had by now taken a light wound from a glancing blow by a spear to go with his horse kick. When he saw his phalanx buckle he did not need to be told that the game was up. With the day clearly lost, he pulled back his cavalry and retreated, leaving the infantry to their fate.
That fate was grim. After the scare they had been given, the Romans were in no mood to show mercy. Macedonians were cut down where they stood, and slaughtered if they tried to surrender. Some waded out to sea to escape the legionaries, and offered their surrender to the Romans who set off after them in small boats. These were either killed in the water or driven deeper until they drowned. Seeing the crush on the shore, the elephant drivers turned their mounts on the crowd, and many more phalangites were crushed either by the elephants or in the attempt to escape them.
Some 3,000 Macedonians mounted a last stand, bravely holding their ranks until the last of them was cut down. Elsewhere the battle had become a massacre so violent that it was reported that blood from the 15 to 20,000 slain made its way into a river over a mile distant. Only nightfall brought an end to the slaughter.
Macedon's army of 40,000 had in an afternoon been reduced to fewer than 5,000 men. Apart from those killed in the battle and aftermath, thousands more were taken captive. Roman casualties were well under 1,000, much of this number made up by the hapless Pelignians. Paulus was deeply concerned about his son who had gone missing, but the young man returned well after nightfall, having gone further than most in chasing and cutting down the fleeing enemy. Apart from the fact that Perseus was still at large, the day had gone as well as Paulus could possibly have hoped. His victory was total, and Macedon was totally and unconditionally crushed