When it grew towards evening, some tell us, Aemilius himself used a stratagem to induce the enemy to begin the fight; that he turned loose a horse without a bridle, and sent some of the Romans to catch him, upon whose following the beast the battle began. Others relate that the Thracians, under the command of one Alexander, set upon the Roman beasts of burden that were bringing forage to the camp; that to oppose these, a party of seven hundred Ligurians were immediately detached; and that, relief coming still from both armies, the main bodies at last engaged. Aemilius, like a wise pilot, foreseeing by the present waves and motion of the armies the greatness of the following storm, came out of his tent, went through the legions, and encouraged his soldiers. Nasica, in the meantime, who had ridden out to the skirmishers, saw the whole force of the enemy on the point of engaging. First marched the Thracians, who, he himself tells us, inspired him with most terror; they were of great stature, with bright and glittering shields and black frocks under them, their legs armed with greaves, and they brandished, as they moved, straight and heavily-ironed spears over their right shoulders. Next the Thracians marched the mercenary soldiers, armed after different fashions; with these Paeonians were mingled. These were succeeded by a third division, of picked men, native Macedonians, the choicest for courage and strength, in the prime of life, gleaming with gilt armour and scarlet coats. As these were taking their places they were followed from the camp by the troops in phalanx called the Brazen Shields, so that the whole plain seemed alive with the flashing of steel and the glistening of brass; and the hills also with their shouts, as they cheered each other on. In this order they marched, and with such boldness and speed, that those that were first slain died at but two furlongs distance from the Roman camp. 
The battle being begun, Aemilius came in and found that the foremost of the Macedonians had already fixed the ends of their spears into the shields of his Romans, so that it was impossible to come near them with their swords. When he saw this, and observed that the rest of the Macedonians took the targets that hung on their left shoulders, and brought them round before them, and all at once stooped their pikes against their enemies’ shields, and considered the great strength of this wall of shields, and the formidable appearance of a front thus bristling with arms, he was seized with amazement and alarm; nothing he had ever seen before had been equal to it; and in aftertimes he frequently used to speak both of the sight and of his own sensations. These, however, he dissembled, and rode through his army without either breastplate or helmet, with a serene and cheerful countenance. 
On the contrary, as Polybius relates, no sooner was the battle begun, but the Macedonian king basely withdrew to the city Pydna, under a pretence of sacrificing to Hercules; a god that is not wont to regard the faint offerings of cowards, or to fulfil unsanctioned vows. For truly it can hardly be a thing that heaven would sanction, that he that never shoots should carry away the prize; he triumph that slinks from the battle; he that takes no pains meet with success, or the wicked man prosper. But to Aemilius’s petitions the god listened; he prayed for victory with his sword in his hand, and fought while entreating divine assistance. 
A certain Posidonius, who has at some length written a history of Perseus, and professes to have lived at the time, and to have been himself engaged in these events, denies that Perseus left the field either through fear or pretence of sacrificing, but that, the very day before the fight, he received a kick from a horse on his thigh; that though very much disabled, and dissuaded by all his friends, he commanded one of his riding-horses to be brought, and entered the field unarmed; that amongst an infinite number of darts that flew about on all sides, one of iron lighted on him, and though not with the point, yet by a glance struck him with such force on his left side that it tore his clothes and so bruised his flesh that the mark remained a long time after. This is what Posidonius says in defence of Perseus. 
The Romans not being able to make a breach in the phalanx, one Salius, a commander of the Pelignians, snatched the ensign of his company and threw it amongst the enemies; on seeing which, the Pelignians (as amongst the Italians it is always thought the greatest breach of honour to abandon a standard) rushed with great violence towards the place, where the conflict grew very fierce and the slaughter terrible on both sides. For these endeavoured to cut the spears asunder with their swords, or to beat them back with their shields, or put them by with their hands; and, on the other side, the Macedonians held their long sarissas in both hands, and pierced those that came in their way quite through their armour, no shield or corslet being able to resist the force of that weapon. The Pelignians and Marrucinians were thrown headlong to the ground, having without consideration, with mere animal fury, rushed upon a certain death. Their first ranks being slain, those that were behind were forced to give back; it cannot be said they fled, but they retreated towards Mount Olocrus. When, Aemilius saw this, Posidonius relates, he rent his clothes, some of his men being ready to fly, and the rest not willing to engage with a phalanx into which they could not hope to make any entrance- a sort of palisade, as it were, impregnable and unapproachable, with its close array of long spears everywhere meeting the assailant. Nevertheless, the unequalness of the ground would not permit a widely extended front to be so exactly drawn up as to have their shields everywhere joined; and Aemilius perceived that there were a great many interstices and breaches in the Macedonian phalanx, as it usually happens in all great armies, according to the different efforts of the combatants, who in one part press forward with eagerness, and in another are forced to fall back. Taking, therefore, this occasion, with all speed he broke up his men into their cohorts, and gave them order to fall into the intervals and openings of the enemy’s body, and not to make one general attack upon them all, but to engage, as they were divided, in several partial battles. These commands Aemilius gave to his captains, and they to their soldiers; and no sooner had they entered the spaces and separated their enemies,but they charged them, some on their sides where they were naked and exposed, and others, making a circuit, behind; and thus destroyed the force of thephalanx, which consists in common action and close union. And now, come to fight man to man, or in small parties, the Macedonians smote in vainupon firm and long shields with their little swords, whilst their slight bucklers were not able to sustain the weight and force of the Roman swords, which pierced through all their armour to their bodies; they turned, in fine, and fled. 

The conflict was obstinate. And here Marcus, the son of Cato, and son-in-law of Aemilius, whilst he showed all possible courage, let fall his sword. Being a young man carefully brought up and disciplined, and, son of so renowned a father, bound to give proof of more than ordinary virtue, he thought his life but a burden, should he live and permit his enemies to enjoy this spoil. He hurried hither and thither, and wherever he espied a friend or companion, declared his misfortune, and begged their assistance; a considerable number of brave men being thus collected, with one accord they made their way through their fellows after their leader, and fell upon the enemy; whom after a sharp conflict, many wounds, and much slaughter, they repulsed, possessed the place that was now deserted and free, and set themselves to search for the sword, which at last they found covered with a great heap of arms and dead bodies. Overjoyed with this success, they raised the song of triumph, and, with more eagerness than ever, charged the foes that yet remained firm and unbroken. In the end, three thousand of the chosen men, who kept their ground and fought valiantly to the last, were all cut in pieces, while the slaughter of such as fled was also very great. The plain and the lower part of the hills were filled with dead bodies, and the water of the river Leucus, which the Romans did not pass till the next day after the battle, was then mingled with blood. For it is said there fell more than twenty-five thousand of the enemy; of the Romans, as Posidonius relates, a hundred; as Nasica, only fourscore. This battle, though so great, was very quickly decided, it being three in the afternoon when they first engaged, and not four when the enemy was vanquished; the rest of the day was spent in pursuit of the fugitives, whom they followed about thirteen or fourteen miles, so that it was far in the night when they returned. 
Aemilius Paulus by Plutarch