This battle looked from the outset as the largest fought in Greece since the time of Alexander the Great, with some 85 000 men facing each other. The area of the battle was a well known battlefield called Cheronea, the exact place where the Macedonian under Philip II had broken Thebes and Athens in 338 B.C., 117 years earlier.
The valley was large in the north and getting shorter in the south, it's flank steeps and preventing any enemy move. The macedonian army formed in the north in a massive phalanx with a line of light infantry to it's front and two troops of light infantry to it's flanks. A force of some 500 horsemen was also put on the left flank to preserve it from enemy cavalry operations while the mainstay of the macedonian cavalry was put on the right flank under the personal command of Philip V who thus took the place Alexander the Great had so famously held for his first major battle. The young king could simply not imagine taking another position...
Opposing them where the allied forces from Aetolia, Athens and Sparta, under the personal command of Kleomenes. The king did not lead his men personally, a fall from his horse preventing him to go with the troops, and he had taken a position a bit behind his lines, on a small height which accorder him a good view of the field, an information that Philip only learned of after the battle.
The right of the phalanx, place of honour, was naturally given to the Spartan heavy infantry, 5000 strong, a veteran force wearing the famous red cloak and carrying in front of them the famous name of their city. Still they were not using the weapons of their ancestor but rather a gigantic 20 feet tall sarissa and a small shield, the famous spartan sword xiphos at their side as secondary weapon. Next to them on their left came the main aetolian phalanx, 5000 men with a high morale who had been training a lot under spartan guidance and had blooded themselves against the achean league's phalanx and were thus confident in their abilities. Then came the athenian phalanx, possibly the worst part of the allied line, the 2500 Athenian not having fought a major battle for a long time. Yet, thanks to the ptolemaic gold, they were stiffened by a force of 2500 veteran mercenaries and they were anchored to the side of the mountain so there was few chances that they could be turned by the enemy's right. Also they were protected by a larger force of light infantry than their counterparts on the right, in order to negate the power of the 1500 horsemen to their front. 500 of the allied light cavalry was also on their left flank to protect them, the 1500 other mounted javelin thrower and horse archer being kept with the heavy cavalry next to the spartan phalanx, under the orders of Eukleidas, Kleomenes' son and deputy commander.
In such a strong defensive position, the objective of the allied was to keep the line, exhaust the enemy forces by holding their ground while the macedonian had to walk under a storm of javelin, stones and arrows, until the cavalry could charge and hit at any weakness on the macedonian's front. All the allied leaders also remembered that Philip II and Alexander's victory in 338 B.C. had come from the overzealous Athenians pursuing the defeated Macedonian left flank, which had broken the front of the phalanx and allowed the victorious cavalry charge of Alexander : here they well intended not to move and not to pursue with heavy infantry as long as the Spartan king did not give the order to all the units.
A secret trick had also been prepared by Kleomenes for his enemy : masked by the huge numbers of light infantry, two shallow parallel trench had been dug just in front of the allied phalanx and some caltrops droped between the two lines in front of the left wing and of the center in order to break any cavalry charge. Thus prepared, the battlefield had been transformed into a death trap for the Macedonians, similar to what Ptolemy I and Seleucos had done to defeat Demetrios at Gaza in 312 B.C.
The battle started early in the morning, about one hour after the sun had arisen on the horizon. The weather was rather cloudy, sparing the men of the usual heat of greek battlefields. Philip V, understanding that his enemy would not move from it's position, ordered the advance of his men. The youthful king wanted this battle to be a major boost to his reputation, already seeing himself as a new Alexander.
First the light infantry of Philip engaged the first line of the allied light infantry, with men running to throw their javelin toward the enemy lines or troops of archer darkening the sky with their arrows or groups of slinger sending their deadly stones and lead projectiles toward the hostile force, the objective of both group being to go through the opponent's screen to be able to pelt the phalanx. Here numbers made themselves felt and the allied first line was repulsed, streaming slowly toward it's lines and regrouping on the flanks on the hills, where they stood their ground, confident that no cavalry would be able to charge them and that the higher ground would allow them to repel the enemy's infantry.
Cheered by this first victory, the Macedonian light forces came against the second line of light forces screening the allied phalanx and stopped. Indeed Kleomenes had mainly put his javelin throwers in the first line, keeping his archers and slingers behind to act as a fixed defence and allow his men the time to pick their target, which they did with deadly efficiency. Soon the battlefield began to cover with the writhing bodies of the wounded or the still corpses of men pierced by arrows or having had their skull broken by heavier projectiles.
Seeing this, Philip V ordered his phalanx to march, confident that this screen of light infantry would not want to be pinned between the two lines of sarrissae. The sight of those 15 000 men walking at a steady pace, in rhythm, their feet raising column of dust into the air, made more than one veteran sweat with fear, but the line held and the light infantry kept firing until the last possible moment. Then came the order to lower the sarrissae and the twenty feet long pikes went to the horizontal as a great cheer came from the lungs of the two armies, and the macedonians accelerated for the last few meters. And there the two trenches, only 50 cm deep and 1m wide, began their destructive office, men stumbling and falling forward under the weight of their ungainly weapons, not having even seen the trap set before them due to them being focused on their enemies and their weapons. Other fell due to the caltrops, cripled, and were tramped by their fellows.
Finally the two lines met and the pushing contest began. Some 30000 men began to try to stab the other with the points of their pikes, almost unable to reach the enemy, taking their two hands to try to control the massive weapon, with the allies having a bit more of an easy time due to the disorganization of the enemy phalanx during the last few meters.
During this time the cavalry of both side had kept quite, no side seeing any opportunity to strike. The plain simply wasn't wide enough to allow a flanking manoeuvre at this point. It was about to change.
Indeed when coming to the contact, the Macedonian phalanx had the natural move toward the right caused by the will of the men to get some protection from their friend's shields : this caused a slight space to open which gave an opportunity for a charge that Eukleidas did not fail to seize, sending his heavy cavalry at the front of his move to open the ranks of the enemy's flanking light infantry and cavalry, his light cavalry following with order to escape from the melee as soon as possible and to strike the back of the enemy phalanx.
In front of them the 500 heavy horsemen Philip had left to protect his flank did their best to keep the allies from breaching their line but failed under the sheer enthusiasm of Eukleidas and his men, also being harassed by allied light infantry coming down from the hills. Breaking under the pressure, they gave the allied cavalry the room to rearrange themselves, the 2500 horsemen intending to strike on the back on the enemy infantry. This was without taking Philip V into account. The king, with his cavalry impotent on the right of his force, was a bit behind his lines and saw what was happening on his left flank. Immediately he ordered his 1500 heavy horsemen to charge to the help of their beleaguered friends and bore down onto Eukleidas and his men. With only about 1000 heavy cavalry on already somewhat tired horses with whom to face them, Eukleidas had to win time and to disorganize their charge. He thus ordered his 500 horse archers, Scythians who usually did the police inside Athens, to attempt to break the enemy force while the 1000 light cavalrymen he still had would attack the rear of the force opposing the spartan infantry, giving the agreed signal for the general push of the allied phalanx, while his outnumbered force would try to neutralize Philip's cavalry with the help of some light infantry.
Immediately the plan was put into motion and thousands of horses began riding toward the fight. The Scythians fired arrows at almost point blank range into the Macedonians and then the two heavy cavalry forces collided in a clash of steel and flesh, horses and men clamoring to the sky.
Nervous macedonian pikemen from the rear files phalanx turned nervous heads toward the momentous clash and saw the masses of allied light cavalry falling down on them, making many panic. As news of the struggle happening in their back came to the ears of the phalangist many let their weapon fall and began to run, hoping to reach the safety of their camp before the cavalry or the fearsome spartans became free to run them down. First a trickle, the move became a flood, a torrent of infantrymen falling back and abandoning their king, the whole left flank of Philip collapsing in a matter of minutes.
Seeing this Kleomenes ordered the general progress for all his line. With a great booming cry the allies took a step forward, then another, pressing into the back of their comrades to help them overcome the enemy phalanx. The pressure grew onto the beleaguered Macedonians of the firsts few ranks who then collapsed, many falling on their back before being trampled or stabbed with the back pike of allied sarrissae.
The general movement could not be checked anymore and the whole macedonian army collapsed, followed by the allied who did not even need to watch their step when they came to their traps because the trampling of thousands of men had raised enough dust to fill the ditches and put the caltrops underground. The light cavalry of Kleomenes went through the fields killing hundreds of fleeing Macedonians.
Seeing the disaster during a brief lull in the heavy cavalry fight, Philip V understood he had lost and ordered the retreat before fleeing north. The victory of Kleomenes was total but still the king did not feel joy in his heart. Indeed his own son had fallen to the Macedonian swords and lay dead on the field, plunging his spartan father in a deep grief.
The massacre went on for hours, the grieving king ordered that no quarter be given to the vaincquished foe, their blood being to flow in sacrifice for his dead son. At the end of the day Philip had lost 300 horsemen, 4000 light infantrymen and 5000 men of his phalanx, a disaster of enormous proportions. Kleomenes had lost some 100 cavalrymen, 1000 light infantrymen and about 500 pike-welding soldiers of his phalanx, among whom only 10 spartan homoioi.
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I did not forget the Achean league, whose forces were also present at Sellasia next to Antigonos, it's future strategos Philopoimen commanding the cavalry forces of the Macedonian army. Yet here I did not introduce him yet because I to introduce him in my next part, which comes right behind this answer to your comments