In 222 B.C. Cleomenes received word that his Egyptian aid was being cut off. The reasons for this are unclear; perhaps the Egyptians were more concerned with the Seleucid threat - Raphia being only five years in the future. Facing an imminent Macedonian invasion of Laconia , he decided to offer battle. In a narrow pass near the Spartan town of Sellasia, flanked by two hills, he drew up his army. Cleomenes probably felt that his numerically inferior forces could adequately defend the hills against the attacking Macedonians. In addition, if Antigonos instead attempted to force the pass, he could wheel and charge down into their flanks from both sides.
The Macedonian forces consisted of 10,000 to form the phalanx, 3,000 Chalkaspides (bronze-shields) armed as peltasts?, and 300 horse. He had besides a 1,000 Agrianians, and a 1,000 Gaul's, while his mercenary force numbered 3,000 foot and 300 hundred horse. The Achaeans furnished 3,000 picked infantry and 300 horse. There were also a 1,000 Megalopolitans armed in the Macedonian manner under the command of Cercidas of Megalopolis. The allies consisted of 2,000 Boeotian foot and 200 horse, a 1,000 Epirot foot and 50 horse, the same number of Acarnanians, and 1,600 Illyrians under the command of Demetrius of Pharos. His total force thus amounted to 28,000 foot and 1,200 horse. Cleomenes, who expected the invasion, had occupied the other passes into Laconia, placing garrisons in them and fortifying them by means of trenches and barricades of trees, and himself encamped at a place called Sellasia, with a force of 20,000 consisting of 6,000 Lacedaemonians armed in the Macedonian fashion, probably 8,000 of perioeci and allies armed as hoplites, 5,000 mercenaries an light troops and 1,000 horse.
At the actual pass there are two hills, one called Euas and the other Olympus, the road to Sparta running between these along the bank of the river Oenous. Cleomenes, having fortified both of these hills with a trench and palisade, posted on Euas the perioeci and allies under the command of his brother Eucleidas, while he himself held Olympus with the Spartans and mercenaries. On the low ground beside the river on each side of the road he drew up his cavalry and a certain portion of the mercenaries. The key to the Spartan position was therefore the hill of Euas. If it was held Cleomenes could pivot on it and if the need arose have a way of exiting.
To confront those on Euas, Antigonus drew up the bronze-shields-shielded Macedonians and the Illyrians in alternate lines, placing them under the command of Demetrius of Pharos. These were drawn up in alternate formation with his Illyrian forces which had occupied the base of Euas the night before. Behind these stood the Acarnanians and Cretans, and in the rear as a reserve were two thousand Achaeans. His cavalry he opposed to that of the enemy by the river Oenous under the command of Alexander son of Acmetus and supported by a thousand Achaean and as many Megapolitan infantry. He himself in person decided to attack Cleomenes on Olympus with the mercenaries and the rest of the Macedonians. Putting a screen of light mercenaries in front, he drew up the Macedonians behind them in two phalanxes 32 deep with no interval between, the narrowness of the space rendering this necessary. Antigonos drew up the allied army in an interesting fashion. He placed his own cavalry opposite the enemy centre under Alexander son of Acmetus.
Antigonus on his arrival observed the great natural strength of the position and how Cleomenes had cleverly occupied the advantageous points with the portions of his force suitable in each case. Antigonus therefore decided to make no hasty attempt to force the position and come to blows with the enemy, but encamped at a short distance with the river Gorgylus on his front, and for several days remained there noting the peculiar features of the country and the character of the forces, while at the same time, by threatening certain movements, he attempted to make the enemy show his hand. But being unable to find any weak or unprotected spot, since Cleomenes always checked him at once by a counter-movement.
The battle led off with the light forces charging up Euas, followed by the heavier infantry. The mercenaries who were supporting the Spartan cavalry noted that the flank and rear of the Macedonian right wing was vulnerable, and the Macedonian cavalry was not making any move. Accordingly, they slammed into the rear of the Macedonian right wing. The cavalry, a mixture of Macedonians, Achaeans, Boeotians, and mercenaries under command of Alexander, made no move. At this point, according to Polybius, Philopoeman of Megalopolis , saw what was happening and urged Alexander to charge. Alexander ignored him, perhaps because of his youth. Philopoeman urged his fellow citizens to follow him and charged the Spartan cavalry. Enough horsemen followed him to create a diversion and force the Spartan mercenaries to turn back to the defence of the cavalry, allowing the right wing to continue their advance against Eucleidas.
Eucleidas in the meantime, was apparently suffering from the same bout of inaction which afflicted Alexander. Instead of counter-charging the right wing as it came up the hill in some disorder, he stood his ground and waited for their attack. This respite allowed the Macedonians to regroup and charge afresh. The staggered formation was able to pin the Perioeci and allies until the heavy infantry could slam into them. Eucleidas defended on the summit of the hill, so once he was forced back, it was downhill and the Macedonians gained momentum. After some savage fighting, Eucleidas’ forces disintegrated into a retreat over broken ground, during which most of them were killed.
On the hill of Olympus, the light units of both kings engaged with equal valour, but neither side could force the other back. Cleomenes, seeing his brother's troops in flight and the cavalry on the level ground on the point of giving way, was afraid of being attacked from all sides and was compelled to pull down part of his defences and to lead out his whole force in line from one side of the camp. Cleomenes now committed his whole phalanx to the attack. A stubborn struggle followed. At one time the Macedonians gradually fell back facing the enemy, giving way for a long distance before the courage of the Lacedaemonians, at another the latter were pushed from their ground by the weight of the Macedonian phalanx, until, on Antigonos ordering the Macedonians to close up in the peculiar formation of the double phalanx with its serried line of pikes, they delivered a charge which finally forced the Lacedaemonians from their position. The whole Spartan army now fled in rout, followed and cut down by the enemy; but Cleomenes with a few horsemen reached Sparta in safety. At nightfall he went down to Gythion, where all had been prepared some time previously for the voyage in view of contingencies, and set sail with his friends for Alexandria.
According to Plutarch, all but 200 Spartans perished, an exaggeration surely.
Should have Cleomenes committed his phalanx now that his position was turned by the capture of Euas. With the hill lost he had no good line of retreat and a victory was the only real alternative. Any chance however small is better than no chance.