In 235 BC, Cleomenes III (r. 235–222 BC) ascended the throne of Sparta and began a program of reform aimed at restoring traditional Spartan discipline while weakening the influence of the ephors, elected officials who, though sworn to uphold the rule of Sparta's kings, had by the time of Cleomenes come to wield extraordinary political power in the Spartan system.

Cleomenes eventually invaded Achaea, seizing control of Corinth and Argos, but was forced to retreat to Laconia when Antigonus arrived in the Peloponnese.

[10] Once this news reached Sparta, the ephors sent Cleomenes out again; he managed to capture the Megalopian city Methydrium before ravaging the territory surrounding Argos.

Meanwhile, Ptolemy III of Egypt, who had been an ally of the Achaean League in their wars against Macedon, shifted his financial support to Sparta.

Ptolemy made this decision after calculating that a resurgent Sparta would be a more valuable ally against Macedon than a failing Achaean League.

The Eleans appealed to Sparta for aid; as the Achaeans were returning from Elis, Cleomenes attacked and routed their entire army near Mount Lycaeum.

Cleomenes contended they could then make the ephors' property common to all citizens and work toward the achievement of Spartan supremacy in Greece.

Expanding the citizen population meant that Cleomenes could build a larger army; he trained 4,000 hoplites and restored the old Spartan social and military discipline.

Cleomenes completed his reforms by placing his brother, Eucleidas, in charge, making him the first Agiad king on the Eurypontid throne.

[20] The Achaeans were demoralized by this battle; Aratus declined the generalship, and when both Athens and the Aetolian League turned down their appeals, they sued Cleomenes for peace.

[21] Initially, Cleomenes advanced only minor requests, but as the talks continued, his demands became greater and he eventually insisted that leadership of the League be surrendered to him.

While marching there, Cleomenes drank too much water, which caused him to lose his voice and cough up blood—a situation that forced him to return to Sparta.

[27] Achaea was now in turmoil, and some cities were close to revolt; many residents were angered at Aratus' decision to invite the Macedonians into the Peloponnese.

Encouraged by this development, Cleomenes invaded Achaea and seized the cities of Pellene, Pheneus, and Penteleium, effectively splitting the Achaean League in half.

[28] The Achaeans, concerned about developing treachery in Corinth and Sicyon, dispatched their mercenaries to garrison the cities and then went to Argos to celebrate the Nemean Games.

Cleomenes sent his stepfather Megistonous to Aratus, asking for the surrender of Acrocorinth—the citadel of Corinth, which had an Achaean garrison—in return for a large amount of money.

[28] He sent a messenger to Aratus proposing that Acrocorinth should be garrisoned jointly by both the Spartans and the Achaeans, and that he would deliver a twelve talent pension.

[33] The hostile Aetolian League occupying parts of Thessaly had threatened to oppose him if he went further south than Thermopylae, despite their neutrality at that point in the war.

When Cleomenes heard of the Macedonian advance through Euboea, he abandoned his siege of Sicyon and constructed a trench and palisade running from Acrocorinth to the Isthmus.

[35] Despite numerous attempts to break through the defensive line and reach Lechaeum, Antigonus' force failed and suffered considerable losses.

Megistonous was killed while assaulting the city, however, and the relief force retreated, leaving the Spartans in the citadel to continue resistance.

Cleomenes ordered his army to take five days' worth of rations and sent his troops toward Sellasia, to give the appearance of raiding the territory of Argos.

From there he went to the territory of Megalopolis; during the night he ordered one of his friends, Panteus, to capture the weakest section of the walls, while Cleomenes and the rest of the army followed.

[45] Walbank describes this raid as being "an impressive demonstration, but it had no effect other than to make it even more clear that Cleomenes had to be defeated in a pitched battle.

Cleomenes positioned his brother, Eucleidas, with the allied troops and the Perioeci on Evas; he stationed himself on Olympus with 6,000 Spartan hoplites and 5,000 mercenaries.

[51] However, at the critical moment, Philopoemon of Megalopolis (who later became one of the greatest heroes of the Achaean League, eventually conquering Sparta), tried to point out the danger to the senior cavalry commanders.

The Spartans' left flank was eventually forced back and thrown from their position and their commander, Eucleidas, killed;[46] they fled the field.

The Spartans, overwhelmed by the deeper ranks of Macedonian phalanx, were routed, but Cleomenes managed to escape with a small group of men.

Under cover of darkness, he fled from Sparta with some friends and went to the city's port of Gythium, where he boarded a ship heading to Egypt.

[55] Within three days, he left Sparta and returned to Macedon to deal with a Dardani invasion, leaving a garrison in Acrocorinth and Orchomenos.