Aratus of Sicyon in the context of "Parallel Lives"

The Cleomenean War (229/228–222 BC) was fought between Sparta and the Achaean League for the control of the Peloponnese. Under the leadership of king Cleomenes III, Sparta initially had the upper hand, which forced the Achaean League to call for help the Macedonian king Antigonos Doson, who decisively defeated Cleomenes in the battle of Sellasia in 222.

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. When, in 229 BC, the ephors sent Cleomenes to seize a town on the border with Megalopolis, the Achaeans declared war. Cleomenes responded by ravaging Achaea. At Mount Lycaeum he defeated an army under Aratus of Sicyon, the strategos of the Achaean League, that had been sent to attack Elis, and then routed a second army near Megalopolis.

Sicyon (/ˈsɪʃiˌɒn, ˈsɪs-/; Greek: Σικυών; gen.: Σικυῶνος) or Sikyōn was an ancient Greek city state situated in the northern Peloponnesus between Corinth and Achaea on the territory of the present-day regional unit of Corinthia. The ruins lie just west of the modern village of Sikyona (previously Vasiliko). An ancient monarchy at the times of the Trojan War, the city was ruled by a number of tyrants during the Archaic and Classical period and became a democracy in the 3rd century BC. Sicyon was celebrated for its contributions to ancient Greek art, producing many famous painters and sculptors. In Hellenistic times it was also the home of Aratus of Sicyon, the leader of the Achaean League.

Cleomenes III (Ancient Greek: Κλεομένης) was one of the two kings of Sparta from 235 to 222 BC. He was a member of the Agiad dynasty and succeeded his father, Leonidas II. He is known for his attempts to reform the Spartan state.

From 229 to 222 BC, Cleomenes waged war against the Achaean League under Aratus of Sicyon. After being defeated by the Macedonians in the Battle of Sellasia in 222 BC, he fled to Ptolemaic Egypt. After a failed revolt in 219 BC, he committed suicide.

The Battle of Sellasia took place during the summer of 222 BC between Macedon and the Achaean League, led by Antigonus III Doson, and Sparta under the command of King Cleomenes III. The battle was fought at Sellasia on the northern frontier of Laconia and ended in a Macedonian-Achaean victory.

In 229 BC, Cleomenes initiated hostilities against the Achaean League, the dominant power of the Peloponnese. In a series of campaigns, Cleomenes was successful in defeating the Achaeans, making Sparta the main regional power. This prompted the chief figure of the Achaean League, Aratus of Sicyon, to approach the King of Macedon, Antigonus III Doson, for military assistance. The Macedonians acquiesced on the terms that the Achaean surrender the formidable fortress of Acrocorinth to them. The Macedonians invaded the Peloponnese in 224 BC at the head of a Greek alliance and by 222 BC managed to hem Cleomenes in Laconia.

The Battle of Mount Lycaeum was fought between Sparta led by Cleomenes III and the Achaean League commanded by Aratus. It was the first major battle of the Cleomenean War. The battle occurred at Mount Lycaeum on the border of Elis and Arcadia and ended in a Spartan victory.