The Cleomenean War (229 BCE/228 BCE – 222 BCE) is fought by Sparta and its ally, Elis, against the Achaean League and Macedon.

The war ends in a Macedonian and Achaean victory.In 235 BC, Cleomenes III (r. 235–222 BCE) ascends the throne of Sparta and begins a program of reform aimed at restoring traditional Spartan discipline while weakening the influence of the ephors, elected officials who wield extraordinary political power, although they have sworn to uphold the rule of Sparta's kings.

When, in 229 BCE, the ephors send Cleomenes to seize a town on the border with Megalopolis, the Achaeans declare war.

Cleomenes responds by ravaging Achaea.

At Mount Lycaeum, he defeats an army under Aratus of Sicyon, the strategos of the Achaean League, that had been sent to attack Elis, and then routs a second army near Megalopolis.

Meanwhile, in domestic politics, he orders the assassination of the ephors.In quick succession, Cleomenes clears the cities of Arcadia of their Achaean garrisons, before crushing another Achaean force at Dyme.

Facing Spartan domination of the League, Aratus is forced to turn to Antigonus III Doson (r. 229–221 BCE) of Macedon and request that he assist the Achaeans' efforts to defeat the Spartans.

In return for Macedonian assistance, the Achaeans are obliged to surrender the citadel overlooking Corinth to Antigonus.

Cleomenes eventually invades Achaea, seizing control of both Corinth and Argos.

When Antigonus arrives in the Peloponnese, however, Cleomenes is forced to retreat to Laconia.

He fights the Achaeans and the Macedonians at Sellasia, but the Spartans are routed.

Cleomenes then flees to the court of his ally, Ptolemy III of Egypt (r. 246–222 BCE), where he ultimately commits suicide in the wake of a failed revolt against the new Pharaoh, Ptolemy IV (r. 221–205 BCE).