III, son of Leonidas, took on the reforms of Agis.
Less squeamish than his predecessor, in 227 the opposition in Sparta were removed in a coup, 4 ephors were
killed and eighty opponents exiled. Aratos was checked in a series of rapid and bold campaigns.
The land was redistributed into 4000 lots and perioeci and hypomeinones were allowed to participate.
The army was reorganized on the Macedonian model and a small subsidy from Ptolemy was paid. This social revolution provided Cleomenes with both the means to restore Spartan greatness, a general support in the rest of the Peloponnese and the reason for its eventual failure.
A united Peloponnese under a reformed and revitalized Sparta could have been a power in the Hellenistic world and a challenge to Macedonia. In 226 the 'New Model' army continued to have success, most of Arcadia, Corinth, Argos and the Argolid succumbed to him or joined him and the Achaeans were routed at Dyme.
The Achaeans under Aratos who had once cleared the Peloponnese of the Macedonians now appealed to Antigonos Doson of Macedon with Corinth as the price. Ptolemy Euergetes now stood aside and stopped the subsidy, Sparta now stood alone. Inevitable it may have been, a Macedonian victory would not be simple.
In 224 Cleomenes fortified the isthmus but his position was turned with the defection of Argos. Much of Arcadia was lost in 223 but Megalopolis was destroyed by Cleomenes. Short of money and men Cleomenes sold 6000 helots their freedom, 2,000 were incorporated in the new style phalanx, and hired more mercenaries. For his last campaign in 222 he faced 30,000 Macedonians with 20,000 at Sellasia on the road to Tegea, despite a well chosen position and skilful handling the Spartans were overwhelmed. Plutarch says that of 6000 Lacedaemonians present only 200 survived. Cleomenes was killed in exile in 219.