After conquering the Acrocorinth, Aratus pursued the Achaean League's expansion. When the Spartan king Cleomenes III conquered the Achaean cities of Argos and Corinth, Aratus succeeded in securing an alliance with his erstwhile enemy, Macedon. Cleomenes III was defeated at the Battle of Sellasia by the joint forces of the Achaean League and Antigonus III Doson, the regent of Macedon. During the Social War against the Aetolian League, Aratus became one of the prime advisors of the new king of Macedon, Philip V. Aratus died in 213 BC, allegedly poisoned by Philip.