For the genus, see Diaeus (genus)
Diaeus of Megalopolis (Διαῖος) (died 146 BC) was the last general of the Achaean League in Ancient Greece before the League was disbanded by the Romans. He served as the League's general from 150–149 BC and from 148 BC until his death.
Diaeus succeeded Menalcidas of Sparta as general of the Achaean League in 150 BC. Menalcidas, having been charged by Callicrates with a capital offence, saved himself by gaining the support of Diaeus, whom he bribed with three talents. Diaeus was generally condemned for this. So to try to divert public attention away from his own conduct, he sought a quarrel with Lacedaemon.
The Lacedaemonians had appealed to the Roman senate over the possession of some disputed land. In response, the Roman senate had said that decisions on all causes, except those of life and death, rested with the great council of the Achaeans. Diaeus decided to ignore the exception included in the Roman response.
The Lacedaemonians accused him of lying and the dispute led to war. However, the Lacedaemonians found themselves no match for the Achaeans, and they sought to negotiate. Diaeus, affirming that his hostility was not directed against Sparta as a whole, but against those who had caused the disagreement, arranged for the banishment of 24 of Sparta's principal citizens. These men fled to Rome and sought Rome's refuge and protection. Diaeus travelled to Rome to oppose their claims, together with Callicrates, who died along the way.
The cause of the Spartan exiles was supported by Menalcides, who assured the Spartans, on his return, that the Romans had declared in favour of their independence, while an equally positive assurance to the opposite effect was given by Diaeus to the Achaeans. In truth, the Senate had made no final decision, but had promised to send commissioners to settle the dispute.
In 148 BC, war was renewed between the parties despite such action being prohibited by the Romans. However, Diaeus, who was again general of the Achaean league in 147 BC, did seek to obey Rome's prohibition by endeavouring to bring over the towns around Sparta by negotiation. When the decree of the Romans arrived, which severed Sparta and several other states from the Achaean league, Diaeus took a leading part in building Achaean indignation and in urging them to the acts of violence which caused war with Rome.
In autumn 147 BC, Diaeus was succeeded as general of the Achaean league by Critolaus. But the death of the latter before the expiration of his year in that office once more placed Diaeus in the key position. (This outcome was based on the law of the Achaeans, which provided in such cases that the predecessor of the deceased should resume his authority.) The size of Diaeus' army was increased with emancipated slaves and by the levy of the citizens, which Diaeus enforced strictly, though not impartially.