Agis III succeeded his father on 2 August338 BC, on the very day of the battle of Chaeronea. His reign was short, but eventful, coming as it did during a low period for Sparta, after it had lost significant borderlands to Philip II of Macedon, father ofAlexander the Great. , Achaemenid satrap of Lydia, against Alexander the Great. In 333 BC, Agis went with a single trireme to the Persiancommanders in the Aegean, Pharnabazus and Autophradates, to request money and armaments for carrying on hostile operations against Alexander the Great in Greece. The satraps agreed to support Agis; however, they could only spare him 30 talents and 10 ships. The news of the battle of Issus in 333 BC, however, put a check upon their plans. He sent his brother Agesilaus with instructions to sail with them to Crete, that he might secure that island for the Spartan interest. In this he seems in a great measure to have succeeded.
Two years after this Spartan success, the Greek states which were in league against Alexander seized the opportunity that had risen from the military disaster of the Macedonian general Zopyrion's campaign against the Scythians, combined with the Thracian revolt, to declare war against Macedonia. Agis was invested with the command and with his Spartan troops, and a body of 8000 Greek mercenaries who had been present at the battle of Issus, gained a decisive victory in the Peloponnese over a Macedonian army under Coragus. Having been joined by the other forces of the league, Agis laid siege to Megalopolis. The city held out until Antipater came to its relief. In the subsequent Battle of Megalopolis, Agis' army fought a larger Macedonian force but was finally defeated, Agis himself died trying to gain his surviving men time to withdraw to safety. On the manner of his death, Diodorus comments: .
He had fought gloriously and fell with many frontal wounds. As he was being carried by his soldiers back to Sparta, he found himself surrounded by the enemy. Despairing of his own life, he ordered the rest to make their escape with all speed and to save themselves for the service of their country, but he himself armed and rising to his knees defended himself, killed some of the enemy and was himself slain by a javelin cast. He had reigned nine years.
