In 362, he showed great courage in the defense of Sparta against the Theban commander Epaminondas. As king, Archidamus supported the Phocians against Thebes in the Sacred War of 355–346. In 346 BC he went to Crete to help Lyttos in their struggle against Knossos in the Foreign War. In 343 BC, the Spartan colony Tarentum asked for Sparta's help in the war against the Italic populations, notably the Lucanians and the Messapians. In 342 BC Archidamus arrived in Italy with a fleet and a mercenary army and fought against the barbarians, but in 338 BC he was defeated and killed under the walls of Manduria. He was succeeded by his son Agis III, and was also the father of Eudamidas I and another son named Agesilaus.