While still a prince, he was the eispnelas (εἰσπνήλας, inspirer, or pederastic lover) of Cleonymus, son of Sphodrias.
Archidamus headed the force sent to aid the Spartan army after its defeat by the Thebans at the Battle of Leuctra in 371 BC, and was commander later during the fighting in the Peloponnese.
Four years later he captured Caryae, ravaged the territory of the Parrhasii and defeated the Arcadians, Argives and Messenians in the "tearless battle", so called because the victory did not cost the Spartans a single life.
[2] In 362 BC, he showed great courage in the defense of Sparta against the Theban commander Epaminondas.
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.