The antecedents of Archidamus are mentioned by the Greek researcher Herodotus of Halicarnassus:
King Leotychidas had a son Zeuxidamus, whom some of the Spartans called "puppy". This man did not become king of Sparta, because he died before Leotychidas, leaving a son Archidamus. After the loss of Zeuxidamus, Leotychidas married a second wife Eurydame [...], by whom he had no male issue, but a daughter Lampito, whom Archidamus the son of Zeuxidamos took in marriage.note[Herodotus, Histories 6.71.]
The year of Zeuxidamus' death is not known, but we know that Leotychidas II was forced to leave Sparta, probably in 475, to live in exile in Tegea, because an expedition to Thessaly had ended without success and he had been accused of accepting bribes. It is not clear whether he remained king, and it is possible that Archidamus' rule started as early as 475. Alternatively, he became king when his exiled grandfather had died in 469.
Because ancient sources usually focus on military events, we know most about Archidamus' military career. In 464, he proved himself a capable commander in the war against the helots, who revolted when a terrible earthquake destroyed Sparta. Immediately after the disaster, Archidamus led the survivors out of the town, organized them as army, and forced a group of rebellious helots, who wanted to attack Sparta, to return to their homes. It is possible that he also played a role in the confused negotiations with Athens: the Spartans asked reinforcements from that city, but once they had arrived, they were sent back, after which the insulted Athenians declared war and sided with the Messenians. The latter held out until 460, and the war with Athens lasted until 445.
Archidamus' colleague as king was Pleistoanax, who was sent into exile in 445. As a result, Archidamus was for some time sole king of Sparta. During the peace negotiations with Athens, he became friends with the enemy leader, Pericles.
The conflict with Athens was renewed in 431. During the negotiations that resulted in the declaration of war, Archidamus warned the Spartans and their allies against ill-conceived judgments and tried to prevent or postpone the conflict, which, he said, "they would bequeath to their children". He pointed out that the Spartans needed a navy, because as long as Athens controlled the sea, the city could not be defeated. Other Spartans, like the ephor Sthenelaedas, were convinced that war was inevitable and argued that Sparta should strike immediately. Athens, they argued, could be defeated if the Peloponnesians wasted the Athenian countryside; it was impossible that their enemy would hold out for more than two or three years.
And the war came. In the summer of 431, king Archidamus led the Peloponnesian forces to Attica, where he laid waste the countryside. This was repeated in 430 and shocked the Athenians so profoundly, that they called the conflict the Archidamian War. However, as Archidamus had already predicted, it was not a successful strategy against a city with large walls that could be supplied from the sea. Sparta needed a navy and the spectacular naval victories of the Athenian Phormio suggested that getting even at sea would be difficuly.
In 429, Archidamus laid siege to Plataea, and in 428, he invaded Attica again. He must have died in the summer of 427, although the winter cannot be excluded. In any case, his son Agis II was on the throne in the summer of 426. He was still young and inexperienced, so a guardian was appointed. It was under these circumstances, with no obvious war leader, that the Spartans embarked upon new strategies. General Brasidas was to invade Thrace, and hit the Athenians where it hurt.
Archidamus had predicted that the war would be bequeathed to the children. He was right. The Peloponnesian War lasted until 404 and in the end settled nothing. Athens was ultimately defeated, but recovered very soon.