Agis was born as the son of king Archidamus II of Sparta, and succeeded him after his death. The precise date is not recorded in ancient sources. Since 431, Agis' father had conducted several campaigns against the Athenians - who called this war the Archidamian War - and is known to have invaded Attica in 428. He is not mentioned in our sources for 427, when the Agiad prince Cleomenes led the annual invasion of Attica; in 426, Agis is mentioned as king. It is common to say that he succeeded Archidamus in 427, but it may have been late in 428 or early in 426.
At the same time, the exiled Agiad king Pleistoanax returned, so Sparta had two kings again, as was normal. But Agis was still young and inexperienced. His first campaign to Attica, in 426, was aborted after evil omens, and the Spartans thought it wise to appoint a guardian.
It was under these circumstances, with no obvious war leader, that the Spartans embarked upon new strategies. General Brasidas was to invade Thrace and cut off Athenian supply lines. At the same time, the Athenian commander Demosthenes had success at Sphacteria, where Spartan soldiers surrendered and became hostages. If Agis had wanted to continue his father's policies, it had now become impossible, and the invasion of Attica of 425 was aborted after fifteen days. Brasidas was to be the hero of the last years of the Archidamian War, but when he was killed in action at Amphipolis, the two warring states were ready to make peace. The Peace of Nicias was signed in March 421; Agis and Pleistoanax were among those who swore the oaths.
The Peace of Nicias marked, in fact, an Athenian victory. The Spartans had gone to war to destroy the Athenian empire, the Delian League, and had miserably failed. In fact, they had also betrayed their own allies of the Peloponnesian League, because the Peace of Nicias was hardly beneficial to Corinth. The Thebans even refused to sign. Sparta's authority had severely suffered, and the Athenians benefited by concluding an alliance with several democratic towns on the Peloponnese, in Sparta's backyard: Elis, Mantinea, and Argos. The architect of this assertive diplomacy was Alcibiades.
Agis counteracted by preparing an attack on a town that the historian Thucydides calls Leuctra and may or may not be identical to the town in Boeotia. If so, it was a strange operation outside Sparta's direct sphere of interests. We will never know what was its aim, because the operation was postponed after unfavorable omens in the early summer of 419. Meanwhile, the Argives attacked Epidauros, who asked for help from Sparta. Agis marched to the Argive plain, and returned after -again- evil omens. Still, the operation prevented the Argives from continuing to ravage the territory of Epidauros, and showed the Athenians that an alliance with Argos could be dangerous. The Athenians understood and in 418, Alcibiades was not elected; instead, Nicias, who had a reputation for cooperation with Sparta, was among the magistrates.
It seems that Agis tried to prevent war with Argos, which could escalate to a war with Athens. Next year, he again followed this policy. He invaded the territory of Argos, and although he was able to isolate the Argive army, he unexpectedly offered an armistice of four months. This was probably a way to open negotiations and lure Argos away from its Athenian alliance. Perhaps the rich, oligarchic-minded people of Argos would even overthrow the Argive democracy. However, the Argives did not respond as Agis had been hoping.
On the contrary. When a small Athenian army arrived, together with ambassador Alcibiades, the Argives listened to his advise to attack the pro-Spartan town Orchomenus, which was situated along the road from Corinth to the southern Peloponnese. The Mantineans and Eleans joined the siege and Orchomenus was forced to surrender. It was absolutely clear that in spite of Agis' moderation, the quadruple alliance was still very much alive.
The Spartans now regretted their policy, for which they blamed Agis. He was fined and would even have lost his house if he had not accepted ten permanent advisers. Without their consent, he could not act.
Meanwhile, the situation was escalating. Late in August 418, the Athenian allies proceeded against Tegea. The Spartans could not allow this town to fall into enemy hands, because that would in fact isolate them in the southern Peloponnese. Diplomatic solutions were no longer possible and Agis marched to the north with a very large army. This forced Athens to choose between either peace with Sparta or its treaty with Argos, Mantinea, and Elis. As it turned out, Athens preferred the second option and after some maneuvers, Agis forced the allies to fight. In September, a battle was fought at Mantinea, and the Spartans overcame the army of Athens, Mantinea, and Argos (text). This was the end of both the quadruple alliance and the peace between Athens and Sparta. From now on, both sides were waiting for an opportunity to renew the war.
The Athenians now attempted to expand their empire. They supported the rebel Amorges against the Achaemenid king Darius II Nothus and tried to conquer Sicily. The Sicilian Expedition, however, was a disaster, and supporting Amorges merely meant inviting Persian help for the Spartans. Worse, in 413 the Spartans saw an opportunity to renew the war (the Decelean or Ionian War). Agis occupied the town of Decelea in Attica, from where he systematically looted the Athenian countryside. Decelea was also a fine base for operations in Central Greece; for example, Agis once marched to the Malian Gulf in the west.
It was rumored that the idea to fortify Decelea had proposed by Alcibiades, who had switched sides and supported Sparta after 413. This is not very plausible: the Spartans were quite capable of developing a strategy without help of an Athenian turncoat. Nor do we have to believe that Alcibiades had an affair with Agis' wife Timaea and was the father of her son Leotychidas, which is mere political gossip. However, the rumors were a political fact, and so was the enmity between Agis and Alcibiades. The Athenian could not win a conflict with a king, and decided to leave Sparta.
By now, Agis was no longer supposed to have ten advisers, but he was apparently not allowed to pursue an independent foreign policy, because he could not make decisions about Athenian peace offers in 411 and 404. Still, he played an important role in the war. The Athenians no longer controlled their countryside, and sometimes, Agis tried to capture Athens by surprise.
In 405, the Spartan admiral Lysander defeated the Athenian navy near Aigospotamoi and the Spartans laid siege to Athens, which surrendered in April 404 (text).
Now that Athens was defeated, the Spartans waged war against the Athenian allies. Agis invaded and ravaged Elis, where he forced the Eleans to allow give more freedom to the towns in their neighborhood. Agis also visited Delphi, where he had dedicated a part of the spoils of the war against Athens to the god Apollo, but on his way back, he died.
Because it was seriously doubted that Agis was the biological father of his wife's son Leotychidas, the young man could not succeed his father. Therefore, Agis was succeeded by his younger half-brother Agesilaus II.