Demaratus was born into the Eurypontid royal house of Sparta, probably in the later 6th century BCE. His father was King Ariston, and his mother had previously been the wife of a Spartan named Agetus. Herodotus makes the story of Demaratus' birth a study in rumour, oath and dynastic uncertainty. Ariston had already married twice without producing a child. He then became captivated by the extraordinary beauty of the wife of his friend Agetus and took her for himself through a deception. When she bore a son less than ten months after the marriage, Ariston counted the months and swore before the ephors that the child could not be his. Later he repented and accepted Demaratus as his son. The child's name was explained as Wished by the People, because the Spartans had long prayed that Ariston might have an heir.

The story did not end there. Spartan gossip offered darker versions. Some said Demaratus was the son of a muleteer or palace servant who had access to the queen. His mother gave a different account when he asked her directly for the truth. She said that on the night she became Ariston's bride she had been visited both by her new husband and by Astrabacus, a local daimon or hero connected with the sanctuary of Helen at Therapne. If the story were true, Demaratus might have had a divine or heroic father as well as a mortal one. Herodotus reports these alternatives without settling them. What matters politically is that a plausible uncertainty existed and could be weaponised years later.

Demaratus succeeded Ariston around 515 BCE as the Eurypontid king. Sparta had two hereditary royal houses, the Agiads and the Eurypontids, each descended from the sons of Aristodemus. The Agiad house enjoyed greater ceremonial prestige because of primogeniture, but both kings held office simultaneously. Demaratus' Agiad colleague was Cleomenes I, one of the most aggressive and controversial Spartan rulers of the age.

The two kings first came into serious conflict over Athens. Cleomenes had helped remove the Peisistratid tyranny, then returned with an army to support Isagoras against the democratic reforms of Cleisthenes. Demaratus either opposed the second intervention or refused to support it. When the two kings led a Spartan force against Athens, Demaratus withdrew his contingent, leaving Cleomenes unable to continue alone. The expedition collapsed, Isagoras lost his power and Athens moved further toward the democratic constitution. Herodotus does not present Demaratus as a principled defender of Athenian freedom; he says the king acted from jealousy and malice toward Cleomenes. Yet the result was an important check on Spartan interference.

Their rivalry sharpened over Aegina. In 491 BCE the Aeginetans gave earth and water to the Persian king, formally acknowledging Persian supremacy. Athens demanded that Sparta punish them. Cleomenes crossed to Aegina and attempted to arrest the leading Medisers, but Demaratus spread hostile reports about the expedition and frustrated his colleague's plan. When Cleomenes returned, he resolved to destroy Demaratus politically.

The instrument was Demaratus' uncertain paternity. Leotychides, a Eurypontid relative and personal enemy, claimed the throne. His hatred had an old cause: Demaratus had seized Percalus, daughter of Chilon, before she could marry Leotychides and had married her himself. Leotychides now argued that Demaratus was not the son of Ariston and therefore not entitled to rule. The ephors had heard Ariston's old oath, and Leotychides summoned them as witnesses.

The dispute went to Delphi, where Cleomenes arranged the answer in advance. Through the influence of Cobon, a powerful Delphian, he persuaded the Pythian priestess Perialla to declare that Demaratus was not Ariston's son. Armed with this oracle, the Spartans deposed him and made Leotychides king. Later the fraud was exposed. Cobon was exiled from Delphi and Perialla lost her office. Many Greeks believed that Cleomenes' subsequent madness and terrible suicide were divine punishment for corrupting the oracle and wronging Demaratus. Herodotus records competing explanations while giving that religious interpretation particular weight.

After his deposition, Demaratus held an ordinary public office in Sparta. During the Gymnopaedia, Leotychides sent a servant to mock him: how did it feel to be a magistrate after being a king? Demaratus answered that he had experienced both conditions and Leotychides had not. He then made a darker prophecy, saying that the question suggested huge calamity rather than huge prosperity for Lacedaemon. Soon afterward he fled.

Herodotus tells the flight as a sequence of narrow escapes and divine signs. Demaratus left Sparta for Elis, pretending that he intended to consult Delphi. The Lacedaemonians suspected his plan, pursued him and seized some of his servants. From Elis he crossed to Zacynthus, where the islanders refused to surrender him. He then crossed to Asia and presented himself to Darius I, who received him generously and granted him lands and cities. Herodotus praises Demaratus as a man renowned among the Lacedaemonians for many deeds and counsels, especially for the time he won an Olympic chariot race but gave the public honour to the Spartan state, something no other Spartan king had done.

At the Persian court Demaratus found a new role. Herodotus says that when Darius' sons disputed the succession, Demaratus helped Xerxes by pointing to a Persian custom: the son born to a king after his accession should stand before an elder brother born before it. Xerxes was the first son born after Darius became king, while Artabazanes was the eldest but had been born while Darius was still a private man. The argument supported Xerxes' claim. Whether the episode is exact history or a courtly anecdote, it shows the tradition that Demaratus remained politically useful after his exile.

When Xerxes prepared to invade Greece, Demaratus was in a position of remarkable danger and influence. He owed his safety to Persia but his identity to Sparta. Herodotus reports that he sent warning to Sparta before the invasion. He wrote a message on the wood beneath the wax of a writing tablet, covered it again and sent the apparently blank object to Lacedaemon. There, according to the story, Cleomenes' daughter Gorgo suspected the trick and advised the recipients to scrape away the wax. The hidden message revealed Xerxes' intention to march against Greece. The anecdote, whether factual or not, became the classic image of Demaratus as the exile who still served his homeland.

During Xerxes' campaign of 480 BCE, Demaratus accompanied the Persian army as an adviser. At Doriscus, Xerxes reviewed his enormous host and asked Demaratus whether the Greeks would dare to resist. Demaratus answered that poverty had always been native to Greece, but excellence was acquired through wisdom and strong law. The Greeks, he said, would resist even if they had only a thousand men, and they would resist most strongly if they had only ten thousand. Xerxes laughed and asked whether free men would fight merely to avoid enslavement by another. Demaratus replied that the Greeks fought for freedom in a way the king did not understand.

His most famous explanation concerned the Spartans. Alone among the Greeks, they would not flee from any host, however large. They were free men, yet not entirely free, for they had Law as their master. This master was stronger than fear of Xerxes. Its command never changed: stand fast, conquer or die. Xerxes accepted the statement politely but did not believe it until the battle of Thermopylae proved it.

Before Thermopylae, Demaratus advised Xerxes to avoid a direct contest with the Spartan force in the pass. Instead, he recommended sending the Persian fleet to seize Cythera, the island off the Laconian coast, and from there to raid Spartan territory. He recalled the saying attributed to the wise Chilon that the island of Cythera was better in Spartan hands than on the mainland and warned that direct conflict would bring a war like no other. Herodotus says the Greeks later believed Xerxes would have conquered the Peloponnese if he had followed this plan. Xerxes rejected it after his brother Achamenes warned that a naval operation around Cythera would distract the fleet at the decisive moment.

When the Spartans and their allies held the pass and broke wave after wave of Persian attacks, Demaratus' earlier warnings acquired force. After the battle, Xerxes called him and asked how many Spartans remained and whether they were all like the men who had died. Demaratus answered that Sparta contained about eight thousand adult male citizens, all equal in quality to the warriors who had fought at Thermopylae. The claim was probably rhetorical, but its function was clear: Sparta's discipline was not the product of a single heroic king or a small band. It was a civic system.

Demaratus again advised the seizure of Cythera. Xerxes refused, and the war continued toward Salamis and Plataea. His advice did not prevent the Persian defeat, and the ancient tradition gives him no heroic role in the fighting. His importance lies elsewhere. He explains Sparta to Persia, Persia to Greek memory and the psychology of the conflict to Herodotus' audience.

After 480 BCE Demaratus disappears from the secure historical record. His death date is unknown. His family, however, remained in Persian service. Xenophon reports that in 401-399 BCE the cities of Teuthrania, Halisarna and nearby territories in Mysia were ruled by Eurysthenes and Procles, descendants of Demaratus. Athenaeus also preserves a reference to Gambreion in their holdings. These dynasts were both Greek aristocrats and Persian vassals, heirs to the strange political identity Demaratus had created.

Later stories attempted to bring him home. Polyaenus and Plutarch preserve anecdotes about his later dealings with Sparta and his descendants, but these tales are difficult to verify. A Demaratus, son of Gorgion, appears in the Hellenistic period and has sometimes been connected with the exiled royal line; he may even have been an ancestor of Nabis, the last king of Sparta. The evidence is too weak to create a continuous family history, but it suggests that the Demaratid name retained prestige long after the Persian Empire had fallen.

Herodotus' portrait of Demaratus is intentionally double-sided. He is a flawed king who acted from envy, a victim of Cleomenes' sacrilegious manipulation of Delphi and a wise exile who speaks truth to the most powerful monarch on earth. He serves Persia while warning Greece, praises Spartan discipline while benefiting from Persian patronage and loses his throne because of an uncertain birth yet displays a nobility that his enemies lack. Deborah Boedeker called this the two faces of Demaratus: the ambitious Spartan politician and the prophetic adviser who understands both Greek and Persian worlds.

His historical reliability requires caution. The details of his birth, the bribery of Delphi, the message beneath the wax and the exact conversations with Xerxes come through Herodotus and reflect oral tradition shaped by moral and dramatic purposes. The deposition itself, however, is secure in outline. So is his exile, his reception by Darius and his presence with Xerxes. His descendants' rule in Asia Minor is confirmed by Xenophon. The anecdotes around him should be read as Herodotean portraits, not transcripts.

Demaratus endures because he embodies the contradictions of the Persian Wars. He is Greek and Persian, king and dependent, exile and patriot, adviser and potential traitor. His speech on Spartan Law became one of antiquity's most influential explanations of Sparta's military character. His warning before Thermopylae shows the Persian court trying to understand a kind of freedom it cannot possess. His descendants carry his divided legacy into the fourth century. In the history of Sparta, he is the king who lost his crown yet preserved his voice.