Demaratus (fl.510-480 BC) was a king of Sparta best known for serving as an advisor to Xerxes I of Persia during his invasion of Greece in 480 BC. His co-ruler, Cleomenes I, was firmly anti-Persian, and this resulted in tension between the two men.
Demaratus was the 15th Spartan king of the Eurypontid dynasty. He was probably king by 510 BC when he and his fellow monarch Cleomenes expelled Hippias, Tyrant of Athens. They were also involved in the Spartan war with Argos. Demaratus was said to have been a very capable ruler, and the only Spartan king to win the four horse chariot race at the Olympics.
In 508/7 BC Cleomenes assembled an alliance of Peloponnesian states to attack Athens. His intention was to install the anti-Persian Isagoras as tyrant in Athens. Demaratus was part of this army, but it all fell apart after Cleomenes revealed his intentions at Eleusis. The Corinthians withdrew from the alliance. Demaratus also withdrew from the army, causing it to collapse.
In 494 BC the two kings were probably united again in the successful Spartan war with Argos that included the crushing Spartan victory at Sepea.
In 491 Darius of Persia sent heralds around Greece to demand that the various communities submit to Persian authority. Some refused and some accepted, but the Spartans, led by Cleomenes, went one step further and murdered the heralds by throwing them into a well. This went against all the rules of diplomacy.
One of the communities to submit to the Persians was the island of Aegina, seventeen miles south of Athens. Aegina was an ally of Sparta, but Cleomenes responded to their act by travelling to the island to try and arrest the pro-Persian leaders. Demaratus supported Aegina in this dispute, sending them a letter in which he suggested that Cleomenes was acting illegally, and had been bribed by Athens.
Cleomenes now decided to try and get Demaratus removed as king. There had always been some concern about the legitimacy of his birth, and Cleomenes bribed the oracle at Delphi to declare him illegitimate. In around 491 Demaratus was demoted to a lower ranking public office, and was succeeded as king by Leotychides (commander of the victorious Greek army at Mycale in 479 BC). The bribery was eventually uncovered, and Cleomenes was also forced into exile.
Soon afterwards Demaratus was humiliated at a public celebration and decided to go into exile in Persia. Darius welcomed him at Susa, gave him land, and welcomed him to his court. Demaratus soon became close to Prince Xerxes, one of several of Darius's sons. According to Herodotus Demaratus was the one who suggested to Xerxes that he should succeed his father, despite not being the oldest son, because he was the first to be born after Darius came to the throne. Xerxes used this argument with his father and became heir to the throne. This may well over-play Demaratus's influence with Xerxes, who had some experience of government before he came to the throne.
After Xerxes came to the throne he began to prepare for an invasion of Greece. Demaratus sent a warning message to Sparta. Herodotus provides two possible motives - first, a genuine desire to warn his countrymen, or second a wish to gloat.
Demaratus accompanied the Persian army during the invasion of Greece, and was often used as an advisor by Xerxes, but his advice wasn't followed. Early in the campaign he gave general information on the nature of the Greeks. After the battle of Thermopylae he suggested that the Persians could defeat Sparta by sending part of the fleet to occupy the island of Cythera off the Peloponnese and using as a base for raids. The Spartans would be unable to help the rest of the Greeks, and could be defeated later at leisure. Xerxes's brother Achaemenes countered this with the argument that it would be foolish to split the fleet, which had suffered heavy losses in storms. Demaratus's plan was ignored.
During the campaign in Attica one of his fellow pro-Persian Greeks, Dicaeus of Athens, reported a supernatural sign that the Persians would lose. Demaratus advised him not to tell Xerxes as he would 'lose his head', suggesting that his position wasn't quite as secure as the rest of the account suggests.
In the aftermath of Xerxes's retreat back to Asia Demaratus was given lands in the Troad. His sons Eurysthenes and Procles were still living there some time afterwards and his descendents were encountered by Spartan troops during the Persian-Spartan War of 400-387 BC.