Demaratus, king of Sparta from c515 until 491 BC of the Eurypontid line, successor to his father Ariston. As King he is known chiefly for his opposition to his colleague the other Spartan king Kleomenes I.
When Kleomenes attempted to make Isagoras tyrant in Athens, Demartus tried unsuccessfully to frustrate his plans. In 501 BC Aegina was one of the states which gave the symbols of submission (earth and water) to Persia. Athens at once appealed to Sparta to punish this act of medism, and Kleomenes I, crossed over to the island, to arrest those who were responsible for it. His first attempt was unsuccessful due to interference from Demartus who did his utmost to bring Kleomenes into disfavour at home.
In retaliation Kleomenes urged Leotychides, a relative and personal enemy of Demaratus, to claim the throne on the ground that the latter was not really the son of Ariston but of Agetus, his mother's first husband. Kleomenes bribed the Delphic oracle, to pronounce in favor of Leotychides, who became king Leotychides II in 491 BC.
After the deposition of Demaratus, Kleomenes visited the island of Aegina for a second time, accompanied by his new colleague Leotychides, seized ten of the leading citizens and deposited them at Athens as hostages.
On his abdication, Demaratus was forced to flee. He went to the court of the Persian king Darius I, who gave him the cities of Pergamum, Teuthrania and Halisarna, where his descendants were still ruling at the beginning of the 4th century.
He accompanied Xerxes I on his invasion of Greece in 484 BC and is alleged to have warned Xerxes not to underestimate the Spartans before the Battle of Thermopylae:
The same goes for the Spartans. One-against-one, they are as good as anyone in the world. But when they fight in a body, they are the best of all. For though they are free men, they are not entirely free. They accept Law as their master. And they respect this master more than your subjects respect you. Whatever he commands, they do. And his command never changes: It forbids them to flee in battle, whatever the number of their foes. He requires them to stand firm -- to conquer or die. O king, if I seem to speak foolishly, I am content from this time forward to remain silent. I only spoke now because you commanded me to. I do hope that everything turns out according to your wishes. — Herodotos vii (trans. G. Rawlinson)